CONTRIBUTIONS OF A LUNAR GEOSCIENCE OBSERVER (LGO) MISSION TO FUNDAMENTAL QUESTIONS IN LUNAR SCIENCE

by LGO SCIENCE WORKSHOP MEMBERS Contributions

Lunar Geoscience Observer Mission Frontispiece: Earth-rise over the lunar highlands. Contributions of a Lunar Geoscience Observer Mission to Fundaments Questions in Lunar Science

LGO Science Workshop Members

March LGO Science Workshop Members

Roger Phillips (Chair), Southern Methodist University Merton Davies, Rand Coworation Michael Drake, University of Arizona Michael Duke, Johnson Space Center Larry Haskin. Washington University James Head, Brown University Eon Mood, University of Arizona Robert Lin, University of California, Berkeley Duane Muhleman, California Institute of Technology Doug Nash (Study Scientist), Jet Propulsion Laboratory Carle Pieters, Brown University Bill Sjogren, Jet Propulsion Laboratory Paul Spudis, U .S. Geological Survey Jeff Taylor, University of New Mexico Jacob Trombka, Goddard Space Flight Center

This report was compiled and prepared in the Department of Geological Sciences and the AnthroGraphics Laboratory of Southern Methodist University.

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Dr. Roger Phillips Department of Geological Sciences Southern Methodist University Dallas, TX 75275 Executive Summary

THE IMPORTANCE OF LUNAR SCIENCE STATUS OF LUNAR SCIENCE DATA AND THEORY The Moon is the keystone in the interlinked knowledge that forms the foundation of our understanding of the silicate Unlike other solar system bodies beyond Earth, the Moon bodies of the solar system. The Moon is remarkable in that it has been intensely studied by telescope, unmanned space- remains the only other planet for which we have samples of crafi and manned missions. Most of the lunar missions known spatial context. Basic considerations are: however were very early in the history of space science and Planetary Crusts: led to later technical advancements that have been used to explore other planets including the Earth. Both the data @ The concept of primary differentiation of planetary quality and coverage of the Moon from orbit, for example, are crusts was established by lunar studies, and this has be- inferior to those for Mars for some primary data sets, such as come the framework for studying the evolution of all imaging. Nevedheless, the availability of returned rock planetary crusts and establishes crustal recycling (as on samples for the Moon has dramatically sharpened scientific Earth) as an exceptional phenomenon. questions about the evolution of our nearest neighbor. The Planetacy Surface Ages and Bombardment Histories: returned lunar samples provide the essential geochemical and @ The Moon, because we have samples of its crust, serves petrological basis for extrapolation of remote sensing data for as the basis for age estimation by crater statistics of all the Moon as a planet. Because of the knowledge gained over other crustal surfaces in the solar system, save Earth. the last twenty years, we are able to pose a set of interrelated and sophisticated questions of planetary origin and evolution Record of Exogenic Processes: that are fundamental in nature and to which a global remote @ Because its igneous differentiation was relatively rapid, sensing survey of the Moon promises to contribute significant the Moon formed a solid crust that retains a record of answers. events that occurred early in solar system history. @ The Moon has recorded more than 4 billion years of exo- genic processes in the solar system. FUNDAMENTAL QUESTIONS IN Baseline Silicate Planet: LUNAR SCIENCE @ The Moon represents a relatively simple silicate system Understanding the Moon's structure, composition and in terms of differentiation, and thus serves as a baseline history will permit us to obtain a much better appreciation of to study more complex planetary processes. fundamental processes that operate on all planets. Some of Key to Early Earth History: the major problems in lunar science are: @ The origin and early evolution of the Earth is inextri- Origin: cably tied to lunar genesis. @ What is the origin of the Moon and how does it relate to the origin and early evolution of the Earth? Utilization of Moon: @ An understanding of the structure and composition of Dgferentiation: the lunar crust is the basis for utilization of the Moon's @ Was there a global magma ocean or was the crust formed resources. by serial igneous intrusion? @ What was the volume fraction of primordial lunar preclude the possibility of a reasonably-sized imaging data melting? set, as well as a robust set of information from VIMS. @ What are the composition and structure of the mantle'! A Radar Altimeter (ALT) and a nearside Doppler- @ 'Is there an iron-rich lunar core? tracking gravity experiment are key geophysical instruments for extrapolating the surface geochemical results throughout Magma tic History: the whole crust. Equally important geophysically are the @ What were the nature and style of highland igneous Magnetometer and Electron Reflectometer (MAG/ER), activity and mare volcanism through time? which are needed to determine, either by direct detection or irhermal Histov: remanent field mapping, if the Moon possesses an iron-rich @ What have been the mechanisms of heat transfer in the core. The existence of a core directly bears on hypotheses of lunar interior over geological time? lunar origin. The geophysical capabilities could be extended @ What is the history of magma genesis in the mantle? by a Microwave Radiometer (MUD) experiment to map @ How did lithospheric thickness vary spatially and surface heat flow and determine the bulk uranium content, through time, and what were the accompanying tectonic and by a Satellite Gravity System (SGS) to measure the styles at the surface? farside gravity field. @ What are the present-day lunar temperature profile and surface heat flow? Impact Processes: @ What was the effect ofthe intense lunar bombardment in RIELATIONSHIP OF LGO TO obscuring primary compositional and lithological varia- FUNDAMENTAL LUNAR PROBLEMS tions in the crust? @ How much of the crustal column was exposed by giant Given the set of instruments described above, LGO can impacts and how were materials dispersed across the contribute to lunar science in at least the following ways: lunar surface? Lunar Origin: @ What are the mechanics of crater and basin formation @ Global compositional estimates principally from the and what is the makeup of ejecta in terms of the XGRS, and possibly from the MUD,should eliminate impacting body and the crustal target? some hypotheses while remaining permissive of others. Paleornagnet ism: @ The MAG and ER instmments should provide limits on @ What are the relative contributions of an internal the radius of a metallic lunar core which, ifdetected, may dynamo and impact processes to the origin of lunar be used in concert with depletions of siderophile ele- paleomagnetism? ments in the lunar mantle to constrain hypotheses of lunar origin. Regolith: @ How do lunar soils mature? Evolution ofthe Crust and IMantle: @ What is the relationship of regolith composition to that of @ Global geochemical and petrological maps from XGRS the underlying igneous crust? and VIMS, coupled with gravity data and topography from ALT should provide (I) evidence for the existence, extent, depth, and differentiation products of a global magnla ocean; and (2) estimates of crustal thickness and REQUIRED SCIENTIFIC CAPABILITIES density variations. OF A LUNAR GEOSCIENCE OBSERVER Magmatic History of the Moon: @ Global geochemical and petrological maps from XGRS The driving scientific basis of a Lunar Geoscience Ob- and VIMS shornld pemit identification of the regional server (LGO) mission centers on questions involving the extent of known and unhown rock types and provide origin of the Moon and the origin and evolution of the lunar insiglit into (I) the nature and duration of highland crust. At the heart of this quest is a capability to map globally igneous activity; and (2) the nature and extent of mare the distribution of minerals and elements on a scale of five volcanism. hundred meters to one hundred kilometers. Three instru- ments provide this capability: the X-Ray and Gamma-Ray Impact Processes: Spectrometers (XGRS) for elemental mapping and the @ Imaging, XGRS, VIMS, ALT and gravity data will Visible and Infrared Mapping Spectrometer (VIMS) for enable studies of crater and basin structure, morphology, mineralogical mapping. In addition, an imaging capability is and composition of deposits. Specifically, these data can required to place the geochemical data in a geological be used to reconstruct (1)pre-impact target composition context, particularly as regards the inte~retationof these and structure; (2) fornational conditions and dimen- results in the face of a crust heavily disturbed by impact sions of the excavation cavity; and (3)post-impact ejecta processes. The data rate of the LGO spacecraft should not deposition and modification. Thermal History: Earth. Continued study ofthe Moon at this time is compelling @ The MUDinstrument should provide estimates of the because we know the right questions to ask and LGO is present-day surface heat flow. High resolution imaging extremely well matched to providing some of the important of tectonic features associated with lithospheric loading, answers. and improved global topography from the ALT instru- The Moon is the centerpiece for the study of all silicate ment and improved gravity data should permit investiga- bodies in the solar system. The study of the Moon profoundly tion of variations in lunar lithospheric thickness. affects our views of the other planets. The knowledge about Together these observations may be used to infer lunar the Moon to be gained from the LC0 mission will be thermal history. immediately applicable elsewhere. We attempt to learn about processes, so we can understand better bow the solar system Lunar Paleornagnetism: came to be the way it is today. The Moon is the best place in @ A combination of the MAG and ER instruments should measure the orientation of regional surface magnetic the solar system to study the processes that affected the larger silicate bodies early in their history, and LGO promises to fields as a function of age. The measurements will help distinguish whether an intrinsic magnetic field, an ex- make substantial contributions to our understanding of the Moon. ternal source, or both, were responsible for lunar paleo- As the world expands its involvement with space science, magnetism. the Moon will undoubtedly continue to be a centerpiece. Regolith Studies: Because of our initial investment and the long-term signifi- @ Imaging, XGRS, and VIMS will establish the distribu- cance of the Moon, both scientifically and politically, it is tion and the composition of the lunar regolith as well as important that we maintain a vigorous program of lunar identifying new rock types, and should provide insight science and exploration. into the processes occurring during regolith formation, and the relationship of regolith to underlying bedrock.

LGO AND OTHER MISSIONS

LGO IN =SOURCE EXPLORATION There are several ways that an LGO mission could AND LUNAR BASE SITE SELECTION interact with other missions, both U.S. and foreign: @ The ISTP (International Solar Terrestrial Physics) The remote sensing instruments on LGO are ideally suited for resource exploration and discovery, and lunar base site program will involve at least four spacecraft (WIND, selection. Examples of known and potential lunar resources POLAR, EQUATOR, GEOTAIL) placed in different Earth orbits, some involving lunar swingbys. to map are: various portions of geospace. One of these ISTP space- @ High concentrations of titanium and of iron in certain craft might serve as a data-relay link to Earth from LC0 basalt Rows. for farside gravity mapping and as a platform for a @ High concentrations of aluminum that would occur in the second magnetometer, which is required to determine major expanses of nearly pure anorthosite. the electrical conductivity profile of the lunar mantle. @ The possible existence of ore bodies (e.g., chromium in @ The Japanese have indicated plans for a lunar orbiter layered magnesium-suite intrusions). mission. They have indicated very informally some @ Possible trapped water in permanently shaded polar interest in a cooperative Japan/U.S. lunar mission. craters. Discussions have occurred which envisage a U.S. LGO spacecraft in low-lunar orbit for global geochemical and In addition, the basic science results concerning the geophysical mapping, and a smaller Japanese spacecraft structure and composition of the lunar crust will lead to in high elliptical orbit, carrying a magnetometer and models of lunar ore genesis, which should greatly aid in the acting as a communication link to LC0 for farside search for new mineral deposits. LGO will aid in lunar base gravity mapping. site selection by mapping site geology, topography, resources @ The Soviets have plans for a two-phase mission. The first and potential safety hazards. phase would put a single spacecraft in low circular orbit for low-resolution global surface mapping. The second CONCLUSIONS phase would have the Soviet spacecraft raised to a high elliptical orbit to allow it to carry out solar observations. Because of the well-studied lunar samples, it is clear that It is conceivable that during this latter phase a U.S. LGO our current understanding of the Moon is at a more advanced spacecraft could utilize the high orbit Soviet spacecraft state than that for any other body in the solar system, save as a path link to Earth for farside gravity measurements.

vii

Contents

Executive Summary ...... v ... Preface: Purpose and Scope of This Report...... xizi 1 Introduction ...... 1 2 Fundamental Questions in Lunar Science...... 3 2.1 Introduction ...... 3 2.2 What is the Origin of the Moon? ...... 3 2.2.1 Intact Capture 2.2.2 Disintegrative Capture 2.2.3 Rotational Fission 2.2.4 Collisional Ejection 2.2.5 Binary Accretion 2.2.6 Tests of Lunar Origin Refractory Elements Magnesium Number 2.3 What is the Evolution of the Lunar Crust and Mantle?...... 5 (Hence What is Their Composition and Structure) 2.3.1 The Magma Ocean Concept 2.3.2 Crustal Thickness and Structure 2.3.3 Depth of Early Lunar Differentiation Event 2.3.4 Structure and Composition of the Mantle 2.4 What is the Magmatic History of the Moon?...... 8 2.4.1 Igneous Activity in the Highlands 2.4.2 Mare Volcanism 2.5 What is the History and Nature of Impact Processes on the Moon?...... 12 2.5.1 Bombardment Effects 2.5.2 Cratering Mechanics 2.5.3 Crater and Basin Ejecta 2.6 Is There an Iron-Rich Core in the Moon?...... 12 2.7 What is the Thermal History of the Moon? ...... 13 2.7.1 Mean Heat Flow at the Surface 2.7.2 Present-day Lunar Temperature Profile 2.8 What are the Origins of Lunar Paleomagnetism? ...... 15 2.9 What is the Nature of the Lunar Regolith...... 15 2.10 Summary ...... 3 Contributions of an LGO Mission to Fundamental Lunar Science...... 19 3.1 Introduction ...... 3.2 Origin ofthe Moon ...... 3.2.1 Refractory Elements 3.2.2 Magnesium Number 3.2.3 Summary 3.3 Evolution of the Crust...... 23 3.3.1 Magma Ocean 3.3.2 Crustal Thickness and Density 3.4 Magmatic History of the Moon ...... 25 3.4.1 Highland Igneous Activity 3.4.2 Mare Volcanism 3.5 Lunar Cratering Processes and History ...... 27. 3.5.1 Basin Processes 3.5.2 Cratering Mechanics 3.5.3 Crater and Basin Ejecta 3.6 Lunar Iron-Rich Core ...... 29 3.7 Thermal History ...... 31. 3.7.1 Lunar Temperature Profile 3.7.2 Surface Heat Flow 3.7.3 Lithospheric Thickness Variations 3.8 Lunar Paleomagnetism ...... -33 3.9 Regolith Studies ...... 34 3.10 Summary ...... 36 4 Role of LGO in Resource Exploration and Lunar Base Site Selection ...... 37 4.1 Why Use Lunar Resources? ...... 37 4.2 Nature and Distribution of Known Lunar "Ores" ...... 37 4.3 Discovery of New Ores ...... 38 4.4 Value of LGO Basic Geoscience to the Search for Resources ...... 39 4.5 Resource Exploration Summary ...... 40. 4.6 Sites for a Lunar Base ...... 40 5 Concluding Remarks ...... 41. 5.1 Introduction ...... 41 5.2 What LGO Can Tell Us about the Moon ...... 41 5.3 The Moon is a Keystone ...... 42 5.4 LGO and Other Missions ...... -42 5.4.1 ISTP Program 5.4.2 Japanese and Soviet Missions Appendix A: Return from LGO Instruments ...... 45 A.1 Global Composition of the Moon ...... 45 A.1.1 Visible and Infrared Mapping Spectrometer A.1.2 X-RayIGamma-Ray Spectrometers A.1.3 Themal Infrared Mapping Spectrometer A.2 Imaging Systems ...... *.48 A.3 Magnetometer/Electron Reflectometer Capabilities ...... 49 A.4 Radar Altimeter ...... +.51 A.5 Gravity Field (Including Spacecraft Gravity System) ...... 51 A.6 Microwave Radiometer Capabilities ...... 53 Appendix B: Apollo Orbital Petrological Mapping Capabilities: Implications for LGO ...... 53 Appendix C: The Present Lunar Photographic and Topographic Coverage and Possible LGO Contributions ...... 57 C.l Introduction ...... 57 C.2 Imaging and Altimetry on LGO ...... 59 Appendix D: A Twin Lens CCD Camera for Improving the Geodetic Control on the Moon ...... 61 D.1 Geodetic Control on the Moon ...... 61 D.2 A Twin Lens CCD Camera for the Lunar Geoscience Observer ...... 61 Appendix E: Heat Flow Mapping by Microwave Radiometry ...... -63 E.l Introduction ...... 63 E.2 Accuracy and Precision for a Microwave Experiment ...... 64 E.2.1 Limitations Caused by Local Properties of the Lunar Regolith E 2.2 Instrument Limitations Appendix F: Lunar Geoscience Observer (LGO) Mission Description ...... 67 F.1 Introduction and Project Characteristics ...... 67 F.2 LGO Science Workshop Summary ...... 68. F.2.1 Introduction F.2.2 Preliminary Results of Workshop F.2.3 Strawman Payload F.3 LGO Mission Design ...... 69 F.4 Spacecraft General Characteristics ...... 70. Appendix G: LGO Instrument Descriptions. Requirements. and Characteristics ...... 71. Appendix H: References ...... 83..

Preface: Purpose and Scope of This Report

This report is the outgrowth of a Jet Propulsion Laboratory After a general introduction, Chapter 2 of this document (JPL) sponsored science workshop that met twice in 1985 to reviews fundamental science questions regarding the Moon address a Lunar Geoscience Observer (LGO) Mission. A without specifying the methods required to provide answers. summary of the results of the workshop are included in JPL Chapter 3 then outlines the contributions of an LGO mission Document No. 642-90 1 (15 November 1985), which in- to providing some of the answers; it will be seen that the cludes the LGO mission design study, a synopsis of which is mission can be extremely scientifically productive. The given in Appendices F and G here. The present report was measurement techniques needed to attack the scientific prepared as an independent effort by the members of the issues also turn out to be highly appropriate for lunar workshop, who desired to set down in some detail the materials resource evaluation and lunar base site selection contributions that an LGO mission can make to answer some (Chapter 4). Various measurement details are provided in of the key questions in lunar science. the appendices.

The Moon occupies three distinct niches in the solar system, except the Earth. Additionally, the concept of system: (I) It is the small end-member of the set of five primary differentiation of planetary crusts was established by terrestrial planetary bodies nearest the sun; (2) with the lunar studies. This provides the framework in which to study exception of Pluto's satellite, Charon, it is the largest satellite the evolution of all planetary crusts and establishes con- in the solar system in terms of mass ratio to parent planet; tinuous crustal recycling (as on Earth) as an exceptional and, (3) because of its close proximity, it is likely to be the planetary phenomenon. location for the first human settlement beyond the Earth. For Because of our understanding of the Moon as the result of these reasons, and because the Moon has recorded 4.5 billion the Apollo program (Figure l.l), we are in a position to years of solar system history, knowledge of its origin, attack fundamental problems in lunar science by asking very evolution, composition and present state provides a true detailed questions, the answers to which can be obtained by window into the events and processes that shaped the solar very specific measurements. In many instances, the mea- system, and may profoundly influence the future of humanity surements needed are significant extensions over the meager in space. spatial and spectral coverage ofthe Moon now available (e.g., The Moon may be the least well understood satellite in Appendix C). These answers will provide direct constraints terms of its origin. It is easier, for example, to understand the on hypotheses of lunar origin and evolution. origin of Jupiter's satellites in terms of accretion in Jovian A large part of our understanding of the Moon is tied up in orbit for the Galilean satellites and capture in orbit for the how well we understand its crust, both in terms of inferring smaller irregular bodies. The origin of the Moon, on the other bulk composition and in understanding the primary crustal hand, is likely tied up with and may have affected the earliest processes of a terrestrial planet. The Moon is very coopera- history of the Earth, so an understanding of the early Earth tive in terms of having an airless unweathered surface, and requires knowledge of the Moon's origin. thus scientific questions regarding the composition of the As a terrestrial planet, the Moon represents a relatively crust are extremely well matched to the remote sensing simple silicate system in terms of differentiation. Thus, we capabilities of a Lunar Geoscience Observer (LGO) mis- are better equipped to understand the multitude of detaiis sion. It follows, therefore, that an Observer-class mission concerning its evolution than we are for the other planetary can address major questions regarding the Moon. The bodies. The Moon serves as a baseline to study more complex spacecraft also provides a platform from which to carry out planetary processes. Additionally, because its igneous dif- key geophysical measurements concerning the presence of a ferentiation occurred early and was relatively rapid, the metallic core, the thermal history ofthe Moon, and the origins Moon formed a solid crust that retains a record of events that of lunar paleomagnetism. occurred early in solar system history. In particular, the In this report we have attempted to summarize some of the Moon is the keystone in understanding the events that shaped major issues in lunar science and assess the role of an EGO the silicate bodies in the solar system. For example, dated mission in answering some of the basic scientific questions. samples ofthe lunar crust serve as the basis for age estimation The role of the mission in resource evaluation and lunar base by crater statistics of all other crustal surfaces in the solar site selection has also been considered. 2 LG8 AND LUNAR SCIENCE

I;igure I.I: Sample collection traverse from the ApolIo 16 landing site at Descartes. Fundaments Questions in Lunar Science

2.1 INTRODUCTION Moon formed, and (4) everything that happened to the Moon and its primary ingredients as it formed. The burning We have barely explored the Moon as a whole, yet we have questions about the origin of the Moon center around the investigated it sufficiently to be able to pose sophisticated Moon's bulk composition, the chemical differentiation that questions about its origin and evolution. Obtaining answers produced the crust, mantle and core, the extent to which the to these questions will allow us not only to understand the Moon was melted when it was assembled, the details of its Moon's history better, but it will permit us to obtain a much thermal history, and the dynamics of the accretion process. better appreciation of fundmental processes that operate on The dry, airless Moon with its lower density, lower mass, all planets, including planetary accretion, differentiation, and smaller radius is clearly quite different from our Earth. volcanism, and cratering. Yet it is a sister to the Earth and may have formed in its In studying such a complex topic as a planet, even a vicinity. How could it have formed, and how did it acquire relatively small and simple planet such as the Moon, we can such different properties from those of Earth? Models of easily ask major questions such as "What is the origin of the lunar origin attempt to answer such questions and are con- Moon?" It is nearly impossible, however, to formulate single tinually being modified, particularly in response to new experiments or make single observations that answer such information. Five hypotheses are currently under con- questions. In most cases, several or many processes have sideration for the origin of the Moon. These are (1) intact contributed to each observable phenomenon and rarely does capture, (2) disintegrative capture, (3) rotational fission, one of those processes dominate to the extent that it can be (4) binary accretion, and (5) collisional ejection. A brief easily recognized and shown to be the principal cause of the outline of the current hypotheses follows. phenomenon in question. Thus, the answer to the question "'What is the origin of the Moon?" may havz to be a lengthy answer, one that restates the question in parts and which 2.2.1 Intact Capture requires answers to many more specific questions before it can be answered definitively. The Intact Capture Hypothesis postulates that the Moon All fundamental questions about the Moon are inter- formed elsewhere in the solar system and was captured intact related, and most require the multidisciplinary efforts of with its present compositional properties. The principal geochemists, petrologists, geophysicists, geologists, and dy- objections to this model are dynamical [e.g., Elansen, 19821, namicists to make substantial progress. Below we provide and on these grounds it is not considered to be viable. There discussion of some of those questions. are no compositional properties capable of constraining this hypothesis which have not already been measured.

2.2 WHAT IS THE ORlGlN OF THE MOON? 2.2.2 Disintegrative Capture The Moon's origin (Figure 2.1) involved numerous pro- cesses operating as planets formed from gas and dust: We As proposed by Wood and Mitler [I9741,Mitler [I9751, must understand (1) where the Moon's raw materials came and Smith [19741, the Moon was formed in circum-terrestrial from, (2) the dynamics of collecting the material together to orbit by accretion of the silicate mantles and crusts of make the Moon, (3) the location in the solar system where the differentiated lunar-sized parent bodies. These lunar-sized LGO AND LUNAR SCIENCE

Figure 2.1: Unless the Moon was captured intact, approximately 4.5 billion years ago the Earth must have been surrounded by a disk of debris that subsequently aggregated to form the Moon. The origin of the circumtenestriat swam is intensely debated. Limits may be placed on this debate by estimates derived from LC0 data of the global concentrations of the refractoeory elements and the bulk Moon Mg/(MgtFe) ratio. From a painting by Bill Hartmann. bodies were originally in heliocentric orbits and were dis- Earth, leading to the ejection of mantle material from both rupted during passes through the Roche limit of the Earth. planets. Estimates of the relative contributions of target and projectile to the ejecta vary from predominantly target [Ringwood, 1979; WGnke and Dreibus, 19841 through roughly equal contributions [Melosh and Soneft, 19861 to 2.2.3 Rotational Fission predominantly projectile 1Cameron, 19861. From a geo- chemical viewpoint, derivation of ejecta almost entirely from The hypothesis that the Moon was formed by rotational the target. (Eapth's marrtle) is a variant of the Fission fission of the Earth was originally proposed by Damin [ 18801. Current advocates include Binder [ 19841 and Dun"- Hypothesis, while derivation of ejecta almost entirely from sen and Scott [I 9841. This hypothesis is by far the most the projectile is a variant of the Capture Hypothesis. readily testable because it predicts that the hullApollo 15 and 16 are too low in contain refractory elements (e.g., U, Ca, Al) in the same both accuracy and precision to enable adequate characteri- proportions relative to each other as we find in the chondritic zation of either spatial variability or average value for the meteorites. These elements do not appear to have separated regions overflown. From the standpoint of precision alone, from each other prior to accretion of individual planets. As a uncertainties in the value of Mg' from the remotely sensed group, however, they may become separated from other data are seldom less than 30%. classes of elements prior to planetary accretion. An impor- The value of 0.80 is compatible with the observed tant question bearing on the origin of the Moon is whether the abundances of Fe and Mg in the lunar basalts, which are bulk concentration of refractory elements is greater in the significantly more iron-rich than most terrestrial basalts [e.g., Moon than in the silicate portion of the Earth. Another is Wanke et al., 1983 1. If the Moon indeed has a different value whether the relative proportions are the same in the bulk of Mg' than the Earth, this would constrain theories of lunar Moon as in the bulk Earth. origin in a substantial way. Either the Moon could not have Immediately after analysis of the Apollo lunar samples been derived ~nainlyfrom Earth, or the mechanism by which began, several investigators concluded that the Moon was it was derived included some complex chemistry. enriched in refractory elements compared to the Earth, an observation inconsistent with simple models for derivation of a substantial portion of the Moon from terrestrial material 2.3 WHAT IS THE EVOLUTION OF THE LUNAR and more consistent with the binary accretion hypothesis. CRUST AND MANTLE? More recent analysis of the abundances of the refractory (HENCE WHAT IS THEIR COMPOSITION elements does not provide convincing evidence for a differ- AND STRUCTUW?) ence between lunar and terrestrial abundances of these elements [e-g.,Drake, 19861. Many refractory elements are "incompatible" during 2.3.1 The Magma Ocean Concept chemical separations within silicate planets; i.e., they enter the less dense, liquid phases rather than concentrating into Intense study of the rock and soil returned by the Apollo residual solid phases when partial melting occurs within these and Luna missions has enabled us to construct a sketchy planets. Thus, as the liquids rise out ofthe planetary interiors, outline of the Moon's evolution. Not only does this outline they extract the incompatible elements and concentrate them need to be filled in with more quantitative details, but some toward the surfaces. Using ratios of incompatible elements aspects of it are Likely to be wrong. This section briefly and estimates of lunar global heat flow from the Apollo 15 describes the canonical view of the Moon's evolution and and 17 heat probes, estimates of the bulk lunar U abundance raises questions concerning it. may be made. Current "best" estimates of bulk U concentra- The surprisingly high abundance of plagioclase feldspar (a tion are 20 parts-per-billion (ppb) for the Earth and 35 ppb for Ca-Al silicate) in the lunar highlands led to the idea that when the Moon [Drake, 19861. There is sufficient uncertainty in the Moon formed it was surrounded by a huge layer of molten the lunar value, however, that it cannot be convincingly rock (magma) in which plagioclase, a mineral with a rela- argued that it differs from the terrestrial one. tively low density, floated to form the original lunar crust. Some large lunar samples are composed, in fact, of almost nothing but plagioclase; these are called ferroan anorthosites Magnesium number. The magnesium number [Mg' = (the adjective "ferroan" refers to the high Fe content of Mg/(Mg + Fe)] is an important indicator of both the initial coexisting mafic minerals). As plagioclase floated, denser composition of a silicate planet and of the extent of its minerals such as olivine and pyroxene sank (or crystallized geochemical differentiation. Mg and Fe are major consti- near the bottom of the system) to form the lunar mantle. The tuents of the Moon and are probably exceeded in abundance magma ocean concept has been reviewed recently by Warren only by Si and 0. Bulk planet estimates of Mg' depend on [ 19851. interpretation of surface concentrations in terms of the extent The whole idea of a lunar magma ocean hinges on the of differentiation of the planet as a whole. composition of the Moon's crust. Some investigators ques- Various workers have argued that the value of Mg' for tion whether there was a mapaocean or not, arguing that a Earth's upper mantle is 0.89; the most recent of these feldspathic crust could be produced by successive intrusions LGO AND LUNAR SCIENCE of large amounts of magma into a primordial crust (Figure 2.2). In each intrusion, plagioclase would tend to concentrate toward the top, thereby producing the plagioclase-rich crust. The main parameters that provide clues to the nature of the crust-forming event are the abundances of plagioclase and incompatible elements. Based on current data, including the remote-sensing devices carried by the Apollo 1 5 and 16 Command Modules and Earth-based spectral reflectance data, we do not know with certainty how much plagioclase there is in the crust or how it varies in abundance laterally or vertically. The same applies to incompatible elements. We also do not know how deep the magma ocean was; estimates range from a hundred kilometers to the entire Moon. Nor do we know how the various igneous rocks in the lunar high- lands, besides those composed of little else but plagioclase, relate to the putative magma ocean, if at all.

Figure 2.2: This diagram [after Walker, 19831depicts formation of the feldspathic lunar crust by a series of intrusions and aluminous 2.3.2 Crustal Thickness and Structure basalt flows.

Geophysics aids the surface measurements of geochemis- try and petrology by extending such information to depth. density variations. Seismic thickness estimates at other sites Crustal thickness has been measured seismicafly, using around the Moon would allow final evaluation of the relative arrival time data from artificial impacts, only in the Mare contributions of Airy and Pratt isostatic compensation of the Cognitum region beneath the Apollo 12 and 14 stations, lunar crust, yielding an optimally accurate estimate of mean where a probable thickness of 55-60 km was deduced crustal density and thickness. [ToksGz et al., 19741. Beneath the Apollo 16 Descartes Seismic velocity models of the lunar crust have been highlands site, a somewhat greater thickness of 75 + 5 km has shown to be generally consistent with laboratory data for been estimated by Coins [1978] via the identification of surface highland samples of anorthositic gabbro (anorthositic possible reflected phases from the base of the crust. Else- norite) [ToksGzel al., 19743. However, rock types ranging where, crustal thichesses must be infened via analyses of from gabbro or norite to anorthosite are also allowed so that orbital gravity and topography, using the few seismic thick- seismic detection of possible vertical composition changes is ness estimates as control points. Unfortunately, high-resolu- not possible with the current data [Liebermann and Ring- tion gravity and topography data are sparse. Coverage is wood, 19761. limited or absent for Large regions of the farside and high- In support of a horizontally stratified crustal model, latitude (>30") parts of the nearside. Conseqiaently , global sample studies have suggested the occurrence of low-K Fra gravity and topography maps are presently limited to very Mauro basalt components in areas where deep crustal approximate spherical harmonic models with wavelengths material sampled by large basin impacts would be preserved, representative of basin-sized or larger features [Bills and e.g. near the rims of the Imbrium and Serenitatis basins Ferrari, 11 977a, 19801. [Ryderand Wood, 19771. Two- or three-layer models have The most detailed analysis of gravity and topography data therefore been proposed in which the most accessible an- to infer lateral crustal thickness variations is that ofBikls and orthositie: gabbro layer comprises less thm one-third of the Ferrari [ 1977 b], who assumed an Airy isostatic compensa- crustal column and is underlain by a 15-20 km thick noritic tion model and showed that crustal thickness would range layer and possibly a third mafic anorthosite layer. Further from 30-35 h beneath the mascon basins to 90-1 10 krn support for the concept of a grossly stratified crust is found in beneath the highlands with a "global" mean of approximately the correlation between estimated transient cavity size 70 km. This model accounts for the center-of-figetre to center- (hence, depth of excavation) and the total fraction of noritic of-mass offset of 2.0-2.5 km [Kaulaet al., 19741by a thicker material identified by orbital geochemical data within the farside crust. Available coverage of orbital gamma-ray mea- ejectaof 1 1 lunar basins [Spudiset al., 19841. Compositional surements of surface Fe, Mg, and Ti abundances are stratification of the upper 10 km of the lunar crust has been consistent with a compensation model that is about 70% Airy clearly documented with high resolution spectral reflectance type if surface compositions are applied to estimate lateral data at Copernicus [Pieters and Wilhelms, 1985;Pieters et density variations in the entire crust [EIaines and Metzger, al., 19851, Aristarchus [Lucey et al., 19861, and Tycho 19801. Increased coverage of both high-resolution gravity/ [Pieters, 19861. topography and crustal composition data would allow more More accurate limits on crustal composition variations detailed and reliable modeling of lateral crustal thickness and with depth would significantly improve current boundary Fundamental Questions

conditions for lunar differentiation models and would more SEISMIC STATIONS firmly establish the crustal contribution to lunar bulk com- position. For example, if a mean crustal composition of norite is more appropriate than anorthositic norite, the crustal Al2O3content is reduced from 25 wt 96 to -17 wt 96 and the crustal contribution to the lunar bulk abundance of AI2O3is reduced from -2.6 wt 96 to -1.8 wt 5% (for an assumed crustal mean density and thickness of 3.0 g ~m-~and 70 km, respectively). Depending on the depth of initial lunar dif- ferentiation, corresponding bulk lunar A1203 abundance estimates would range from values consistent with chondritic and terrestrial mantle abundances to values twice as large as the chondritic abundance 1-5.0 wt 96;Kaula, 19771. It is therefore evident that more accurate estimates for bulk lunar A1203abundances are needed and can be obtained only when crustal vertical composition variations, mean crustal thick- ness, and depth of differentiation are more effectively con- strained.

2.3.3 Depth of Early Lunar Differentiation Event

Although the existence of a thick plagioclase-rich crust implies differentiation of at least the outer few hundred lam of Figure 2.3: Structure of the lunar interior as inferred from Apollo the early Moon [see reviews by Warren, 1985; Wood, 19861, seismic data [after Nakamura, 19831. the depth of that diRerentiation event has not been estab- lished. As a result, models for the bulk structure and composition of the deep lunar interior remain controversial. to observational uncertainties and interpretations are sorne- Some models have adopted a relatively shallow (<500 km times nonunique, any resulting compositional inferences are deep) outer differentiated zone overlying a primordial un- of interest in constraining bulk composition models and the differentiated interior on the basis of (1) a consideration of depth of early mantle differentiation. available heat sources for melting the deep interior; (2) upper Determination of precise limits on lunar mantle seismic limits on global thermal contraction and expansion derived velocities has been hindered by the small number and areal from an apparent lack of surface compressive tectonic distribution of Apollo stations, by the finite available data set, features (Soloman, 1977; but see Binder and Lange, 1980, and by a low signal-to-noise ratio in the received seismic for a dissenting view]; and (3) interpretations of mantle signals due in part to a near-surface scattering zone. The focal seismic velocity models. Atthough the heat source for depths of deep moonquakes range from about 800 to 1 100km producing an initially molten or partially molten Moon and impose a basic limit on the maximum depths to which continues to be obscure, a similar problem exists for the velocities are usefully deteminable at the present time. parent bodies of certain differentiated meteorites and remains The most recent mantle seismic velocity model [Naka- generally unresolved. Some current geochemical arguments mura, 19831 is based on the complete five-year data set based on the volume of parental material needed to produce a acquired when the four Apollo seismometers were sirnul- given concentration of elements in the crust favor whole- taneously operative and includes significantly more deep Moon differentiation [Taylor, 19821, while other arguments moonquake signals than were available in earlier analyses. In based on interpretations of mare volcanic glasses suggest the the upper mantle (<500 km depth), the model is in approx- possibility of a primordial undifferentiated interior (Delano imate agreement with earlier velocity models leg. Goins, andlindsley ,19831. Thus, from the present limited informa- 19781 and would be consistent with extrapolations of labora- tion, it appears that models for restricted and unrestricted tory data for olivines and pyroxenes with Mg' values of depths of initial lunar differentiation are equally uncertain. 0.65-0.80. In the middle mantle (500-1000 km depth), Nakamura's velocity model differs substantially from earlier models, demonstrating the greater uncertainties at these 2.3.4 Structure and Composition of the Mantle depths and the sensitivity of derived model velocities to the deep moonquake data base. Specifically, a velocity increase Geophysical bounds on lunar mantle structure and com- is inferred to occur below 500 km, whereas earlier models had position, including evidence for changes in mineralogical suggested a velocity decrease at these depths. The inferred phase and density, are derived primarily from seismic velocity increase implies either an increase in Mg' for olivines velocity models (Figure 2.3). Although the latter are subject and pyroxenes below 500 km, a phase change, presumably LC0 AND Id UNAR SCIENCE involving the transition from spinel- to garnet-bearing assem- 2.4.2 Mare Volcanism blages, or simply a transition from an aluminum-poor to an aluminum-rich region. Distinguishing among these possibili- The dark, relatively smooth areas on the Moon were ties would be an important goal because quite diRerent formed when basaltic lavas erupted and flowed across its implications follow regarding the depth of mantle difkren- surface, filling low areas, including most of the giant impact tiation, the bulk mantle Mg', and the size of a possible basins on the nearside. Although volumetrically estimated at metallic core needed to satisfy the moment of inertia only 1% of the lunar crust [Head, 19761, mare basalts are constraint. imporlant because they formed by melting of the lunar It is clear that any substantial progress on the question of mantle. Consequently, they serve as probes of the chemical mantle composition and structure reqtalres establishing a composition and mineralogy of othewise inaccessible re- global seismic network on the Moon. However, other obser- gions of the lunar interior. Experiments at hi& temperature vations are also relevant and must be considered in evaluating and pressure suggest "Eat mare basalts fomed by partial mantle composition models. Improved limits on crustal melting at depths of at least 200 km, deeper than even the density and thickness combined with independent limits on largest impact excavated. Geochemical models for mare the size of a metallic core from elec"comagnetr^esounding data basalt evolution suggest that the rocks in the lunar mantle that would further restrict the range of mantle density models partially melted to fom the mare basaltic mamas may have through the mean density and moment of inertia constraints. been the dense if;e/Mg-bearing minerals that would have sunk In addition, probable limits on bulk lunar A1203abundance in the magma ocean. Mare basafits, therefore, may contain derivable from cmstal composition estimates and differen- infc~rmationabout the initial lunar differentiation. tiation models would help to determine whether garnet is Basalts collected from maria by the Apollo and Luna likely to exist in the middle mantle in suficient quantities to missions range in age from about 3,9 to 3. li b.y. The low explain seismic velocity increases. abundance of craters on some maria indicates that significant volcanism continued until about 2 b.y. ago [Boyce et aE., 2.4 WHAT IS THE MAGMATIC HISTORY 1974.1, or possibly until It b.y .ago for minor amounts (Figures OF THE MOONi? 2.4a,b; see Schultz and Spudis, 1983). However, not all basalts are found in the marii(Figure 2.5). Glasts in breccias 2.4.1 Igneous Activity in the Highlands from the I?i&lands, especially from Apollo 14, indicate that mare-type volcanism staarted at least as long ago as 4.2 b.y. Afier the fornation of the primordial cmst (and the mama [iraylor et al., 19831. Surfaces of such old flows are not ocean had solidified), the Moon's mantle or lower cwst began apparent now because impacts early in lunar history have to remelt and a vast array of magmas are thou&t to have obliterated them jRyder and ;Taylor. 1976: Ryder and intmded the crust during the period from 45 to 4.0 billion Spudis, 1980;Be11 and Hawke, 19841 and/or they are not years ago [C-Varwn,19861.Studies of lunar samples have led common. to the identification of at least eight distinct highland cwstal Besides their large range in age, mare basalts also vary rock types (Appendix A, Table A.1) besides ferroan an- greatly in chemical composition (Appendix A, Table A.2). orthosites, and more are recognized every year, Lithologic TiO,, for exmple, rmges from less than I wt 96 to 13 wt %in abundances in the lunar sample collection of pristine samples the returned smples. Furthemore, spectral data of the lunar relatively unaltered by the impact history suggest that rocks nearside maria obtained by telescopic observations from consisting of about half plagioclase and half either low-Ga Earth [Pieters, L 9'781 indicate that we have sampled fewer pyroxene (norites) or olivine (troctolites) mi&t be wide- than half of the types of mare basalts that erupted on the spread in the lunar cmst, rivaling ferrcaan anorthosites. These Moon. We know vidually nothing about farside maria. Early rocks are collectively called the "Wg-rich suite." It is not suggestions of a systematic relation between age and com- known how these rock types relate to one another, to position of lunar basalts were shown to be wrong as additional apparently less common rock types. or to the mapaocean or compositional information was obtained througli detailed products supposedly produced from the mama ocean. studies of lunar samples and remote measurements. Conse- Considering that new lunar rock types are still being found quently, we have an incomplete picture of mare basalt origin as fragments in complex, impact breccias after years of study, and, thus, a skimpy knowledge of the lunar interior and of it seems safe to say that many other rock types exist on the processes operating inside the Moon that could have modi- Moon's surface, but are as yet unsampled, or even detected fied mare basalt compositions prior to eruption. by remote sensing. In addition, photogeologically and spec- Another style of lunar volcanism is the pyroclastic erup- trally distinctive domes of non-mare materials are seen in tion of glassy materials, such as the "orange glass" sampled several places on the Moon [Headand McCod, B 9781, yet by the Apollo 17 crew and the "green glass" of Apollo 15. their compositional afinities with the sample collection are Other possibilities have been identified in soil samples and unknown. It is also clear that the propo~ionsof rock types in soil breccias and at unsampled areas [e.g., Gaddis et al., the Apollo co1Iection are not representative ofthe lunar crust 19851. Although they are probably volumetrically minor as a whole. surface units, these materials are important because they are mental Questions

Figur 2.42: The bright-rayed crater Lichtenberg (20 km diameter) in Oceanus Procellarum. Dark areas south and east of the crater are due to a mare ~asaltflow that post-dates the crater. These basalts are the youngest recognized on the Moon; crater statistics indicate an age conte nporaneous with Copernicus, which may be as young as 900 m.y. Scale bar 10 km, north at top, photo LO lV-170-PI 1. From Schiltltz and Spud i [I9831. Rectangle outlines area shown in Figure 2.4b. LGO AND LUNAR SCIEN(

Ere 2.46: Low-sun angle, high resolution view of the mare basalts that embay Eichtenberg (top left). North at top, scale bar 5 km, pho 5-369 and 370. From Sehultz and Spudis 119831. Figure 2.5:Distribution of visible lunar maria (black) and highland light plains (stippled) that display numerous dark-halo craters, which have excavated mare basalt from beneath the plains. Because such highland plains are -3.85 to 3.95 b.y. old, the buried mare basalts must predate them, suggesting extensive mare volcanism prior to the end of the heavy bombardment (-3.85 b.y.1 From Schziltz and Spudis [1983]. LGO AND LUNAR SCIENCE

compositionally primitive, most likely the lavas least altered StiiJter ~f a(., 19751. A simplified analytic approximation to by fractionation processes within the Moon. and so contain the cratering Row field [the ""Z-model"; Maxwell, 19771 the most information about the lunar interior. We know little describes the padicle motions during the later stages of crater about the distribution, age, composition, and relation to mare groivth. after impact kinetic energy has been transferred to the basalts of such deposits on the Moon, but because they may target. The 2-model has been applied to lunar cratering represent relatively volatile-rich magmas or sites of extended prc~blems,with the specific goal of estimating the maximum eruptions (Head and Wilson, 19791, their identification and depth of excavation of craters and basins [Grofl,1980,1981; mapping are of fundamental importance. kYp~ddkL~et al . 1984: StGffleu, 1981 1, Recogizing the efkcts of gravity on cr:iker growth, the important distinction has been established between the gravity-controlled excavation crater 2.5 WHAT IS 'THE HISTORY AND NATUW, Ok and the strength controiled collapse zone [Ivanov, 1976; IMPACT PROCESSES ON THE MOON? (2rieve el a/., 1981; Cro$, 19811. 2.5.1 Bombardment Effects 23.3 Crater and Basin Ejecta The scarred lunar highlands attest to the intense bcrrn- bardment the Moon received early in its histov. The returned A coraseyuerace sf the formation of an impact crater is the rocks reflect this fusillade: most are breccias, complex excavation of material from the target and its deposition on mixtures of rock and mineral fragments. In fact, many ofthe the surrounding terrain. 'rhe details of this process are size rock fragments in breccias are themselves breccias. dependent, consisting of a simple blanket of debris for smaller The most intense bombardment took place between 4.5 (c 10 krn diameter) craters, but becoming quite complex at and 3.9 billion years ago; there might have been a peak in the basin scales [Obevbeck, 19751. A crucial question at these impact rate between 4.0 and 3,9 b.y. The impact of huge larger scales is the quatity of material mixed into the primary kodies excavated craters hundreds of kilometers across. The ejects from the underlying, pre-existing substrate. 'This crust was reworked to varying degrees locally and globally, problem has long vexed those attempting to understand the creating a megaregolith at least 2 km thick on the highlands provenance of samples collected from basin ejecta blankets surface. Materials must have been mixed laterally and $e.g., Apoflo 14-Fra Mauro), but is also relevant to the vertically, perhaps obscuringprimovdkal compositional and s~aterpretationof remote-sensing data for basin ejecta as it is lithologic variations, but it is not known to what extent this desirable to reconstruct crater and basin targets to add the happened. Even with the limits of current remote sensrng third dimension to models of lunar crustal composition. data, however, distinct lithologies have been idea~tified The ejeeta produced by an impact event may have wide associated with major basins [e.g., Spudis et al., 19843 as variations in shock level, although it appears that the most well as with large and small impact craters [e.g.Pieter,r. 1983, highly-shocked material remains in the crater cavity [Stuner 19861. ~aba&,, 19751. This effect predicts that most impact melts (the Giant impacts may have confused the record of igneous most highly-shocked crater ejecta) remain within the crater, evolution of the highlands crust, but they may also have although apparently increasing amounts of melt are ejected revealed the nature of the lower cmst and afford us the from the cavity with increasing crater size [Orphal et al., opportunity of studying how large impacts disperse materials 19801, Impact melt sheets in terrestrial craters tend to be across planetary surfaces. Furthermore, the impact mc%t chemically homogeneous [Grieve et al., 19771, but whether deposits so clearly visible in relatively young craters prcrvlde this tendency persists during the fornation of large, lunar- us with average compositions of the materials melted during basiar impact melts has not been established. Thus, this the impact. Determining their compositicpns can help us process n~vstbe understood if basin melt sheets are to be used deternine the average composition of the Bunar cn1s.t in the as probes of the average emstal composition in the lunar target areas, and this would be particularly impoflant for the target regions, relatively unflooded Orientale and Humboldtianurn basins, whose melt sheets may represent averaged compositions of several tens of kilometers of the local crustal column. 2.6IS THEm AN IRON-NCH COmIN THE MOON?

2.5.2 Cratering Mechanics From the observed mean density sand absence of volatiles, it has long been realized that the Moon must be severely Attempts to understand the formational meckxanlcns of depleted in iron in comparison to other terrestrial bodies, so impact craters borrow from explosion analogs, laboratov- t81at a metallic core must be small or non-existent. Histor- scale experiments, terrestrial craters and photoggaphic ob- ically, this observation has encouraged special lunar origin sewations of lunar and planetary craters. The existence of models including, for instance, selective accretion of iron- cratering flow fields has been predicted through theow and poor diKerentiated planetesimals under the influence of a documented through experiments [Orphal et at., 1980; nearby Ea&h or formation from te~restrialmantle material by dynamically plausible mechanisms. More recently, the ob- upper limit on the core radius from tirne-dependent induction served depletion of lunar siderophiles (e.g., W, Mo, Ir) studies is near 500 km. Orbital measurements of the lunar relative to G 1 chondrites and the terrestrial mantle has been induced magnetic dipole moment in the geomagnetic tail interpreted in terms of an indigenous lunar metal-silicate suggest the presence of a core of radius 439 k 22 km with a fractionation event, suggesting the existence of at least a minimum conductivity of approximately 10 Sm- ' [Russellet small metallic core [Newsom and Drake, 1982; 19831. al., 1981 1. However, neglecting measurement error sources, Although the alternate hypothesis of a distribution of metal the minimum conductivity lower limit can still be explained pods cannot be obsemationally eliminated, gravitational by a molten silicate core as well as by a metallic core. Further segregation of the metal fraction through a suficiently plastic refinements of the conductivity lower limit as well as the mantle to allow core formation would be a plausible end radius estimate should therefore be attempted using a future result. Geochemical data cannot be applied to derive the independent satellite data set. Finally, laser ranging mea- segregated metal fraction and hence the core size, since this surements of physical libration parameters indicate a larger depends on unknown initial siderophile abundances and on dissipation of rotational energy in the Moon than would be the assumed depth of differentiation. expected in the absence of a fluid core f Ybder, 198 1; Ferrari Thus, a geophysical determination of the core size would et al., 19801. The observed parameter is a small (0.2") help to distinguish among lunar origin models [Newsom, advance in the lunar rotation axis from the Cassini alignment. 19841. If the Moon formed entirely from terrestrial mantle This advance implies a larger dissipation of tidal energy per material, which is already depleted in siderophiles relative to flexing cycle than would be expected for the Moon based on chondrites, then core segregation of only 0.1-1% metal mantle seismic Q-values and comparisons with other bodies. would be required to produce the lunar siderophile abun- Thus Uoder has proposed viscous friction at the interface dances; the radius of a predominantly iron core would be between a fluid metallic core and the solid mantle as an <285 km. If the Moon formed entirely from undifferentiated alternate means of explaining the observed rotation axis solar nebula material, then Newsom estimates that segrega- advance. For particular laminar and turbulent core-mantle tion of at least 2% metal would be required for the lowest coupling models, he concludes that turbulent coupling is plausible degree of silicate partial melting and the expected preferred and derives a rough estimate of 330 km for the core iron core radius would be 2360 krn if whole-moon differen- radius. tiation is assumed. In reality, it is possible that only part ofthe Future demonstration of the existence and radius of a lunar Moon differentiated, resulting in a smaller core size than the metallic core can be most directly achieved via a global predicted limits. Alternatively, although the Moon may have seismic network. The electromagnetic sounding technique formed primarily from material derived from the Earth's should be applied as well, using orbital and surface mag- mantle, metal may have been added to the protomoon from netometers to independently constrain the core radius. Earth-orbiting planetesimals or from the postulated large impactor of the Collisional Ejection Hypothesis (see section 2.2.4). Thus an exact evaluation of the lunar metdlic core radius and mass may not unequivocally distinguish the mode 2.7 WHAT IS THE THER of lunar origin. Nevertheless, the siderophile data do allow OF THE MOON? very useful quantitative tests of particular origin models under different assumptions if the core mass is known, so a The most fundamental geophysical problem in post- determination of the core radius and density continues to be accretion studies of the Moon is that of thermal evolution. of basic interest. What is the temperature distribution inside the Moon over Several observations resulting from Apollo-related inves- geologic time, and what have been the modes of heat transfer? tigations have suggested but do not prove the existence of a This leads to a set of subsidiary questions of importance to the lunar metallic core. Possible seismic evidence for a 170-360 evolution of the planet: When and over what depth interval km radius low-velocity core was tentatively suggested by did major differentiation events take place? What were the Nakamura et al. [I9741 from an early interpretation of a global tectonic styles (expansion, compression, volcanism?) single P-wave arrival from a farside meteoroid impact. in response to lithospheric heat transfer? Ifthere is a core, was However, no additional detectable farside impacts occurred there sufficient thermal energy to drive a dynamo to generate during the remainder of the 5-year operation period of the an internal magnetic field? Geophysicists attempt to recon- seismic network. Consequently, this observation cannot be struct the thermal history of the Moon by independently regarded as definitive. Time-dependent electromagnetic answering some ofthe subsidiary questions and then applying sounding data have been used to calculate an upper limit of these results to constrain the larger question. For example, 435 km on the radius of a highly electrically conducting core successful thermal history models must account for the [Plrobbset al., 19831. However, uncertainties resulting from timing and source depth of the mare basalts. They must be accuracy of the theoretical model used to interpret the data consistent with the timing of global expansion and contrac- and possible inter-calibration errors between the surface and tion of the lithosphere, as revealed by the distribution of orbiting magnetometers suggest that a more conservative tectonic features (e.g., graben, wrinkle ridges) on the surface. LGO AND LUNAR SCIENCE

Lunar thermal history had a direct effect on the evolution larger of these two bulk Moon uranium abundances is of lithospheric thickness, which, in turn, was a key factor in significantly larger than terrestrial and chondritic values, dictating tectonic style. Conversely, studying the tectonic supporting the view that the Moon is generally enriched in response to loads emplaced at or near the surface at various refractory elements. However, the lower of these abundances times in lunar history provides information on lithospheric would not imply a siqificant lunar enrichent. Thus, evolution and, ultimately, the history of the internal tem- establishment of the true lunar global mean heat flow value is perature state. For the Moon, multi-ringed basins are the needed before any final conclusions can be reached. most sipificant of the lithospheric loads, and have been used in several studies to estimate lunar lithospheric thickness in space and time [e.g., Solomon and Head, 1979, 1980; Bullan and Lambeck, 1980, 198 1). Obtaining reliable 2.7.2 Present-Day I,uraar Temperature Profile results is difficult because of uncertainties in load magnitude, Limits on the present-day lunar geothem are of basic subsurface configuration, and stress state at various stages in interest for distirrpishmg among thermal evolution models. basin history. A significant improvement in understanding For example, models that assume the dominance of sub- the evolution of the lunar lithosphere would be expected with solidus convective heat transpod in the lunar interior below a high resolution global gravity, topography and imagery. Of few hundred km predict relatively cool! central temperatures particular importance is data acquisition from the spectrum and a flat profile below the conductive zone, while models of unfilled to volcanicalIy filled lunar basins. that assume the dominance of thermal conduction at all Two obvious and powerful constraints on the Moon's depths predict a continuous increase to central temperatures thermal history are the present-day heat flow and radial approaching or exceeding the soliidus. temperature profile. The mean heat flow can be used to The surface heat flow constraint on the lunar geothem is estimate the bulk uranium content ofthe Moon, which in turn, currently weak. Although Keihm and Langseth [ 19771 by its refractory nature, constrains theories of lunar origin estimated temperatures near 380 km depth in the range 880- (see section 2.2.6). A successful estimate ofthe bulk uranium 1100°C using the steady-state balance assumption and a content requires a correct description of the modes of interior global heat flow of 1.8 pW/cm2, the actual allowed range is heat transfer. For example, we need to understand the history larger than this because of uncertainties, among other factors. of mantle convection over the last few billion years. in the mean global heat flow. A second constraint on the temperature profile in the upper mantle is provided by the high average seismic Q-values (400&7000), implying tem- 2.7.1 Mean Heat Flow at the Surface peratures well below the solidus at these depths. In addition, the apparent maintenance of mascon anisostasy over 34b.y. The true value of the global surface heat flow represents a is consistent witit relatively cool upper mantle temperatures. fundamental boundary condition for thermal evolution An attempt to qumtilFy the mascon maintenance constraint models [e.g., Toksiiz et al., 19781. Heat flow probe mea- was made by Bullan and Lambeck [1980], who applied surements were successfully obtained at only two of the laboratorydetermined creep laws and calculated stress dif- Apollo landing sites, Apollo 15 and 17, yielding final ferences for the interior beneath the mascons to estimate a estimates of 2.1 and 1.4 pW/cm2, respectively [Langsethet temperature upper limit of about 800°C at 300 h depth. In al., 19761. Derivation of globally representative averages the middle mantle, Q-values > 1000 together with the loca- from these isolated measurements is necessarily difficult and tions of deep moonquake foci between 800 and 1100 km values ranging from 1.8 pW/cm2 [Larzgseth el al., 19761 to (implying the accumulation oftidal stresses and the existence 1.1 pW/cm2 [Rasmussen and Warren, 19851 have been of large-scale heterogeneities) are again most consistent with proposed. The former authors extrapolated the observed temperatures well below the solidus although the reduced Q values to compute global averages using orbital measure- may indicate increased temperatures. Finally, the possible ments of surface thorium abundances and inferred crustal sharp increase in shear wave attenuation below -1 100 km thicknesses. Assuming nominal radioactive element ratios where deep moonquake sources either cease to exist or are and a steady-state balance between heat production and loss, undetectable would be consistent with temperatures closely mean surface rates of 1.1 and 1.8 iu,W/cm2 would imply bulk approaching or exceeding the solidus in the deep interior. moon uranium abundances of 29 and 46 ppb, respectively, However, it should be noted that current views on the origin of compared to 14ppb for C 1 chondrites and 20 ppb for the bulk the reduced Q (increased attenuation) in the terrestrial low- Earth. However, parameterized convection calculations velocity zone indicate that par"lial melting is not necessarily have shown that the concept of a steady-state balance required. between heat production and loss is not appropriate for In principle, infemed bounds on the mantle electrical planetary bodies; i.e., a fraction of the heat escape is due to conductivity profile combined with laboratory measurements secular cooling. With this consideration, the above global of conductivity versus temperature for relevant mineral heat flow estimates may be consistent with a bulk uranium assemblages provide an alternate means of deducing limits on abundance of as little as 20 and 35 ppb, respectively. The lunar temperature as a function of depth. Current limitations of this approach include (I) uncertainties in lunar mantle units and the Reiner Gamma-type swirls (Figure 2.6: see composition: (2) lack of suficiently complete laboratory Hood and Cisowski, 1983). To some extent, the association measurements for relevant mineral compositions; (3)uncer- of orbital magnetic anomalies with impact-produced ma- tainties in how physical properties (e.g., fabric, grain bound- terials is explicable in terms of properties of ferromagnetic aries, intracrystalline defects) affect the conductivity at depth carriers. In particular, iron grains in the single domain size in the Moon; and (4) uncertainties and nonuniqueness in the range. which predominate in weakly annealed breccias and determination of the mantle electrical conductivity profile. soils, carry the most intense stable magnetization of any Consequently, current attempts to translate electromagnetic sampled materials and would be expected to produce the sounding limits on the lunar electrical conductivity profile strongest orbital anomalies. However, the primary natural into corresponding limits on the lunar geotherm are pro- remanence component of these materials is not clearly visional at best and comparisons with thermal history models themoremanent in origin and shock magnetization effects are still in a preliminary stage [Hoodet al., 1982b; Sonett, appear to be impofiant. Thus it is possible that transient fields 19821. In addition to limiting mantle composition models generated in hypemelocity meteoroid impacts may be re- more accurately via improved seismic velocity data, future sponsible for a substantial part of the mametization observed work should concentrate on completing laboratory electrical from orbit [Hoodand Vickery ,19841. Other possible surface conductivity versus temperature measurements for a wider mechanisms for generating steady local fields include ther- variety of mineral compositions and on determining more moelectric currents between cooling surface magma ex- accurately the lunar electrical conductivity profile. The latter posures and subsurface dynamos [Daily and Dyal, 19793. is currently known only to within one or two orders of One empirical means for evaluating the evidence for global magnitude at a given depth /Hood et al., 1982a; Sonett, scale versus local surface sources of the lunar magnetizing 19821. field involves determination of approximate bulk directions of magnetization for major anomaly sources using orbital magnetometer data. If a steady core dynamo was responsible for the magnetization of these anomaly sources, then sources 2.8 WHAT ARE THE ORIGINS OF of comparable age should ideally be rnagnetized along field LUNAR PALEOMAGNETISM? lines oriented in the shape of a dipole centered in the Moon. The pervasive magnetization of lunar surface materials, if Unfortunately, the coverage of orbital magnetometer data is ascribable in whole or in part to the existence of a former core presently limited to narrow equatorial bands on the nearside dynamo, would imply the presence of a metallic core and may and to a few limited areas on the farside. Consequently, a allow further inferences regarding lunar internal and dy- total of only 28 sources with widely varying probable ages namical history. Alternatively, if local surface processes such can currently be studied in this manner so that the directional as impact are found to be the sources of a substantial evidence for or against a core dynamo is difficult to evaluate. component of lunar paleomagnetic fields, then important When all 28 independent sources are considered together, the implications for paleomagnetic studies on similar airless corresponding distribution of paleomagnetic pole positions is silicate bodies in the solar system would follow. Relevant not significantly different from random [Hoodand Cisowski, obsehational data include magnetization properties and 19831. Also, adjacent or nearby anomaly sources are gen- paleointensity estimates derived from returned samples, erally inferred to be magnetized in very different directions inferences from surface magnetic field measurements, cor- suggesting a very local scale size for the magnetizing field. In relations of orbital anomalies with surface geology, and contrast, Runcorn 119831 finds that three subsets of the 28 directional properties of large-scale magnetization inferred anomaly sources do have significantly clustered paleopoles from orbital magnetometer measurements. and he proposes that these may represent sources magnetized At present, the major lines of evidence for a former core during three different epochs of lunar history when the dynamo are (I) possible identification of the stable natural rotational pole was greatly displaced from its present posi- remanence component of many mare basalts as thermo- tion. Thus it remains dif'ficult to eliminate the possibility of a remanent in origin (implying the presence of a relatively former core dynamo on the basis of the presently limited data steady magnetic field during formation and cooling of the set. basalt); and (2) possible evidence for a time variation of maximum paleointensity amplitudes with the strongest fields occurring in an interval between 3.9 and 3.6 b.y. [Cisowskiet al. , 19831. On the other hand, significant paleointensities have been derived for at least one young (<100 m.y. old) 2.9 WHAT IS THE NATURE OF impact glass sample which is unlikely to have been magne- THE LUNAR WGOLITH? tized by a global-scale field [Sugiura et al., 19791. Also, orbital magnetic anomalies detected with magnetometers and The lunar regolith is a global veneer of debris generated by the electron reflection technique are most often associated from underlying bedrock by meteorite impacts. It is a with impact-produced surface features such as basin ejecta dominant surface feature whose development covers most of LGO AND LUNAR SCIENCE

THE RElNER Y ANOMALY 12 "

S: -+% B- Total (gammas) \s. 2 10" /-10.5

Figure 2.6: Correlation of the strongest single magnetic anomaly (lower panel) detected by the Apollo subsatellite magnetometers with Reiner Gamma (upper panel), an unusual albedo marking on western Qceanus Procellarum. The origin of Reiner Gamma, as well as other similar albedo markings found elsewhere around the Moon, remains unresolved [From Hood et al., 19791.

the Moon's history. Because the signals detected from regolith" from the "regolith.'The megaregolith comprises at geochemical remote sensing instruments will emanate almost least the outer >2 km thickness of the hiblands crust, and entirely from the regolith, understanding the properties of this may be as thick as 40 km. Its fomation is dominated by huge thin layer is extremely important to deducing crustal proper- impact events. The regolith is thinner, comprising the upper- ties from surface measurements. most 2 to 30 meters of the Moon; it is a fine-grained layer of The flux and size distributions of impacting bodies debris produced by impacting bodies much smaller than the changed substantially during lunar history. For example, all ones responsible for producing the megaregolith, including impact basins larger than 200 krn formed prior to 3.8b.y. ago. micrometeoroids having masses less than 1Om6g. Many of its Consequently, many lunar scientists distinguish the "mega- components (mostly mineral and rock fragments and glassy objects produced by micrometeoroid impacts) contain ele- 2.10 SUMMARY ments implanted by the solar wind. This demonstrates that these materials once resided on the very surface of the Moon. This section discusses the regolith; the megaregolith and Fundamental questions in lunar science which remain related topics are discussed in section 2.5. unresolved 14 years after Apoilo 17 are: Most of the regolith has been reduced to powder. Bom- ( I ) What is the origin of the Moon? This question involves bardment by micrometeorites recombines portions of this establishing estimates of the global lunar abundances of powder into glassy aggregates of rock and mineral fragments. elements such as U and Fe in order to constrain extant These particles, called agglutinates, are larger than the hypotheses. fragments they contain, so small-scale impact melting (2) What is the evolution of the lunar crust and mantle? opposes continued comminution of rock. This process causes This question involves establishing (a) whether the Moon the glass content to increase with time in a given volume of really undement an early phase of global melting to yield a regolith, but agglutinates build up slowly. Consequently, magma ocean, (b) the thickness of the lunar crust, (c) the freshly produced soils, for example ejecta from craters that depth of early lunar differentiation, and (d) the structure and penetrate through the regolith, are deficient in these glassy composition of the mantle. particles. Instead, fresh ejecta contains rock fragments from (3) What is the magmatic history of the Moon? The underlying "bedrock" (which in the highlands might itself be question involves establishing (a) the nature and duration of an impact deposit in the megaregolith). How such fresh, or igneous activity in the highlands, (b) the effects of early immature, soils evolve to degraded, or mature, soils is not intense bombardment, and (c) the nature and duration of known in detail. mare volcanism. We know from the Apollo samples that the compositions (4) What is the history and nature of impact processes on of soils at a given location may or may not match those that the Moon? This question involves (a) determining the depth could be produced by mixing of local breccias or igneous of impact mixing in the highland crust (the megaregolith), (b) rocks. Also, the compositions of sampled highlands soils do establishing the size and shape of complex crater and basin not correspond to those of simple mixtures of the most excavation cavities, (c) determining the ratio of locally- prevalent, well preserved ('pristine") highlands igneous derived material to primary ejecta in basin continuous rocks. We must determine whether this merely reflects biased deposits, and (d) establishing the homogeneity or lack thereof sampling or biased survival of certain rock types in the Apollo in large basin impact melt sheets. collection or whether it implies derivation of breccias and (5) Is there an iron-rich core in the Moon? This question associated soils from different lunar provenances that contain involves establishing (a) the siderophile element abundances lithologies unsampled by Apollo. Much remains to be learned in the Moon, and (b) geophysical techniques for direct about the relationships between the compositions of regolith detection of a core. and the underlying igneous crust. Careful studies of craters, (6) What is the thermal history of the Moon? This question surrounding regolith, and exposed rocks will improve the data involves establishing (a) the mean surface heat flow, and (b) base for these studies. the present lunar geotherrn(s). Regolith mixing has been studied with Apollo samples, but (7) What is the origin of lunar paleomagnetism? This details such as transport rates and efficiency of vertical and question involves establishing (a) direct detection of a lunar horizontal mixing have not been worked out accurately. core, and (b) the orientations of regional surface magnetiza- Mixing is accomplished almost entirely by impacts that either tion as a function of surface age. dredge material up from beneath the surface or disperse (8) What is the nature of the lunar regolith? This question material laterally across the Moon's surface. Both processes involves improving our understanding of (a) vertical and complicate interpretation of remotely-sensed data, so it is lateral mixing in the regolith, and (b) the details of regolith essential to understand them in detail. maturation.

Contributions of an LGO Mission to Fundaments Lunar Science

3.1 INTRODUCTION imaging system that is compatible with the LGO mission and spacecraft constraints is discussed in Appendix D. Also In the previous chapter, a number of fundamental ques- summarized in Appendix is the question of geodetic tions about the Moon are discussed. In a sense, some of these contrO' On the Appendix presents the are general questions that can be raised about any of the surface heat flow mapping from orbit by microwave radi- terrestrial planets. Answers to the questions would provide Ometry. Appendix i~ a summaqof the mission, and an important advance in our of the Appendix G is a set oftables regarding the characteristics and system. spacecraft requirements of candidate LGO instruments. An LGO mission is scientifically attractive because it promises to make significant progress on some basic issues of 3.2 ORIGIN OF THE MOON lunar and planetary science. This is so for three reasons: (1) The Moon appears to be a relatively simple planet in terms of The principal instruments that can be carried on LGO that evolution, and thus we have a chance of deciphering its are relevant to issues of lunar petrology and geochemistry are history. The Moon serves as a baseline for the study of more the X-Ray and Gmma-Ray Spectrometers (XGRS) and the complex planets. (2) We have a database of returned lunar Visual and Infrared Mapping Spectrometer (VIMS). As samples and their analyses in which to place remote sensing examples of how the data from these instruments are measurements in context in terms of absolute chemical and expected to bear on theories of lunar origin, considered below mineralogical composition and in terms of age. (3) The crust are lunar abundances of refractory elements and the relative of the Moon is unweathered by dynamic atmospheric and lunar amounts of iron and magnesium. Use of these data for biological processes, so remote sensing measurements will the purpose described requires that we know how to relate yield primary crustal compositions. observed surface compositions to those of the bulk Moon, Contributions of LGO to lunar science are discussed and our ability to do this depends in part on the geological below in terms of candidate instruments. By focusing on context of the measurements and our understanding of the identified questions, the discussion is intended to be illus- Moon's chemical evolution. The Magnetometer (MAG) and trative but not exhaustive. The sensitivities and data return of Electron Reflectometer (ER) instruments bear on lunar individual instruments are described in Appendix A. Of origin in that they provide evidence for or against an iron core. particular interest there are tables describing the elemental This question is discussed later in the chapter. and mineralogical abundances of various lunar rock types, along with tables showing the measurement accuracies obtainable from geochemical remote sensing instruments. 3.2.1 Refractory Elements Appendix B describes the ability to map rock types with the limited Apollo orbital geochemical data sets and the very The LGO can produce an accurate global map of the positive implications for LGO. Further discussion of imaging surface spatial distribution for two refractory incompatible and altimetry are found in Appendix 6.A specific solid state elements, Th and U, and for the more volatile, incompatible LC0 AN11 LUNAR SCIENCE element I&. The precision of these measurements depends on mapping can determine what materials were excavated from ah@ concentrations of Th, U, and K, and on the length of the largest basins and provide information on crustal layering measurement time, as described in Appendix A. Concentra- and on lateral heterc~geneityat these scales (see section 3.5). tlons for an anorthositic norite cornpositlorn typical of lunar The proposed Gargantuan Basin (3200 km diameter) and the highlands are 0.6 ppm Th, 0.2 ppm U, and 400 ppm K. The Big Backside Basin ( L 900 km) may have penetrated into the precision of measurement (at 30' latitude) for these elements lunar mantle IGrofi, 1981 1. Compositional inbrmation from is 7'%, 7(h,and 5(%,respectively, leading to uneedainties, for LGO experiments will not only help geologists understand the ratios csf two elernents, on the order of 10%- Such the results of basin formation, but it may also provide a precision is: adequate to enable good comparisons between unique glimpse into the stratigraphy sf the lower crust. If, for terrestrial and lunar values, as well as to reveal: sipificant example, ejecta close to basin margins. where the materials differences among drperent regions of the lunar surface. The excavated from greatest depth are located, are found by LGO precision will be even higher for some lunar materials. to be systematically k~werin concentrations of K, &I, and Th The concentrations of these elements have been used by than more distant makerial, it would suggest that the crustal various workers [e.g., Taylor, 19821 to estimate global inventory of those elements is less than that estimated from a abundances of refractory elements. Limiting factors on uniform crust model. Estimates could be adjusted accord- estimates such as Taylor's include (I) u~rcertaintycon- ingly. Conversely, evidence that compositions were essen- cerning global surface distribution, (2)uncedainty of concen- tially uniform with depth would rernove a major uncertainty tration with depth, (3) uncedainty in lunar cmstal thickness, in estimating bulk lunar composition. and (4) uncefiainty in efficiency of extraction of refractory ejernents from the lunar mantle. Data gem the LGO will imprave our knowledge of at least three of these factors, 3.2.2 Mawesium Number For example, at present we do not know whether our Apollo-derived near-equatorial compositions and element The improved resolution of the XGRS will enable the Mg' ratios are representatit e of the lunar surface or whether they of individual regions of high Mg and Fe concentrations to be may differ significantly at higher latitudes (Figures 3.1a,b). determined to within about 1396, and averages over broader Also, the precision of measurement sf K and U by the Apollo regions of lower concentrations to that precision or better. If gamma-ray experiment was so limited by the now-obsolete the largest Iunar craters really did penetrate ~ntothe lunar technology of the instrumentation that we do not know to mantle, we rnay be able to measure the Mg' in the e~ectafrom within 50% the ratios of concentrations of those elements to the mantle, providing us with a more direct detemination thorium or to each other for most regions sampled. The much than simply one assuming a aven extent of paditioning better precisions for these elements with the LGO spec- during melting in the mantle to produce the liquids that reach trometer will allow us to obtain the desired values. Also, the surfice. Apollo data for these elements had to be averaged over huge patches of the highlands along the command module ground track to yield enough data for deconvolution of the spectra. 3.2.3 Summary Empofiant spatial resolution was lost, This resolution will be improved in the I,GO mission, with areas as small as 88x80 Refining estimates of refractory element abundances and km yrelding data of high quality for these elements. Thus, we the Mgkre two examples of the contribution LC0 can lnake will readily be able to detect significant spatial variability in to discriminating among hypotheses for the origin of the the ratios. Moon. Present '"best'" estimates of global U concentration Most estimates of bulk Moon concentrations of refractory are approximately 20 ppb for Ear;th and approximately 35 elements are based on the assumption that the lunar crust is ppb for tkle Moon [Drake, 19861. There is, however, homogeneous throu&out its tl~ickness,while recognizing considerable uncedainty in the lunar value in pa~icuiiar,and that this assumption is unlikely to be correct in detail [e.g,. one cannot exclude the possibility that the global concentra- Drake, 19861. For example, evidence from sample studies tions of tJ in the Eadh and Moon are both approximately 20 (1F"yder arfd Wood, 19771 suggests that the crust is hetero- ppb. EGO should be capable of improving the lunar estimate geneous and perhaps layered. Ea.rlh-based near-IR spectral such that it may be possible to resolve the current ambiguity. measurements have shown the cmstal mineralogy at 5-1 0 km Most estimates of the Mgkf the bulk Moon (8.80) are lower depth (exposed by large craters) to be vev heterogeneous than the Mg' of the upper mantle of the Earlh (8.89), with [Pieten, 1983, 19863. Some craters exhibit a distinct stra- only the most extreme lunar estimates overlapping terrestrial tigraphy in the upper crust; for Copernicus the stratigraphy values. A lower Mg' for the Moon is consistent with the includes a thin basalt cap, underlain first by an extensive observation that lunar basalts, in general. are significantly noritic layer, then a troctolite zone by 10 km depth [Pr'eters more FeO-rich than terrestrial basalts [Wankeet a/,, 19831. and WilheEms, 1985; Pieters el al., 11 985j. Large basins kG0 should be capable of improving the lunar estimate penetrate the lunar crust to depths of at least a few kilometers based on regolith analyses such that it may be possible to and perhaps a few tens of kilometers. Global geochemical confirm or eliminate the higher values of Mg'.

LGO AND LUNAR SCIEC'EI Contributions of LGO 23

Data from LGO for the elements will not conclusively A1 and low Fe, Mg, and U, and will have spectral properties prove any one hypothesis, although pure fission of the Earth lacking signatures of the rnafic minerals, olivine or pyroxene may be eliminated if the refractory content and Mg' of the (see Appendix A). Even for areas not consisting of nearly Moon prove to be different from the Earth, as suggested by pure plagioclase, the instruments carried on LGO will present knowledge [Drake,19861. Table 3.1 summarizes the certainly be able to determine the fractional abundance of hypotheses permitted by various outcomes of the LGO plagioclase. A better estimate of whole crustal composition mission. Note that it is assumed that in the Collisional provides better constraints on lunar bulk composition and, Ejection Wypothesis, the contribution of target and projectile particularly in the case of A1203,lends insight to the extent of to the ejecta that subsequently coalesce to form the Moon is initial lunar melting, and to the relative enrichment, or lack 50,as 100% ejecta from the target (Earth) is geochemi- thereof, of refractory elements in the Moon compared to the cally indistinguishable from the Fission Hypothesis while Earth. 100% ejecta from the projectile is geoehemically indistin- Mixing by huge impacts has undoubtedly confused origi- guishable from the Intact Capture Wypothesis. nal rock relationships. This is more of a problem in assessing the distribution of known rock types and in defining as yet undiscovered ones (see 3.4) than it is in determining how 3.3 EVOLUTION OF THE CRUST much plagioclase has been concentrated into the Moon's crust. The goal is to obtain a total plagioclase inventory in the The XGRS and VIMS instruments provide compositional crust (Figure 3.2). A reasonable estimate of how plagioclase information on the nature of the crust and on the evidence for varies with depth can be obtained from geophysical con- a magma ocean. Altimetry (ALT) and Doppler-tracking siderations of crustal thickness and density and from chem- gravity data provide constraints on crustal thickness and the ical and mineral abundances on the floors, walls, and ejecta vertical distribution of elements and minerals within the blankets of large craters and impact basins. crust. An imaging system (SSI) is needed to provide the geological context (e.g., ejecta blankets) of remotely sensed data. 3.3.2 Crustal Thickness and Density Modeling of crustal thickness and density variations 3.3.1 Magma Ocean combined with information on vertical composition and structure provided by LGO geochemical mapping of ma- The concept that an ocean ofmagma surrounded the Moon terials excavated in major basin impacts will help to resolve when it formed was sparked by the apparently exceptional the issue of bulk crustal composition. Geochemical informa- abundance of plagioclase in the highlands. Available data tion provides an indirect means of estimating locally the from rock analyses and remote sensing support the notion of a density of the crust. In particular, XGRS measurements can plagioclase-rich upper crust, but we do not know the global provide abundances of major rock-forming elements and average plagioclase abundance or how deep the plagioclase- VIMS can provide abundances of major rock-forming min- rich region is. This lack of information hinders us from erals. These quantities in turn can be converted to mean rock proving or disproving the existence of a magma ocean and densities. Translating this information to meaningful crustal prevents us from determining, if there was a magma ocean, densities depends on a number of factors, including the how deep it was. relationship of surface elemental abundances to underlying The combination of the XGRS and VIMS can unam- bedrock and the vertical inhomogeneity of crustal composi- biguously determine the plagioclase abundance on the sur- tion. Haines and Metzger [1980] used the limited Apollo face in the highlands. Areas dominated by ferroan anortho- gamma-ray data to construct crustal density models for the site will have characteristic chemical compositions, e.g., high Moon along the Apollo groundtracks. Using an assumption

TAB= 3.1 Earth/Moon Data Comparisons: Data from LGO Permissive of Hypotheses of Lunar Origin Mg' Same Mg' Diff. Mg' Same Mg' Diff. U Same U Diff. U Diff. U Same Intact Capture - - - - Disintegrative Capture No Yes Maybe Yes Fission Yes No No No Collisional Ejection (50:50) No Yes Maybe Yes Binary Accretion No Yes Yes Yes

Same = same as Earth; Diff. = different from Earth. LGO AND LUNAR SCIENCE

ANORTHOSITE IN THE LUNAR CRUST AT BASIN TARGETS Crustal thickness exaggerated

(I" Serenitatis 8.5t0.2 Imbrium B.5k0.3 ectaris 21r3

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Orientale 4025 hii 18t3 Mantle ; Core (?I

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Hemisphere cutaway South Polar View

Figure 3.2:Schematic cartoon showing the relative thickness of a hypothetical anorthosite layer in the lunar crust, derived from the composition of basin ejecta. An estimate of the total amount of plagioclase in the lunar crust is a key test of the Magma Ocean Hypothesis. For details, see Spudis and Davis [1985]. Contributions of LGO of isostasy, they were able to calculate the relative con- highland rock types relate to one another. For example, there tributions of the Pratt and Airy mechanisms. Topographic might be concentrations of Mg-rich lithologies within or elevation versus density is the key relationship. If elevation adjacent to the areas of nearly pure anorthosite that probably varies inversely with density (as suggested by Apollo geo- exist on the Moon. This would suggest that the Mg-rich rocks chemical remote sensing), this is strong evidence that a Pratt had formed from magmas that had intruded into the anor- mechanism is part of the isostatic compensation. More thosites, or had formed as volcanic flows in limited areas. importantly, it says that elemental differences measured at Another possibility is that in broad areas of anorthosite, Mg- the surface may be representative of "whole crustal" pro- rich rocks are found only on the floors of large basins. This cesses and there is reasonable hope of inferring crustal might indicate that the Mg-suite formed as intrusions in the composition. The superior coverage and performance of the lower crust. XGRS and Radar Altimeter (ALT) instruments on LGO, The LGO instruments are certainly capable of distin- plus the addition of mineral phase information from VIMS at guishing known rock types from each other, and the com- higher spatial resolution, can provide data for estimates of bination of XGRS and VIMS is more adroit than either one local crustal density and its variation with depth. alone. These instruments should be able to discern areas Gravity data provide additional constraints to the question dominated by one rock type and to provide data necessary to of crustal composition and density, although coverage de- estimate the proportions of the several rock types making up a pends on the type of gravity measurement system carried by mixture. LGO (see 3.5.1 ). With crustal thickness known in one area To comprehend fully the evolution of the lunar crust we (as from the Apollo seismic network), then the Pratt and Airy must identify all of the major lithologies and determine how mechanisms can be separated [Haines and Metzger, 19801, they formed and relate to each other. Considering that new and gravity could then be used, for example, to estimate types of rock are found each year in the complicated breccias density gradient in the crust. This, in turn, can be related to returned by Apollo, which were obtained from a restricted the distribution of major rock-forming oxides and the nature geographic area on the Moon, it seems almost certain that of the magma ocean. other types exist in unsampled crustal regions. The problem of identifying and characterizing unknown rock types is somewhat more difficult than discerning the presence of 3.4 MAGMATIC HISTORY OF THE MOON known rock types. If areas of hitherto undiscovered Iitholo- gies exist on the lunar farside, for exmple, then their The geochemical remote sensing instruments, XGRS and chemical and mineralogical character can be identified if VIMS, are the major LGO instruments contributing to those rocks have compositions that are not intemediate questions on the magmatic history of the Moon. Imaging between those of known types, which would mimic mixtures. (SSI) and altirnetric (ALT) information are necessary to The limitations on how well this can be done will not be set by place these data in geological and topographical context, the adequacy of the LGO instruments. They will be set by the respectively. jumbled nature of the lunar crust, the areal extent of occurrences and the distinguishing characteristics of un- known rock types. 3.4.1 Highland Igneous Activity

In principle, the LGO offers a chance to obtain field relations among the rocks composing the lunar highland 3.4.2 Mare Volcanism crust. Unfortunately, impacts have obscured much of the original relations and the mapping of rock types is more The Apollo program sampled fewer than half the types of difficult than determining bulk plagioclase amounts or Mg'. mare basalts identified by Earth-based remote sensing Nevertheless, data from the Apollo XGRS and from Earth- [Pieters, 19781. To obtain a complete picture of mare basaltic based spectral measurements demonstrate that some areas evolution and thus learn more about the lunar mantle and the on the lunar surface are dominated by one rock type [Pieters , Moon's thermal history, we must fill in the gaps (Figure 3.3). 1983, 1986;Davis and Spudis, 1985; see also Appendix B]. LGO can distinguish different types of mare basalts (Ap- This seems to be particularly common on the floors of large pendix A), even though regolith formation and mixing has craters, offering hope that variation of rock abundances with somewhat obscured original flow compositions. We expect to depth can be discerned by the LGO. More detailed relations, obtain a more complete record of mare volcanism by mea- such as intrusions of Mg-suite magmas in an anorthosite surements of both the mare soils and the recently exposed crust, require the higher spatial resolution data from VIMS material of fresh craters. and SSI to unravel the lateral and vertical mixing and general Although, LGO will not be able to address directly the demolition of the crust caused by immense impacts. Varia- question of magma alteration by crystallization or reaction at tions in chemical and mineralogical compositions across the depth in the Moon, it can help determine the extent of lava lunar surface can help unravel the mixing to some degree, homogeneity from the vent and how much fractional crystal- perhaps giving some insight into how assorted kinds of lization or assimilation took place as lava flowed across the LGO AND LUNAR SCIENCE

Figure 3.3: An area in southwestern Mare Imbrium, showing a complex of overlapping lava flows. ]In this low-sun (about 4") photograph, individual flows can be recognized as elongated lobes bounded by steep scarps (Apollo Metric 15-1701). Contributions of LGO lunar surface. Flows hundreds of kilometers long have been the nature of impact processes. Multi-ring basins excavated mapped from Apollo photography. Determining the varia- many kilometers of the lunar crust and complete coverage of bility of the chemical and mineralogical compositions of the even one basin would give us an improved understanding of regolith developed on such flows will show the extent to crustal composition, possible layering, lateral heterogenei- which fractionation processes operated. ties and thickness, in addition to providing important infor- Studies of breccias from the lunar highlands have demon- mation on the nature of the basin-forming process itself. strated that mare volcanism began before the end of the Gravity data and topography provide key information to heavy bombardment. No discrete flows of mare volcanism deduce basin structure (Figures 3.5a,b;Philllps andDvorak, have been observed in the highlands, but there are chemical 1981). The Radar Altimeter (ALT) would provide global hints that they existed, such as dark halo craters (Figure 2.5) high resolution topography data. Earth-based Doppler track- and locally high Fe and TiOa concentrations. Because of the ing would limit high-resolution gravity coverage on major compositional differences between highland and mare LGO to the lunar nearside. Spherical hamonic models of rocks, the LGO ought to be able to spot areas where mare very long-wavelength uavity fields can be derived for the volcanism was especially concentrated in the highlands, even entire Moon from nearside tracking data [Ferrari and though avisible record of the flows may have been obliterated Ananda, 19771 although the accuracy of such models is by impacts. limited. The inclusion of a passive tracking experiment The margins of some maria are mantled with dark, glassy (SGS) would allow high resolution mapping of the lunar deposits thought to be formed by pyroclastic eruptions [e.g., farside. Such an instrument is discussed and described in Gaddis et al., 19851. One such deposit was sampled at the Appendices A and G. At present, nearside coverage above Apollo 17 landing site. These materials are special for two 30" N. latitude and below 30" S. latitude is essentially non- reasons: First, they may represent magmas that experienced existent. There was no coverage from Apollo and the Lunar little alteration as they traveled from the lunar mantle to the Orbiters I-V had such large eccentricities with periapsis at the surface. They thus may contain invaluable information about equator that the resolution at higher latitudes was lost. With the interior of the Moon. Second, the surfaces of lunar detailed topography from ALT and significantly higher pyroclastic glasses are enriched in volatile elements that are quality (a factor of 5 to 10) and higher resolution (lower otherwise rare in the Moon. Examples are Pb, S, Cl, Cu, Zn, altitude) gravity observations outside of the Apollo band, Ag, Au, and Na. These volatile elements could prove to be detailed modeling of all frontside ringed basins can be valuable resources in the context of permanent lunar evaluated and compared, especially for Mare Orientale and settlement. Mare Imbrium. Pertinent questions are: Is the internal The combination of VIMS, which can detect the presence structure of Mare Orientale significantly different from that of Fe-rich glass (Appendix A), and XGRS will allow the of Mare Imbrium? Wl~atis the structure of Oceanus Procel- distribution of pyroclastic deposits to be mapped, their range larum? The present coverage and resolution of this area are of compositions to be assessed, and basaltic Rows that might very poor. Inclusion of the SGS experiment in the payload be related to be identified. will extend these types of analyses to the farside and allow studies over a wide variety of filled and unfilled basins. The effect ofthe thick farside highland crust on basin substructure would be determined. 3.5 LUNAR CRATENNG PROCESSES In summary, LGO offers the opportunity to understand AND HISTORY better the structure of large impact basins, and this has application to the solar system as a whole. In addition, a Imaging (SSI), altimetry (ALT) and gravity are the major better understanding of lunar basin structure and the impact data types related to studies of cratering and basin processes. process guides our interpretation of sample provenance in Geochemical information obtained from XGRS and VIMS geochemical remote sensing. will greatly aid in understanding the nature of ejecta deposits.

3 -5.1 Basin Processes 3.5.2 Cratering Mechanics

Through the integration of XGRS-VIMS compositional The Moon may be used as a natural laboratory for the data with results from photogeology and cratering mechanics, study of cratering mechanics by integrating information from we would hope to obtain a fairly reasonable knowledge of the compositional, gravity, altimetric and imaging experiments lunar crust and its evolution through time. The most obvious (Figure 3.6). Some lunar craters (e.g., Aristarchus) have task is to use craters and basins as natural "drill-holes" into formed in regions where a gross, pre-impact target stra- the lunar crust. Through this technique, the pre-impact target tigraphy can be deduced from regional stratigraphic relations. may be reconstructed and a three-dimensional picture of the By looking at the composition of various ejecta facies, the lunar crust obtained (Figure 3.4a,b). Conversely, howledge relative contribution to the ejecta from their stratigraphic of the composition of geologic units provides us with clues to precursors may be estimated. In bulk, this would yield LGO AND LUNAR SCIENCE

Figure 3.4a:Copernicus crater (95 km in diameter) has locally exposed a variety of rock types documenting that at least this part of the Lunar crust is notably heterogeneous on a scale of a few kilometers. Information frorn near-infrared spectra obtained for the small areas 3 tn 6 km in diameter identified with arrows indicates a noritic composition frorn the wall and floor areas a and b (material excavated from beneath a thin mare basalt cover) and a troctolitic composition for the central peak area d (deep seated material from perhaps a 10km deprtif. Area c is a region of impact melt and contains components of Fe-bearing glass, pyroxene and crystalline (recrystallized) plagioclase. Contributions of LGO

clues in deciphering cratering mechanics. Unusual depar- tures from the expected circular plan of basins (e.g., Orien- tale) and preservation of pre-impact features provide important constraints on the dimensions of excavated cavi- ties and conditions of the target during crater and basin fomation. Imaging further contributes to cratering studies by identifying deposits that are both typical and atypical and this may be significant for the understanding of anomalies that will probably be found in the orbital compositional data.

3.5.3 Crater and Basin Ejecta

The LGO compositional experiments can directly address issues relative to the quantity of local material incorporated into crater and basin continuous deposits. Some lunar craters (e.g., Aristillus; Dionysius) possess spectrally-distinct rays that may largely consist of primary ejecta. The high-resolu- tion spectral data to be provided by VIMS can look directly at such distinct material and an estimate of the primary ejecta fraction residing near the surface can be derived; this technique has already been employed in Earth-based remote sensing studies [Pieters et al., 19851.Compositional studies of crater and basin floor deposits can directly address the problem of impact melt homogenization by specifically looking for compositional variations within the melt sheet, the distribution of which is identified by photogeology. If ejected WAVELENGTH (urn) melt deposits are found around craters and basins, it is important to establish their compositional affinities to the main melt sheet. These data would tell us both about the process of melt homogenization and probable particle Figure 3.4b: Near-infrared spectra obtained with Earth-based movements of material during excavation cratering flow. telescopes for the area indicated in Figure 3.4a. These spectra are In summary, LGO data will permit us to directly test used to infer the bulk mineralogy at each location and are displayed to enhance residual absorption features (after removal of an esti- various hypotheses for cratering mechanics and ejecta forma- mated continuum). The absorption band centered near 0.9 1 pm for tion and deposition by using compositional information in the areas a and b indicates the presence of low-Ca pyroxene (ortho- context of its geological setting. pyroxene); the weakness of the band indicates a minor pyroxene abundance (- 10%) in the bulk surface material. The broad multiple band centered beyond 1.0 pm for area d indicates a significant olivine component with little or no pyroxene (<5%). Area c displays a number of superimposed mineral absorption features: pyroxene (near 0.9 pm), crystalline plagioclase (near 1.3 pm),and Fe-bearing 3.6 LUNAR IRON-NCH GORE glass (broad band near 1.0 ,um). One of the primary objectives of Magnetometer (MAG) and Electron Reflectometer (ER) investigations on LGO is to detemine limits on the radius and electrical conductivity of information on the size and shape of the excavated cavity, a highly conducting (and thus metallic) lunar core. This which could then be tested against analytical models of would assist us in distinguishing among models for lunar bulk cratering flow fields. Compositional identification of various composition and origin, as well as among theories of origin of morphological members of multi-ring basins may give infor- lunar paleomagnetism. Knowledge of the size of a lunar mation on such topics as the pre-impact stratigraphic position metallic core will allow limits to be placed on the percent by of inner rings (which are predicted in some models to be weight of metal that could have fractionated from the silicate structurally displaced from deep in the crust), the exposure of portion of the Moon during early differentiation. The percent pre-basin material in the main basin ring. and the possible by weight of metal in a core, combined with the observed chemical identification of pools of ejected impact melt. depletions of lunar siderophiles elsewhere in the Moon, then Imaging coverage of better resolution and lighting con- allows quantitative tests of models for lunar origin, including ditions than currently exists is needed to utilize morphologic those which suppose that the Moon formed primarily from LC0 AND LUNAR SCIENCE

Figure 3.5~:Earth-based radar topographic map of the Grimaldi basin. The contour interval is 400 rn; the vertical resolution of the orignal data is -100 m. From Phillips and Dvorak [1981]; original data provided by S. Zisk, Haystack Observatory.

terrestrial mantle material [lvewsorn,1984; see also 2.2,3.2]. plasma environment. Ancillary electron flux measurements Furthermore, constraints on the core radius limit the range of by the Electron Reflectometer (ER) instrument will be mantle density models that are allowed by the moment of critical for distinguishing intervals when quasi-vacuum con- inertia constraint. ditions exist in the geomagnetic tail. Sufficiently lengthy Measurements of the induced moment will be obtained on observation periods (>30 hours) during which the Moon is LGOusingmethods similar to those developed for analysis of exposed to a nearly spatially uniform and constant amplitude Apollo 15 and 16 subsatellite data (Figure 3.7;Russell et al., magnetic field would allow more restrictive limits on the 1981 ). A minimum-cost effort would utilize a single orbiting radius and minimum electrical conductivity of a highly magnetometer to measure the induced dipole moment in the conducting lunar core. If the required conductivit,y is demon- geomagnetic tail, together with improved theoretical model- strably greater than 10 S m--', then a metallic rather than a ing of electromagnetic induction in a finite density molten silicate composition would be indicated. Contributions of LGO

INNER RING OUTER RING 100 km (200 km)

I MARE FILL EJECTA -BRECCIA ZONE 60 km

figure 3.5b:Cross-section along radial from center of Grimaldi basin for model that best matches topography and Doppler gravity data. From Phillips and Dvorak [I98I].

3.7 THERMAL HISTORY 19821. Part of this uncertainty is due to non-optimal inter- instrument calibration on the magnetometers used in the LGO can contribute to the question ofthe Moon's thermal Apollo experiments. If the LGO mission has a second, history by constraining the present temperature profile and presumably high altitude, magnetometer available, then an surface heat flow and by estimating the temporal and spatial experiment could be designed that would significantly im- evolution of the thickness of the elastic lithosphere. The prove accuracy of the transfer function estimates. In turn, the principal data types involved are from the Magnetometer range of allowable electrical conductivity profiles will be (MAG), the Microwave Radiometer heat flow experiment greatly diminished; and, with improved electrical conduc- (MRAD), gravity (Earth-based and SGS) and the Radar tivity-temperature data from the laboratory, reasonable Altimeter (ALT). The details of the MUDexperiment are bounds on the lunar temperature profile could be established. discussed in Appendix E.

3.7.1 Lunar Temperature Profile 3.7.2 Surface Heat Flow

As discussed in the previous chapter, the lunar electrical The objectives of a Microwave Radiometer heat flaw conductivity profile can be inverted in principle to obtain the (MRAD) experiment on LGO are (I) to measure more temperature profile. The electrical conductivity profile is accurately the global mean surface heat flow and (2) to obtained by modeling the frequency-dependent transfer investigate the nature of lateral variations in surface heat function between two magnetometers placed at different flow. Realization of the first objective will provide a firm radial distances from the Moon's surface. The lunar electrical boundary condition for all viable radial thermal models and conductivity is currently known to only one or two orders of will allow more accurate estimates for the total. amount of magnitude at any given depth [Hood et al., 1982a; Sonett, long-lived radioactive isotopes in the Moon. Estimates for the 32 LGO AND LUNAR SCIENCE

Figure 3.6:Radar altimetric profile and image covering about 30"of longitude along an Apollo 1 7 ground track on the farside of the Moon. The cratered nature of the topography is evident. The crater just to the left of center is Aitken. Data is from the 5 MHz channel of the Apollo Lunar Sounder Experiment. Horizontal axis is ground distance; vertical axis is topographic elevation (bright line).

bulk uranium abundance are needed to test suggestions that pW/cm2 (20% of' the expected global value of about 2 the Moon is enriched in refractory elements and will therefore pW/cm2). Measurements to this accuracy for, say, 100 assist in distinguishing among lunar origin models (see 3.2.1 ). suitable sites around the Moon would allow a global mean Lateral variations in surface heat flow are expected to occur heat flow estimate to an error of <20%. This would represent due to local variations in the ab~lndmcessf radioactive a significant improvement in current determinations from the isotopes in the upper 100 h.In the unlikely event that two Apollo surface measurements [Langseth et al., 1976; magmatic activity persists at present in the lunar upper Rasmussen avld Warrevl, 19851. To achieve the required mantIe, locaions of such activity could also be identified, For accuracy, individual sites must be moderately flat on a scale example, a mama at T = 1188°C at 100 km depth would of 100km, must have deep regoliths (>3m), and must possess create a heat flow of about 6 pW/cm2 (using a reasonable subsurface rock and clump distributions similar to the Apollo value of thermal conductivity), which is a factor of 3 or 4 and Surveyor landing sites (Appendix E). Significant but greater than the expected mean value. Demonstration of the achievable instrument developments are required to produce absence of such magmatic sources would also be a wo&h- a broadband antenna and a calibration system to achieve the while result. low bias level. The experiment would be greatly improved by The suggested goals for an LGO MMDexperiment are a having an IR themal radiometer (A = 20-50 pm) to monitor measurement accuracy of +1 pW/cm2 for each 100 h surface temperatures over a lunation, allowing inference of surface element, with an intrinsic bias no larger than 0.4 subsurface themal parameters, particularly the themal Contributions of LGO

Moon in vacuum exposed to a steady-9- spotia- e - * - . unitorm magnetic field

Figure 3.7: Cartoon illustrating the perturbation in a spatially uniform ambient magnetic field produced by a highly electrically conducting metallic core in the Moon. The LGO magnetometer will obtain improved measurements of the induced magnetic moment using methods developed for analysis of Apollo subsatellite magnetometer data [Russell el al., 19811.

conductivity. However, our knowledge of this parameter The degree to which LG0 will be able to carry out studies from Apollo measurements may be sufficient for a meaning- in lithospheric thickness variations in space and time will be ful experiment. A successful lunar remote heat flow experi- payload dependent. High resolution imaging is needed to map ment would provide a basis for future applications to similar tectonic features associated with lithospheric loads as well as dry, airless or nearly airless surfaces in the solar system, map the areal distribution of such loads. For the most including those of Mars, Mercury. large asteroids, and outer definitive results, the Spacecraft Gravity System (SGS) is planetary satellites [Keihrn, 19841. needed to obtain the gravity anomalies associated with farside features. In the absence of this experiment, improve- ment over the present lithospheric model is expected because 3.7.3 Lithospheric Thickness Variations of the availability of global topogaphy (ALT experiment) and high latitude nearside gravity from Earth-based tracking. The base of the elastic lithosphere at any time in the history of the Moon was controlled by the operative temperature gradient through the Row laws for the relevant geological materials. If the lithospheric thickness can be estimated at 3.8 LUNAR PALEOMACNETISM particular places and for particular times in lunar history, then a temperature structure in the outer part of the Moon can A second primary objective of the MAG/EW investigation be estimated. Lithospheric thickness estimates are obtained is to deternine the most probable origin(s) of lunar paleo- by estimating a lithospheric load (e.g., basalt flows, impact magnetism. This objective is directed toward an improved feature) at a particular time and the corresponding tectonic understanding of paleornagnetism on airless silicate bodies in response (e.g., gaben, wrinkle ridge) due to the imparled load the solar system and may yield further information on lunar stresses. The position of the actual tectonic feature relative to internal and dynamical history if a former core dynamo the load is directly related to lithospheric thickness [Solomon is found to be responsible for part of the observed and I-lead, 1979, 19801. magnetization. LGO AND LUNAR SCIENCE

The methods for investigating the origin(s) of lunar are significantly clustered for sources of comparable age, paleomagnetism build on results obtained with the magne- then evidence for a former intrinsic lunar magnetic field tometer and charged particle instruments on the Apollo 15 would be obtained; further investigation of the historical and 16 subsatellites. Direct magnetometer coverage in orientation of the lunar spin axis and possible time variations plasma-free environments and at sufficiently low altitudes to thereof would also be possible [Runcorn, 19831. Alterna- permit useful mapping (<100 km) is presently limited to a tively, if paleofield directions for contemporaneous sources total of less than 8% of the lunar surface (Figure 3.8). A are randomly oriented, then local field generation mechan- nearly polar orbiting LGO with a periapsis altitude of -50 km isms would be preferred. Possible local field generation for a minimum of two lunations would increase useful mechanisms could then be investigated to evaluate impli- coverage to approximately 90% of the Moon. Both the cations for paleomapetism on other airless silicate bodies, Electron Reflectometer (ER) and the Magnetometer (MAG) such as asteroids and the planet Mercury. will map the distribution of surface magnetic fields and determine the correlation of magnetic anomalies with surface geology. MAG will measure the field amplitude and direction 3.9 REGOLITH STUDIES at the spacecraft altitude during time intervals when ambient plasma disturbances are minimal. The electron reflection VIMS, XGRS and SSI will contribute to regolith studies. instrument will measure field strengths as small as 0.1 nT Conversely, rational interpretation of VIMS and XGRS data ( Gauss) at the lunar surface and will associate magnetic will depend on knowing how impacts mix materials in the features with surface coordinates to an accuracy of about 2 regolith. For example, LGO will be able to measure the km for an orbit altitude of 100 km. Measurement of fields at compositions of regoliths developed on mare basalt flows in two vertically separated altitudes will allow some resolution regions not sampled by Apollo landings or orbital data. The of the source depth versus lateral size ambiguity. An optimal most confusing complication when interpreting these data magnetic survey of the Moon can be accomplished only with will be caused by the presence in the regolith of material both the MAG and the ER instruments. derived from strata beneath the basalt flows. The mixed-in Previous work has shown that crustal magnetic fields are lithologies might be basalts with compositions much different generally weak across the maria and are stronger in the than those close to the surface, or they could be non-mare, highlands. Anomalies tend to be correlated with basin ejecta aluminous rocks derived from greater depth. Determining the units such as the Fra Mauro Fomation and with the Reiner compositions of ejecta and crater floors in mare regions will Gamma-type swirls. One potentially important anomaly provide clues to the nature of the contaminating rocks. In detected via the electron reflection technique is associated turn, this will help decipher the history of the regolith. with &ma Sirsalis, an extensional graben-like feature south- Because of its excellent spatial resolution, VIMS will be the east of Grimaldi [Andersonet al., 19773. Ifthe magnetization most useful instrument for determining the nature of ejecta responsible for the anomaly is directly associated with the and crater floors. VIMS is clearly capable of distinguishing rille, then a deep-seated source implying slow cooling in the between mare and non-mare rock types (Appendix A). presence of a steady, large-scale magnetic field would be Bombardment by micrometeorites creates glassy agglu- indicated. This would be in contrast with other orbital tinates in the regolith. Freshly produced soils are deficient in anomalies that appear to be associated primarily with these glassy particles. Young, or immature, soils can be surficial materials that are predominantly of impact origin. detected by the VIMS; the locations and frequency of their Future availability of lower-altitude magnetometer data over occurrence will inform us about transport rates of lunar Rima Sirsalis may significantly improve our understanding of surface materials. the origin of this anomaly and of the paleomagnetism in Exotic materials might exist in the regolith. En soils in general. Identification of other anomalies associated with permanently shaded areas in polar craters. water or other deep-seated structural features would increase the proba- volatile substances from meteorite and comet impacts may be bility that a large-scale magnetic field such as may have been trapped; the XGRS can detect hydrogenous materials and provided by a former core dynamo was responsible for a part aan detect chlorine and sulfur if present in sufficient concen- of the observed magnetization. Alternatively, if anomalies trations. We know from the Apollo sarnples that sulfur and are invariably associated with impact-produced materials, chlorine, as well as a series of less abundant. relatively then transient magnetic fields generated by impact plasmas volatile elements, all presumably indigenous to the Moon, are would be most plausible [Hood and Vickery, 19841. concentrated in some impact-produced breccias and on the In order to distinguish objectively between local-scale and surfaces of some of the glass spherules that we think represent global-scale lunar magnetizing fields, bulk directions of pyrocIastic products of fire fountains. In special cases, these magnetization must be detemined for a large number of elements may have been concentrated in the regolith to an major anomaly sources with comparable ages around the extent that could be detected by the XGRS. These are merely Moon. Assuming that any former intrinsic lunar magnetic a few examples of unexpected substances that might appear field was dominantly dipolar, paleornagnetic pole positions in the regolith; there will be surprises, but not necessarily any can be calculated for each anomaly source. Ifthese positions of these. Contributions sf EGO LGO AND LUNAR SCIENCE

3.10 SUMMARY versus local ejecta in crater and basin continuous deposits and the degree of large-impact melt homogenization. The LGO is expected to address the following funda- (5) Lunar Iron-Rich Core. The MAG and ER instruments mental questions in lunar science. should provide limits on the radius of a metaHic lunar core (1) Origin of the Moon. Global compositional estimates which, if detected, may be used in eoncerl with depletions of principally from the XGRS, and possibly from the MUD, siderophile elements in the lunar mantle to constrain hy- should eiiminate some hypotheses while remaining per- potheses of lunar origin. missive of others. (6)Thermal History. The (two magnetometer version of) (2) Evolution of Crust and Mantle. Global geochemical MAG and the MUDexperiments should provide estimates and petrological maps from XGRS and VIMS, coupled with ofthe lunar geothem and present-day surface heat Row. High gravity data and topography from ALT should provide (a) resolution imaging of tectonic features associated with litho- evidence for the existence, extent, depth, and differentiation spheric loading, improved global topography from the ALT products of a global magma ocean; (b) estimates of crustal instrument, and high-resolution gravity (Earth-based thickness and density variations; and (c) insight into basin- tracking and SGS) should permit investigation of variations forming processes. in lunar lithospheric thickness in space and time. This result (3) Magmatic History of the Moon. Global geochemical may be used to infer lunar thermal history. and petrological maps from XGRS and VIMS should permit (7) Lunar Paleomagnetism. A combination of the MAG identification of new and existing rock types and provide and ER instruments should permit direct detection of a lunar insight into (a) the nature and duration of highland igneous core if present and should measure the orientation of regional activity; and (b) the nature and extent of mare volcanism. surface magnetic fields as a function of age. The measure- (4) Impact Processes, Imaging, XGRS, VIMS, altimetry ments will help distinguish whether an intrinsic magnetic and gravity will enable studies of basin structure, mor- field, and external source, or both, were responsible for lunar phology, and composition of deposits. These data may be paleomagnetism. used to reconstruct: (a) the pre-impact target composition, (8) Regolith Studies. Imaging, XGRS, and VIMS will stratigraphy and structure, (b) formational conditions and establish the distribution and the composition of the lunar dimensions of the excavation cavity, and (c) post-impact regolith as well as identifying new rock types, and should eject a deposition and modification. In addition, composi- provide insight into the processes occurring during regolith tional data will allow assessment of fractions of primary formation. e of LGO in Resource Exp oration and Lunar Base Site Se

4.1 WHY USE LUNAR ESOURCES? element on Earth, are the Moon's prime known ores (i.e.. the most economical sources of raw materials for manufacture As our practical uses of near-Earth space expand, we will and construction). build large structures in space and use large amounts of At present, plausible schemes for making products for propellant there. To supply these materials from Earth, even construction or propellant from lunar rocks and soils may just to low-Earth orbit, we pay a stiff tariff in the form of assume as feedstocks only those materials known to be very energy because of Earth's strong gravity and atmosphere. An common at Apollo landing sites. To plan for actual use of any alternative would be a nearby source ofmaterials on a body of ofthese "ores," we will have to know that they are sufficiently lower gravity. This limits our early prospecting to the Moon abundant at the specific site chosen for a lunar base. For and the Earth-crossing asteroids. Ofthese, we know the most example, one scheme for producing oxygen requires the about the Moon, and the Moon, being always nearby, is much mineral ilmenite, from which water can be extracted by more readily accessible than the asteroids. Thus, the Moon is reaction with hydrogen imported as a recyclable reagent from the logical source of extraterrestrial materials for earliest use Earth. Should this method be chosen for lunar use, the in space (Figure 4.1). landing site would need to have soil rich in ilmenite. Such soils derive from high-titanium mare basalt. We would need to know the locations of deposits of such soils large enough for our proposed mine. This is more detailed knowledge than is 4.2 NATURE AND DISTRIBUTION OF currently available, but the LGO mission can provide it KNOWN LUNAR "ORES99 readily. Alternatively, if the principal product were to be Al, a soil From the Apollo and Luna missions, we know that certain produced mainly from feldspar-rich anorthosite would be types of rock are common on the Moon and that the Moon has needed. Anorthosite is an abundant type of rock in the an easily mineable regolith. For example, there are basalts highlands, as evidenced from the Apollo samples. We know with as much as 8% Ti in the form of minerals such as of some major expanses of pure anorthosite, but we do not ilmenite. Some basalts contain as much as 15%Fe, mainly in know how abundant soils consisting of nearly pure anortho- the mineral pyroxene. Anorthosites from the lunar highlands site are on a global basis. The LGO mission can determine contain as much as 17% A1 in the mineral feldspar. Most their distribution and assist us in locating the most suitable lunar rocks and soils contain about 20% Si, mainly in sites for feedstock for aluminum production. feldspar and pyroxene. Calcium is also abundant in feldspar. If knowledge of ore distribution is essential to selection of Nearly all lunar minerals contain oxygen, the most abundant the site for the first lunar base, the LGO mission must provide chemical element at the lunar surface. All of the oxygen is it. For the initial base, we could opt to use one of the Apollo tightly bound in the minerals, however; there is no atmo- sites, whose materials and characteristics we know. We sphere of free oxygen as on Earth. These common minerals might prefer a site, however, where an ilmenite-rich soil and in common types of rocks, which we would not smelt for any an anorthosite-rich soil were close enough together that we 38 LGQ ARrD LUNAR SCIENCE

Figure 4.1:Artist's concept of lunar ilmenite mine and oxygen plant. Automated front end loaders scoop lunar soil at the mine and transport it to a conveyor belt, which introduces it into an electrostatic mineral separator. At this stage, the mineral ilmenite (iron-titanium oxide) is concentrated in a pile, while the residue is returned to a dump in a previously excavated area. The concentrated ilmenite is introduced into the tubular reactor, where it is heated and reacted with hydrogen gas, yielding water (steam) and a mixture of metallic iron and titanium oxide as a solid residue. The residue is stored for potential later recovery ofmetal. The steam is introduced into an electrolysis unit that separates hydrogen and oxygen as gases. The hydrogen is recycled to the reactor, so is only slowly used up. The oxygen is sent to the liquifier, the larger quarter- cylindrical structure at the right. Oxygen is stored in the tanks in the foreground; hydrogen is stored in the stack of tanks behind the chemical plant. A truck carries liquid oxygen tanks to the launch pad where they may be transported to space for use there as a propellant. could exploit both. No Apollo site offers this combination, the Apollo and Luna samples. We may reasonably expect to Such sites, if they exist, could be identified by the LGO find other types of rocks. for example, ores of Cr, Ti (of higher mission. Alternatively, the EGO data might reveal the grade than mare basalt), and Mn. Magmatic processes of presence of an undiscovered type of resource, such as water crystal accumulation which produce such ores are common ice trapped in permanently shaded craters at the lunar poles. on Earth, and we suspect that they have operated on the Thus, from a resources standpoint, results of the LGO Moon as well. The LGO mission should be able to locate geochemical surveys of the surface could contribute impor- such ores, which would be valuable for producing higher tantly toward optimization of the site of the first lunar base. quality steels than we could othenxrise make on the Moon. In fact, all naturally occurring chemical elements are found on the Moon in some form, but most of them are dispersed at 4.3 DISCOVERY OF NEW ORES low concentrations in common types of rocks. Whether there are ores for these elements remains a mystery, one to whose The above examples considered mainly materials known solution the geochemical sunreys of the EGO will contribute to be abundant on the lunar surface, based on our studies of substantially. We cannot expect all of the same types of ores Resources/Lunar Base Sites on the Moon as we find on Earth. Many tenestrial ore- Antarctica, brought surprises in terms of new rock types or producing processes involve leaching of ordinary rock by significant extensions of compositions of known types that water or other volatile substances to concentrate minor and had not been expected or predicted. All of the presently trace elements. The necessary volatile substances seem available samples represent a miniscule portion of the never to have been present in the Moon in sufficient quantities Moon" surface, and the crude remotely sensed data from the to produce ores, the postulated trapped water ice in per- Apollo 15 and 16gamma-ray and X-ray spectrometers cover manently shaded polar craters being of probable extemal only another 20% and 1096, respectively. The LGO will origin (e.g., from comets ). However, we observe in the Apollo provide the still missing, but fundamental, global exploration samples that some type of geochemical process has acted to of the Moon's surface from which the strategy for any more concentrate many trace elements in some lunar rocks to levels detailed searches for lunar resources could be intelligently that are orders of magnitude greater than their average lunar derived. concentrations. The nature of these lunar processes is not known, so it is difficult to predict whether they might have Ied to ores. Possibly, the LGO will discover regions of unusually high 4.4 VALUE OF LGO BASIC GEOSCIENCE concentrations of relatively rare elements. The broad region TO THE SEARCH FOR RESOURCES around Mare Imbrium is already known to abound in Th and, thus, probably in the entire suite of so-called "KEEPy trace Even the fundamental science resulting from the global elements." Following computer enhancement of the spatial geochemical and geophysical surveys of the LGO will resolution of the rather crude data from the Apollo 15 and 16 enhance our knowledge of lunar resources. Increasingly on orbiting gmma-ray experiments, we have already raised our Earth, our exploration for ores is guided by our models for initially estimated concentrations of Th in some localities by Earth's overall nature and workings, our knowledge of the a factor of two. We expect that there are still higher processes by which Earth's crust develops and changes, and concentrations in smaller regions; if so, the LGO can find our concepts of how Earth's crustal materials extracted them. The LC0 could also detect anomalously high concen- themselves from its mantle. Our knowledge of lunar differen- trations of other elements normally present only in trace tiation processes is in a much cruder state, To improve it, we quantities. need to know the spatial pattern of abundances of common The importance of using the LGO to seek ores of elements lunar rocks and soils. This information will be a principal or substances that we must currently regard as dispersed or product of the LGO mission and will increase significantly rare becomes evident when we consider the potential value of our understanding of lunar processes, We will then be able to near-surface water, which may be trapped as ice in per- make better estimates of lunar resource potential. (If the manently shaded craters near the lunar poles. Water would present pattern of better understanding of terrestrial rock- be invaluable on the Moon as a raw material from which to forming processes stemming from questions raised by the produce propellants, as an extractant and a coolant for study of lunar rocks continues, we will be better able to manufacturing processes, and for life support. estimate terrestrial ore potential, too.) Consider the importance of water to manufacture of For example, we need to know whether the general tenets propellants. Hydrogen-oxygen propellant is used in the of the "magma ocean" hypothesis for early lunar differen- Space Shuttle, and its use is anticipated for the Orbit Transfer tiation to produce a thick crust of nearly pure plagioclase are Vehicle (OTV), the proposed reusable vehicle for moving essentially correct. If so, the many different types of rocks we satellites between low-Earth and geostationary orbits. The find in lunar highland soils and breccias must be part of a thin, OTV is expected to account for most of the traffic beyond contaminating veneer. Alternatively, the lunar crust might low-Earth orbit during the next two decades. Its use as now have derived mainly by repeated intrusions of magma from conceived depends on our hrnishing from Earth the pro- beneath and outpourings of lava. If so, we might expect more pellant required to operate it. Given frozen lunar water, we variety in rock type and more opportunity for production of could produce the necessary propellants for the OTV in specialized ores than predicted from the magma ocean model. space, probably at relatively low cost. Without water, which Also, the variety of materials on the surface might then be appears to be absent in lunar rocks, it would be necessary to characteristic of the Moon's entire upper crust. extract oxygen from lunar rock, a quite feasible but more Perhaps there was an early, Moon-wide melting event that complicated task. Possibly, hydrogen that originated in the produced a magma ocean and an anorthositic crust, and this solar wind and became entrapped in particles of the lunar early crust was disturbed by intrusions and extrusions of fresh soils can be extracted as well, but very large quantities of soil magma, so Moon's rocks and ores might be derived from would have to be processed to furnish it. processes envisioned in both hypotheses, Such fundmental Whether it discovers mineable water ice or not, the LGO knowledge of how the lunar crust formed is thus important to undoubtedly will find unexpected rock types or mineral our ability to predict the possible types and abundances of concentrations of interest as ores. Samples from every Apollo ores as well as to satisfy our general curiosity about how small and Luna mission, even the lunar meteorites collected in planets differentiate. LC0 AND LUNAR SC/I:'NCE

4.5 RESOURCE EXPLORATION SUMMARY Since lunar dust sticks to space suits, some of it may get into the astronauts' quarters or pressurized working areas. Re- The LGO mission is important to resource exploration for gions of high concentration of potentially hazardous ma- a variety of reasons. We need it to help us locate optimum terials could be detected by the LC0and precautions against sites for those types of resources that we know are present as a the effects of such materials could be taken in base design, if result of studies of lunar rocks. We can use it to discover the necessary. presence of suspected resources such as water and ores ofCr. Important contributions to site safety might come from We can use it to discover unsuspected resources. We will use high-resolution imaging. While we do not expect the LCO to its data on the areal and depth distributions of different rock be equipped to map the entire lunar surface at high resolution types to guide our understanding of the Moon's geochemical (i.e., one to three meters), we may he able to obtain high- differentiation and lead us to more educated speculation resolution images of selected locations. Potential sites that about what resources we can expect and where to look for currently seem especially suited to the multivariate uses of a them. lunar base or that are discovered during the LGO mission on the basis of LGO observations might be imaged at high resolution during the last portion of the mission, after primary 4.6 SITES FOR A LUNAR BASE mission goals had been met. In this manner, we could learn about boulder sizes, general landing site characteristics, or We have already pointed out the importance of the LGO the presence of special features such as lava tubes that might mission to our understanding the types, locations, and serve as protective shielding. quantities of "ores" available at a potential site for a lunar Engineering considerations are also important in site base. The LGO survey can contribute in other ways to site selection. The chosen site must have suitable locations for selection. For example, our scientific objectives may require habitats, roadways, and tunnels. Its regolith must be easily specific types of locations, such as access to a mare-highlands moveable, and perhaps contain more coarse particles than boundary region or to a particular type of crater. Radio usually found. The LGO can assist us in finding regolith low telescopes may need to be out of the line of sight with Earth. in glass content, an indication of soil immaturity; young soils The base may need to be near a source of volatiles. Our may have larger average particle size than normal. The scientific activities may need to be separated from our mining regolith must be deep enough to enable easy excavation or and manufacturing activities by a protective ridge or hill. gathering of fill. LGO compositional data can supplement While we can choose some low latitude sites with these photographic information about the extents and eharacter- physical features from existing photographs, we cannot know istics of ejecta blankets, which may be locations of deep the overall characteristics of those sites or make optimum regolith. Better photographic coverage than that available choices regardless of geographic location until we have the from Lunar Orbiter photography can help us to define slopes LGO data. Conversely, we may not have adequate photo- of surfaces and to estimate soil cohesiveness. graphs to select suitable sites in regions that the LGO The lunar base and subsequent human activity will cause geochemical sensors indicate as good ore locations. changes in the Moon's pristine environment. The LGO Safety will be a major consideration in site selection. This survey will provide a baseline for monitoring changes in lunar may restrict the landing area to a particular range of longitude surface properties, both near to the site of the base and remote and latitude, as for the Apollo missions. If so, sites we choose from it. It may also assist us in locating sites where the extent as potentially interesting on the basis of our current scientific of changes will be minimized, e.g., mining inside of a suitable rationale or needs may be inaccessible and our choices may crater or among surrounding hills. be more limited than we would like. LGO data could assist us Of course, we could set up a lunar base without the data in seeking a larger number of interesting sites within the that the LGO mission will provide. We could carry out regions acceptable for location of the base. interesting scientific studies and manufacture oxygen, Fe, or Compositional information provided by the LGO could Al (given suitable smelting techiques) at such a base as a prove impodant to the safety of astronauts at a lunar base. result of our knowledge from the Apollo missions. However, For example, soils at some landing sites could have incon- this would require our returning to an Apollo site, a limiting veniently high concentrations of radioactive uranium and strategy that would optimize neither the scientific nor the thorium. Others may contain inconvenient quantities of industrial output from the first lunar base. The global surveys compounds such as ferrous chloride (the mineral lawrencite), of the LGO might merely confirm the wisdom of returning to known to be present in some breccias. On exposure to moist an Apollo site, but more likely will open our vision to more air, lawrencite gives off poisonous hydrogen chloride gas. promising locations. 5 Conc uding Remarks

5.1 INTRODUCTION 5.2 WHAT LGO CAN TELL US ABOUT THE MOON

The LGO mission can contribute substantially to the It is clear that our current understanding ofthe Moon is at a solution of major questions regarding the Moon. The level of more advanced state than that for any other body in the solar its contribution is, of course, payload dependent. As defined system, save Earth. Some may argue that we should expend by the SXEG (Appendix F), a baseline LGO mission is our next efforts and resources on objects that we know much primarily a geochemical-petrological remote sensing mission less about. We argue that continued study of the Moon at this with VIMS, XRS and GRS instruments. A Radar Altimeter time is compelling, however. We are poised with a set of (ALT) would provide the topographic context of the geo- interrelated and sophisticated questions ofplanetary origin chemical-petrological mapping and, along with Doppler and evolution that are truly fundamental in nature and to tracking for gravity, would allow studies of nearside crustal which a global remote sensing survey ofthe Moon promises structure. We consider, however, that the addition of an to contribute signz3cant answers. imaging system (SSI) is essential to the baseline mission in The questions we are asking about the Moon could be order to place the remotely sensed chemistry in its proper asked of each terrestrial planet: (1) What is the origin of the geological context. Moon? (2) What is the evolution of the lunar crust and The baseline SSEC mission (+SSI) would provide many mantle? (3) What is the magmatic history of the Moon? (4) of the science contributions discussed in Chapter 3. Elemen- What is the bombardment history and the nature of the tal and mineralogical abundance maps of the Moon would cratering process on the Moon? (5) Is there an iron core in the address questions of lunw origin, crustal evolution, and Moon?(6)What is the thermal history of the Moon? (7) What mamatic history. Addition of more geophysically*riented are the origins of lunar paleomagnetism? (8) What is the instruments would, however, place even tighter constraints nature of the lunar regolith? on origin and evolution. The inclusion of a Magnetometer1 Most ofthese questions lead to the formulation of alternate Electron Reflectometer experiment (MAGIER) might well working hypotheses that, at our level of understanding, then provide a firm estimate on the radius of an iron-rich core lead to very detailed questions that can be answered with which, along with the known lunar siderophile depletion, specific measurements. Some of the competing hypotheses would help distinguish among lunar origin models. The regarding the origin of the Moon, for example, can be measurement of surface heat flow (MMD experiment) separated by determining bulk refractory element abun- would help constrain the bulk uranium abundance, and thus dances and bulk Mg'. The nature of the putative lunar magma the refractory element inventory of the Moon. An SGS ocean, as another example, can be determined by mapping experiment for farside Doppler gravity tracking would give us the igneous rock types in the lunar highlands and placing them a much more detailed and complete picture of crustal in correct vertical sequence in the crust. In addition to structure, an importmt complement to the geochemical and answering specific questions, the global surveys of the Moon petrologic data in understanding lunar crustal evolution, can be expected to show unsuspected features and trends that In our discussion of the science questions that will be amount to completingthe exploration stage of lunar study and addressed with LGO we have not explicitly discussed data- will lead to new hypotheses about major planetary processes. rate tradeoffs that may be required by mission or spacecrak LC0 AND LUNAR SCIENGE limitations. In particular, both the VIMS instrument and any measurement of energy flow via fields and particles from the solid-state imaging system will produce valuable data at a sun through the Earth's space environment. The ISTP very high rate. Although most of the lunar science objectives Program will involve at least four spacecraft (WIND, can be addressed at some level with the low data-rate of a POLAR, EQUATOR, GEOTAIL) placed in diRerent baseline Observer mission (32 kbps peak), we recommend Easth orbits, some involving lunar swingbys, to map various that careful consideration be given to the science output that portions of geospace. One of these ISTP spacecraft might would result from an enhancement of the data-rate for the serve as a data-relay link to Earth from LGO so that farside lunar mission. As discussed in this report, the potential gravity data could be obtained. It could also provide the science that would result from the technical capabilities of second magnetometer for the electromagnetic sounding ex- even a nominal LC0mission is very impressive; it would be a periment to detemine the lunar conductivity profile. Fur- scientific embarrassment if the output is limited by an themore, if a very capable Mars Observer class spacecraft artificial bottleneck in returning the data to Earth. were available, both the LGO mission and at least one ISTP In all, answers to the important questions require complex mission (e.g., WIND) could be combined and the objectives and interrelated measurements and it is not the purpose ofthis of both missions met with this same spacecraft with a larger report to attempt to prioritize such measurements. We merely payload. In the combined mission, WINDILGO would first wish to emphasize that, within the constraints of an Observer- go into a high elliptical lunar orbit to achieve WIND class mission, it is possible to make substantial headway in objectives (solar wind plasma and energy input upstream our understanding of the Moon and evolution of the terrestrial from Earth), followed by low circular lunar orbit for meeting planets. At the same time, the data obtained will make the LGO objectives outlined in this report. It is not possible, significant contributions to lunar resource evaluation and however, for an ISTP class spacecraft to achieve the LGO lunar base site selection. objectives.

5.3 TI-IE MOON IS A KEYSTONE 5.4.2 Japanese and Soviet Missions

The Moon is the centerpiece, or keystone, for the study of The Japanese and the Soviets have indicated individual all silicate bodies in the solar system because it has preserved plans for lunar orbiter missions. The Japanese have indicated 4.0 a record of over billion years ofsolar system history, is the very informally some interest in a cooperative Japan/U.S. basis of age estimations of nearly all solid surfaces in the solar lunar mission. Discussions have occurred which envisage a system, is a baseline for studying the more complex silicate U.S. LC0 spacecraft in low-lunar orbit for global surface systems of other planets, and is strongly tied to the earliest geochemical and geophysical mapping, and a smaller Japa- evolution of the Earth. For these reasons, study of the Moon nese spacecraft in high elliptical orbit carrying a mag- profoundly affects our views of the other planets. The netometer for monitoring the interplanetary magnetic field for knowledge about the Moon to be gained from the LGO the electromagnetic sounding experiment and acting as a mission will be immediately applicable elsewhere. We communication link to LGO for farside gravity mapping. An attempt to learn aboutprocesses, so we can understand better additional feature that has been discussed is the launch by the how the solar system came to be the way it is today. The Japanese spacecraft of surface penetrators that would install Moon is the best place in the solar system to study the a seismic network on the lunar surface. processes that affected the larger silicate bodies early in their The Soviets have plans for a two-phase mission. The first history, and LGO promises to make substantial contributions phase would put a single spacecraft in low circular orbit for to our understanding of the Moon. global surface mapping very similar to the LGO mission, but of shorter duration. The second phase would have the Soviet spacecraft raised to a high elliptical orbit to allow it to carry 5.4 LGO AND OTHER MISSIONS out solar observations. It is conceivable that during this latter There are several ways that an LGO mission could phase a U.S. LGO spacecraft could benefit by the high orbit interact with other missions, both U.S. and foreign, or be Soviet spacecraft acting as a path link to Earth for farside combined with other U.S. missions. gravity. Or, during the early low-orbit Soviet phase, a U.S. lLGO could be in high orbit and act as a relay. In summary, two spacecraft, regardless of national origin, 5.4.1 ISTP Program and operating during the same epoch, could enhance a lunar mission by (1) providing a relay for farside gravity data, (2) The ISTP (International Solar Terrestrial Physics) Bro- providing a second magnetometer for deep electromagnetic gram involving Japanese (ISAS), European (ESA), and U.S. sounding of the Moon, and (3) installation of a seismic (NASA) participants will have as its prime objective the network. Appendices

Appendix A: Return from LGO nstruments

A.1 GLOBAL COMPOSITION OF THE MOON km or larger in dimension. This high spectral resolution ca- pability is essential for the detection and measurement of Although the Apollo and Luna missions sampled only nine absolption features characteristic of key lunar mineral locations on the lunar surface, the returned lunar samples species. Compositional information that can be measured provide a rough ovewiew of the types of rocks that are or may with visible/near-infrared spectra is summarized in Table be important on a global scale to answer the unresolved A.3. Primary lunar minerals (orthopyroxene, clinopyroxene, questions of lunar geochemical evolution (Chapters 2 and 3). olivine, plagioclase, glass, etc.) can be identified and their Geochemical properties of such typical sampled mare and modal abundances estimated. These data are fundamental to highland rock types are presented in Tables A.1 and A.2. the identification of known (and new) lunar rock types. Discussed briefly below are the capabilities of a Visible1 (2) The multispectral mapping capability allows the Infrared Mapping Spectrometer (which measures mineral- spatial extent ofdistinet rock and soil types to be identified ogical abundances) and a X-RayIGamma-Ray Spectrome- and mapped at 0.5 km surface resolution. Important rock ter (which measures elemental abundances) as they relate to types are exposed at these small local scales (e.g., walls of these types of lunar materials. These two instruments provide craters). The composition of individual units will be charac- complementary data necessary to detemine the local, re- terized using the mineralogical infomation derived from the gional and global composition of the surface of the Moon. high spectral resolution data and the elemental abundance data derived from XGRS. These combined data provide the basis for a global assessment of lunar geochemical units at A. 1.1 Visible and Infrared Mapping Spectrometer relatively high spatial resolution. Data from a Visible and Infrared Mapping Spectrometer (VIMS) are used to characterize the mineralogcal composi- A.1.2 X-WayIGmma-Ray Spectrometers tion of a surface through the detection and measurement of superimposed mineral absorption bands in visible and near- These two instmments measure the elemental abundances infrared reflected radiation. The estimated technical capa- of surface material. Although the spatial resolution is lower bilities of a lunar mapping spectrometer (VIMS) allow high than that obtained with VIMS, they provide fundamental precision (<0.5%) reflectance spectra to be obtained at a information on the chemistry of the lunar surface and are resolution of 0.5 km on the lunar surface. These capabilities essential for developing maps of the regional and global provide the basic data for two important interrelated contri- surface composition of the Moon. The X-Ray Spectrometer butions to understanding the global petrology and geo- measures X-rays emitted at characteristic energies by K- chemistry of the Moon. shell electrons (e.g., Ca, Mg, Al) that have been excited by (1) The near-infrared spectroscopic capability (about 250 solar X-rays. The Gamma-Way Spectrometer measures spectral channels) allows specific mineral species to be gamma rays emitted either as a product of natural radio- identified for individual geological surface units that are 0.5 activity (e.g., Th, U, K) or as the result of interactions with 46 LC0 AND LUNAR SCIENCE

TABLE A. 1 Compositions of Highland Rock Types

Ferroan Gabbro- KWEP Alkali Magnesium Anorthosites Norites Troctolites Norites Dunite Basalts Anorthosites Anorthosites Granites

Mineral Abundances (vol %) plagioclase >90 40-75 50-75 0-30 olivine 0-10 <1 20-50 <1 (fayalite) low-Ca pyroxene 0-10 25-60 <5 high-Ca pyroxene

Element Abundances (wt %) Si 20-2 1 22-24 A1 16-19 8-1 4 Mg 0.05-1 .O 4-9 Fe 0.15-2.5 2-5 Ca 13-14 8-1 1 Cr <0.015 0.10-0.30 n?m <0.03 0.04-0.13 Ti <0.08 0.06-0.20 Na 0.2-0.3 0.20-0.30

Element Ratios Weight Al/Si 0.80-0.90 0.30-0.60 0.40-0.80 Atomic Ga/(Ca+Na) 0.94-0.98 0.89-0.96 0.94-0.97 Atomic Md(Mg+Fe) 0.40-0.75 0.68-0.82 0.8 1-0.92

'Also has: -40% Quartz (or other silica minerals), 3040% K-feldspar. *One sample (67667) consists of 50% olivine. "bout half is pigeonite in KKEEP basalts.

high energy galactic cosmic rays (e.g,, Fe, Ti, H, Si). The about one hour of counting time for each 80x80 h area in precision of measurements are strongly dependent on the equatorial re~ons.However, counting times are longer, counting statistics via the duration of the measurements and hence measurements more precise, at higher latitudes. With- o bsewing geometry. in 10 degrees of the poles, counting times are about ten (1) X-Ray: The effective spatial resolution of the X-Ray times greater than at the equator, which leads to a factor of the Spectrometer depends on counting statistics and on solar square root of ten better precision. Estimates of the precision output. The nominal "quiet sun" resolution along the ground- of elemental abundances are listed in Tables A.4 and 14.5 for track is about 30 km. Estimates of one-sigma errors for Si, Al, the rock compositions listed in Tables A. 1and A.2, respec- and Mg are about 10% of the amount present. During solar tively. Spatial resolution is inversely related to precision. flare events, the X-ray system may also be capable of Changing the resolution area from 80x80 km to 160x 160 measuring Fe, Ca, Ti, and K. Because one cannot predict km, therefore, improves the precision by a factor of two. how many solar flare events will occur during the LGO (3) XGRS Capabilities: The ability of the XGRS instru- mission (nominally one year), we have not estimated the ment to distinguish mong known lunar rock types can be precision for these elements. seen by comparing the error estimates in Tables A.4 and A.5 (2) Gama-Ray: Both the precision of measured elemen- with rock compositions in Tables A. 1and A.2. An element's tal abundances and the spatial resolution are controlled by concentration will not be detemined accurately if the values counting time. Counts accumulate over many orbits for each in Tables A.4 and A.5 exceed 20%. Although the XGRS resolvable pixel of area, which is about 80x80 km for most cannot measure accurately all elements in each rock type elements at an altitude of 100 km. A 359-day mission yields (e.g., Mg is too low in ferroan anorthosites for a precise TABLE A.2 Compositions of Typical Mare Basaltsl + Very-low Ti Low-Ti High-Ti Aluminous Volcanic Glasses 5 L-24 A-17 A-12 8-12 A-12 A-15 A-15 A-11 A-11 A-17 A-17 A-14 A-14 G16 A-15 A-17 5 very low high fb pig. oliv. ifm. pig, oliv. low K high K high-Ti REE REE green orange 5*

Mineral Abundances (vol. 96) plagioclase 39 32 olivine 10 5 pyroxene 49 6 2 opaque oxides 2 1 Element Abundances (wt. 96) Si 21 22 Al 7.3 5.3 Mg 3.8 7.3 Fe 16.0 14.4 Ga 9.1 7.1 Cr 0.1 0.5 Mn 0.2 0.2 Ti 0.5 0.5 Na 0.2 0.1

K (PP~) 180 100 560 460 500 500 330 500 3000 500 400 60 1400 1700 170 750 U (PP~) - 0.2 0.2 0.3 0.1 0.1 0.2 0.8 0.1 0.06 - - - 0.02 0.2 Th (ppm) 0.2 0.8 0.8 1.O 0.5 0.4 0.7 3 0.4 0.2 0.4 2.3 1.4 0.08 - Element Ratios Weight AI/Si 0.35 0.24 0.26 0.21 0.25 0.23 0.21 0.29 0.26 0.27 0.26 0.28 0.32 0.33 0.19 0.18 Atomic Mg/ (Mg+Fe) 0.36 0.54 0.37 0.52 0.38 0.44 0.46 0.38 0.40 0.44 0.49 0.37 0.32 0.38 0.59 0.54

'L = Luna; A = Apollo. 48 LGO AND LUNAR SCIENCE

TABLE A.3 Lunar Compositions Identified with VIMS Spectra HIGHLANDS: Unweathered Exposure Soils General Rock Pjpes Identified (Craters, etc.) Pyroxene composition Pyroxene composition Anorthosite Ca/(Ca + Fe + Mg): 55% Ca/(Ca + Fe + Mg): t5% Noritic Anorthosite Fe/(Ca + Fe + Mg): + 10% Fe/(Ca + Fe + Mg): tlO% Anorthositic Norite Pyroxene abundance: t5% Pyroxene abundance (~5%relative) Norite Olivine abundance: + 10% Olivine abundance (+ 10% relative) Gabbroic Anorthosite Plagioclase abundance: Plagioclase abundance Anorthositic Gabbro Anorthosite (<5% pyroxene) (110% relative) Gabbro Other rock types Agglutinate abundance Troctolitic Anorthosite (+ 10% plagioclase indirect) (t1'0% relative) Anorthositic Troctolite Glass (Fe-bearing): + 10% Troctolite (Impact melt) MARE: Unweathered Exposure Soils General Basalt Types Iden tzlfied (Craters, etc.) Pyroxene composition Ti02 content: k2% High-Ti Basalt Ca/(Ca + Fe + Mg): 33% -1- 1% (relative) Law-Ti Basalt Fe/(Ca + Fe + Mg): k10% Pyroxene composition VLT Basalt Pyroxene abundance: +5% Ca/(Ca + Fe + Mg): t5% Aluminous Basalt Olivine abundance: rt 10% Fe/(Ca + Fe + Mg): +10% Olivine Basalt PlagiocIase abundance: t 10% Pyroxene abundance (t5% relative) Pyroclastics (glassy) Olivine abundance (t10% relative) Plagioclase abundance (+ 10% relative) Agglutinate abundance (relative) Glass (Fe-bearing, pyroclastic): i10%

measurement), it seems clear that the instrument will be able radiometric precision of measurement. However, require- to distinguish all of the rock types listed in Tables A. 1 and ments for a TIMS instrument consistent with specific scien- A.2 from each other. It also appears that the data will be tific goals and known physical and chemical state of the lunar precise enough to identify mixtures of these rock types. surface need to be determined.

A. 1.3 Thermal Infrared Mapping Spectrometer A.2 IMAGING SYSTEMS

Characteristic molecular emission and absorption features The quality and coverage of the image collection for the occur in the thermal infrared between 6 and 20 pm for most lunar surface is very non-uniform and inadequate for many rocks. Of particular interest in this spectral region are the Si- scientific applications. The current photographic coverage 0 resstrahlen vibrational bands for bare rock, as well as the for the lunar surface is summarized in Appendix C and the primary Chistiansen emission peaks for dusts, both of which status o falunar geodetic control net is presented in Appendix are indicative of silicate mineralogy. The direct silicate D. With the exception of images taken with Earth-based mineralogy data provided by a Thermal Infrared Mapping telescopics, none of the useful lunar image collections are in Spectrometer (TIMS) imaging data is independent and digital form. complementary to VIMS, which supplies mineralogical Many of the scientific questions concerning geological information based on the electronic absorption bands of the processes (impact cratering, volcanism, tectonism) discussed transition metals, such as Fe. 'ITIMS spectra in principle can in Sections 2 and 3 require analysis of high quality images of provide a remote means for identification of primary silicate the surface. Furthermore, the geological context under which minerals. The precision of identiltlcation and the extent to the geochemical measurements of VIMS and XGRS are which mineral abundances can be recovered for both lunar made will have significant implications for their interpre- soils and rocks will increase with spectral resolution and tations; e.g., is the origin of a geochemical anomaly related to Return from LC0 Instruments 49

TABLE A.4 One-Sigma Counting Errors (Percent of Amount Present) for Rocks Listed in Table A.1' Ferroan Gabbro- KmEP Alkali Magnesium Anorthosites Norites Troctolites Norites Dunite Basalts Anorthosites Anorthosites Granites 4-4.5 4.6-4.9 4.7 3.5-4.1 75-* 44-5 1 42-48 70 11-22 22 25-80 22-* 21-22 47-* * 29-* 40-50 24-29 23-25 48-* * * * *

'Errors calculated assuming one-hour counting time and 80x80 km spatial resolution. Errors are lower at high latitudes, where counting times are ten times longer. Errors decrease linearly with degradation of spatial resolution (i.e., divide above values by two for 160~160krn resolution). Asterisks indicate values below detection limit (taken to be counting errors of 100%). excavation from depth by a fresh crater or to a localized altitudes above the surface. The directional measurements mantle of material on a mountain? will be essential for evaluating theories for the origin of the Specific imaging systems for EGO are defined in Appen- paleomagnetisrn as discussed in Chapter 3. Measurements dices D and G. Any imaging system is expected to easily will be obtained during intervals when solar wind plasma- fulfill the following needs: related disturbances are minimal, i.e. during times when the (I) Obtain high quality images of unimaged lunar areas Moon is in the geomagnetic tail lobes or when the Moon is in (e.g., poles, Orientale, etc.) the solar wind but LGO is in the Iunar wake. It is critical that (2) Map selected areas for which only poor quality images the LGO orbit periapsis altitude be no higher than approx- exist. imately 50km (ideally, 30 km) for a period oftwo lunations in (3) Target areas of high scientific importance. order to obtain optimal (near 100%) coverage, as compared to the ~8%useful coverage obtained with the Apollo subsatellite magnetometers [e.g., Hood et al., 19811. A.3 MAGNETOMETERIELECTRON The ER instrument will provide complementary indirect RIEFLECTOMETER CAPABILITIES measurements of surface magnetic field strengths during periods when the lunar surface is exposed to significant The Magnetometer (MAG) and Electron Reflectometer energetic electron fluxes. A magnetometer is a necessary (ER) instruments have as their primary objectives (1) a supplement in order to measure the mbient field orientation determination of bounds on the size of a highly electrically along which the electrons are guided. Surface field strengths conducting metallic core; and (2) a determination of the most inferred by the ER instrument are essentially independent of probable origin(s) of lunar crustal paleomagnetism. As spacecraft altitude. In addition, measurements are possible discussed in Chapters 2 and 3, the size of a possible metallic during a wide variety of ambient plasma conditions. Cur- core is a basic constraint on lunar origin models while the rently, ER maps of lunar surface fields are characterized by origin of the paleomagnetism is a basic unresolved issue low spatial resolution and extend to, at most, t$0" of latitude, raised by Apollo orbital and sample studies. The increased database to be expected from an LGO mission With respect to the first objective, the MAG instrument would significantly improve spatial resolution as well as will measure the induced dipole moment of the Moon when coverage. the Moon is in the quasi-vacuum environment of the geo- Finally, we note that the above summary assumes that magnetic tail lobes [Russell et al., 1981; and references only a single LGO spacecraft will exist in lunar orbit during therein]. In order to obtain measurements of optimal accura- the LGO mission. If, through international collaboration or cy, simultaneous electron flux data obtained with the EW through collaboration with other U.S. missions, a second instrument will distinguish intervals when ambient plasma spacecraft with a magnetometer and plasma instrument is densities are minimal, present, then several augmentations of the above objectives With respect to the second objective, MAG will map the would be possible. First, a second magnetometer would allow amplitude and direction of crustal magnetic fields at low electromagnetic sounding of the lunar electrical conductivity LC0 AND LUNAR SCIENCE

d2k f-.c., v, d d g 4 2

.cc 4sl 0 2 i-J 4 El 2 E4,Na, mh 4 2 L-2 P

;I;;" 6" e.Cf

L/! Return from LG0 Instruments profile in a manner similar to that done with the Apollo l 2 and global abundances of elements mapped by the XGRS Explorer 35 magnetometers le.g., Sonett, 19821, As dis- experiment. Gravity and altimetry data combine in litho- cussed in Chapter 2, these limits can in principle be applied to spheric flexure studies, and this is one ofthe primary means of constrain the iunar temperature profile. Second, plasma estimating the thermal history of the Moon. Gravity field instruments on the second spacecraft could assist in identi- mapping on LGO will take place by the traditional method of fying plasma-free time intervals for the induced moment Earth-based Doppler tracking. Good data will be acquired measurement. Third, a second magnetometer could measure for the first time for latitudes higher than 30" on the nearside. ambient field time variations in the geomagnetic tail allowing As with the MAGIER experiments, it is important for more accurate mapping of crustal mametic fields by the LC8 periapsis altitude to be lowered to 50 km during part of the magnetometer. Thus, in addition to allowing acquisition of mission, so as to maximize resolution. farside gravity, a second spacecraft would significantly It is possible to obtain farside gravity data with the enhance the MAGIER investigations on LGO. Spacecraft Gravity System (SGS) mentioned in the main body of this report. This proposed system uses the same principles as that of traditional Earth station to spacecraft A.4 RADAR ALTIMETER tracking. The SGS uses the LGO spacecraft to track a released corner reflector. The reflector requires no power, no The Radar Altimeter (ALT) serves a number of scientific attitude control, and no electronics, only a surface to reflect investigations on LGO, specifically: (I) When combined the radar signal transmitted by the spacecraft. The system with gravity data, it forms the basis for mapping density was originally proposed for the Lunar Polar Orbiter mission; distributions in the lunar interior; (2) when combined with it would interface directly with a radar altimeter of the type ~avitydata, it also provides the basis for estimating litho- presently proposed for LGO. A significant portion of the spheric loads in flexure studies; and (3) when combined with altimeter electronics and the stable oscillator would be used. elemental abundances determined by XGRS at the surface, it Since data rates are relatively low for both experiments, there provides a basis for detemining crustal isostasy and extrap- would not be any conflict in data return. olating compositional measurements to depth. Additionally, an altimeter, in concert with imaging, XGRS and VIMS, is a key in reconstructing the geological history of the surface. It is used, for exmple, to deternine the A.6 MICROWAVE RADIOMETER CAPABILITIES thickness of ejecta deposits, the depth of craters, and the volume of individual mare basalt flows. The global figure of As discussed in Chapter 3, the objectives of a Microwave the Moon, as derived from the full ALT data set, will allow Radiometer (MUD)experiment on LGO are (1) to more investigations into such topics as the departure of the Moon accurately measure the global mean surface heat Row and (2) from a hydrostatic state and the center-of-figurelcenter-of- to investigate the nature of lateral variations in surface heat mass ofl'set. flow. The desired measurement accuracy is II pW/cm2 for Further discussion of altimetry is found in Appendix C. each 100 km surface element with an intrinsic bias no larger than 0.4 pW/cm2. Measurements to this accuracy for a large number of suitable sites would allow a global mean heat flow A.5 GRAVITY FIELD (INCLUDING determination to an error determined only by the maximum SPACECWAFT GMVITY SYSTEM) intrinsic bias. This would represent a significant improve- ment over current estimates derived from the two Apollo Gravity field measurements, when combined with topog- surface measurements which range from 1. I to 1.8 pW/cm2 raphy, allow determination of the Moon's internal density [Rasmussen and Wctrren, f 985, andhangseth et al., 1976, distribution. The best known gravity anomalies on the Moon respectively]. To achieve the desired accuracy, individual are associated with mascons; these are positive free-air sites must be moderately flat on a scale of 100 km, must have gravity anomalies associated with multi-ringed basins and deep regoliths (>3 m), and must possess subsurface rock and are apparently caused by a thick basalt fill in these basins clump distribution comparable to the Surveyor landing sites combined with structural uplift of the underlying dense (see Appendix E). Significant but achievable instmment mantle. This concept is basically a hypothesis, and if LGO developments would be needed to produce a broadband can provide a set of free-air gravity data over a spectrum of mtenna and a calibration system to achieve the low bias filled to unfilled lunar basins, then this issue should be level. The experiment would be improved substantially by resolved. having an IR thermal radiometer to allow modeling of Gravity and altimetry data can also be used to map subsurface thermal conductivity. Alternatively, estimates of variations in crustal thickness under certain assumptions, this parameter derived from Apollo measurements would be This is of fundamental importance in determining absolute used.

Appendix B: o Orbita Mapping Capabi ities: Imp ications for LGO

The only geochemical data currently available for un- Norman and Ryder (19809 showed that most lunar pristine sampled lunar areas are results from the Apollo X-ray and rock types are separated into distinct fields on a plot of the gamma-ray experiments of the lunar equatorial region and Sm/Ti ratio vs. the Mg/(Mg -t Fe) ratio. This separation information derived from near-infrared reflectance spec- suggested to Davis and Spudis [I9851 that the lunar orbital troscopy and multispectral imaging of the nearside using geochemical data might be similarly employed, replacing the Earth-based telescopes. Althougb both the maria and lo- LIL element Sm with Th, which is of similar geochemical calized areas in the highland crust are known to exhibit incompatibility, and the Mg/(Mg + Fe) ratio with the Fe significant petrological heterogeneities on the scale of a few to concentration alone. Both Th and Fe were obtained with several tens of kilometers as measured by reflectance data moderate accuracy from the Apollo orbital gamma-ray [e.g.,Pieters and McCord, 1976;Pieters, 1978;Lucey el al., spectrometer. 1986; Pieters et al., 1985;Pieters, 19861, the complexity of Plots of the Th/Ti ratio vs. Fe abundance obtained from the impact record and regolith formation in the hiblands pristine rocks and from orbital data are shown in Figures B. 1 might lead one to believe it will be difficult to distinguish and B.2. The rock types anorthosite, norite, and KEEP/ reglbnal petrological affinities with the XGRS instruments. Mg-suite display excellent separation. The sigrnificriulce of For many of the goals outlined in Chapters 2, 3, and 4 to be this plotting method is that not only can regions of relatively successfully addressed by LGO, it is necessary not only to "pure9' rock types potentially be seen within the orbital data, demonstrate that impacts on the Moon have not globally but orbital points that plot between fields of pure rock types homogenized the regolith, but also that there exist distinct may represent regions of mixtures that can be quantified. terrae surface regions of spatial scale comparable to that The orbital geochemical data in Figure B.2 corresponding attainable by XGRS. to the rock-sample data of Figure B.1 encompass most of In this context, recent petrological maps extracted from the known highland rock compositions. Much of the orbital the spectrally obsolete but spatially comparable Apollo 15 data lie close to anorthositic compositions: but orbital and 16 X-ray and gamma-ray instmments are of interest. The data also correspond to KREEP, noritic and extreme rock proposed XGRS instmments to be carried aboard the Lunar compositions such as troctolites and KMEP-granites. The Geoscience Observer will produce global maps of elemental vast majority of orbital data points could be interpreted as ablandances for certain key elements (Al, K, Th, Ti, Fe, and mixtures of these pristine rock types, as they tend to fall Mg, at a minimum). These same elements were measured by within the mixing triangle of moflhosite/basalt/Mg-surite, the Apollo 15 and 16 orbiting spacecrafts. Such a data base although they may represent pure rock types of intermediate will directly create a geochemical map of the lunar crust but composition or mixtures of yet undiscovered rock types. will not directly address the petrological makeup of the lunar The orbital data may be subdivided into petrologic units, highlands. However, Longhi and Boudreau [ 19793 and based first on correspondence with known pure rock types LGO AND LUNAR SCIENeE

Figure B. 1:Plot of pristine rock sample data of Fe (wt.%) and Th/Ti, normalized to C 1 chondrites. Data from the literature. Symbols: Open circles-anorthosites; open diamonds-alkalic and troctolitic anorthosites; open squares-anorthositic norites; closed circles-Mg norites; closed diamonds-anorthositic troctolites; closed squares-troctolites; stars-gabbros; crosses-KEEP-rich basalts, QMD and granites; triangles-mare basalts. Apollo Petrological Mapping

Figure B.2: Plot of hi&land orbital pixel data of Fe (wt.96) versus ThITi ratio (normalized to C I chondrites) and petrologic unit boundaries arbitrarily established for the Fe-(lE'h/Ti)c plot.

and then into an arbitrary series that represents quantified The success of the primitive Apollo orbital gamma-ray and mixture categories (Fig. B.3). The product is a petrologic X--rayspectrometers in identifying distinct chemical units in map of the Moon for which the following properties are the terrae establishes the feasibility of producing a global known: (1) its mean composition and standard deviation in petrological map from LGO, convolving the detailed chem- terms of Th/Ti and Fe; (2) its compositional afinities to both ical compositions obtained with XGRS at relatively poor pristine rock types and mixed-rock types and soils; (3) its spatial resolution with the mineralogical identification made total areal extent; and (4) its geogaphic distribution on the by VlMS at relatively high spatial resolution (see A.1.). Moon. As is seen in Figure B.3, some units surprisingly These combined data sets should be capable of addressing the correspond to apparently pure pristine rock compositions major scientific and programmatic issues raised in Chapters [e.g., unit 1 is composed of almost pure (-95%) anorthosite]. 2, 3, and 4. LGO AND LUNAR SCIENeE Appendix C: The Present Lunar Photographic and Topographic Coverage and Possib e LGO Contributions

G.1 INTRODUCTION scarp of the Orientale basin, has photographic coverage at only highly oblique viewing angles near the terninator, or Contrary to widespread belief, the photographic coverage under the zero-phase ligbting conditions observed by the of the Moon is incomplete and inadequate. There are many Soviet probe Zond 8. This gap is simificant; from what we reasons for this, but it is largely a consequence of the can see of the western rim ofthe Orientale basin, it appears to piecemeal exploration plan of lunar exploration devised to be quite different in morphology from the well-photographed support the Apollo missions. Because spaceflight was a new eastern half of the basin. The spectacular series of photo- and fledgling technology in the 1960's, the logic of lunar graphs of the eastern half of the Orientale basin have been exploration was developed from an engineering perspective. used to develop several hypotheses of multi-ring basin The effect of this exploration plan was to produce a Iunar formation [Head, 1974; Moore et al., 1974; Chao et al., photographic data base consisting of a wide variety of 1975; Wilhelms et al., 19771. It is highly probable that good formats, including television, analog-electronic, digital, and quality photographs of the western portion of the Orientale hard-copy, in a non-systematic pattern of coverage and basin will cause us to revise and modifl some of these models resolution (Figure C.1). With the technology of spaceflight for basin development. This is just one example of the perfected, we were able to design a series of planetary importance of a good quality photographic data base to missions, ranging from flybys to orbiters, that have mapped elucidating the processes that have shaped the Moon's some terrestrial planets and satellites of the Jovian planets. surface and, by analogy, the surfaces of other terrestrial This has produced the ironic situation that Mars, a planet planets. with almost four times the surface area of the Moon, The Apollo ""JIbmissions (15-17) carried high quality possesses a consistent, cartographic-quality photogaphic cameras developed for both positional accuracy (mapping data base (average resolution of -100 m), which is in digital camera; -50 m resolution) and high resolution (pmormic form and can be electronically enhanced and processed for camera; -1 m resolution). Because of the near-equatorial various studies. About half of the Moon has been photo- orbital characteristics of these missions, the resulting pho- gaphed at this resolution level, but none of these data are in tography covers only about ten percent of the lunar surface in digital form. stereo (mapping camera) or in high resolution (pan camera). In addition to the lack of a photographic data base in a Even this photography is less useful than needed, however, in systematic format, coverage of the lunar surface is also that almost 40 percent of it was taken under nearly zero- incomplete. The most glaring blank spot is in the south polar phase lighting conditions, such that subtle topographic fea- region, where a gore consisting of over 200,000 k.m2has no tures cannot be identified. High-sun Iunar photographs are an photographic coverage whatsoever. Another key region on excellent source for albedo contrast data, useful in numerous the nearside western limb, including the entire western outer studies such as the mapping of dark mantle pyroclastiq LGO AND LUNAR SCIENCE Present Photographic Coverage deposits, but they cannot be used for such rudimentary Observer-class missions. However, a very substantial geo- photogeologic studies as crater counting (to establish relative logical imaging experiment is possible for the LGO and can ages) and recognition of ancient topography (to map, for yield important data and help fill in the gaps in our lunar example, old basin rings). Thus, even though extensive photographic base. In the foHowing discussion, a one-year coverage exists for the eastern hemisphere equatorial region, nominal mission is assumed, and the camera system is the more photographs are needed under favorable lighting Geodesic Imager (GSSI) described in Appendices D and G, conditions. with 36 m/pixel resolution and at least 2:1 noiseless data Other inadequacies in the current lunar photographic data compression. With this LGO imaging experiment, we may base include poor coverage of the lunar farside. The Hum- expect at least the following: ( I ) complete stereo coverage of boldtianum basin, located near the extreme northeastern limb both polar regions down to about 70" latitude; (2) selected, of the Moon, is a multi-ring basin of at least equal, if not contiguous coverage in the mid-latitude band (30"-70"); and greater, interest than the better known Orientale basin. It (3) isolated coverage (spot frames) in the near-equatorial appears to have a relatively well-preserved ejecta blanket, region. and many of the morphologic facies seen at Orientale can be One of the main photographic gaps on the Moon is a large recognized in Humboldtianum. It has even been suggested region near the south pole; this gap could be easily filled that this basin was the product of a double or triple impact during the LGO nominal mission. The importance of com- [Lucchitta, 19781. The importance of the Humboldtianum plete photographic coverage near the poles may be con- basin lies in the fact that it is older than the Orientale basin but siderable. One hypothesis to be tested by LGO is the is comparably well preserved. This would be an important presence of water ice within the permanently-shadowed point of reference to infer, for example, the nature and role of regions of both poles [Arnold, 19791. If this proposal turns lunar lithospheric growth in the development of multi-ring out to be correct, a complete cartographic data set for these basin morphology. regions would be essential for site selection studies for a polar Other lunar farside photographs, taken mostly by Lunar lunar base that would use this resource. Additionally, several Orbiter, have widely varying viewing geometries, lighting sites of geological interest also occur near the poles, including conditions, and resolution. Coverage of the interesting South the following: ( 1)the south rim and interior of the South Pole- Pole-Aitken (Big Backside) basin is incomplete, even though Aitken basin; (2) the northern edge of the Imbrium basin Fra this basin is both the locus of most farside mare deposits Mauro Piomation (ejecta deposits); and (3) the northern half [Stuarl-Alexander, 19781 and major geochemical anomalies of the Wurnboldtianum basin. These targets alone will [f-lawke el al., 19851. provide impor"&ant new insights into lunar geological In addition to supporting gravity analyses, topographic processes. data provide a key information resource in almost all lunar geological studies. Topographic maps are used in structural We may also expect selected, contiguous coverage in the studies (e.g., the nature of basin ring patterns and mor- medium latitude band (30"-60"), where several key features phologic evolution), and in estimating crater volumes and of geological interest occur. In particular, the southwestern lava flow thicknesses. Topographic data provide the quanti- scarp of the Orientale basin lies between 20" and 40" south tative information necessary to model the processes of crater latitude. This feature has not been adequately photographed formation, lava flooding, and lunar lithospheric growth. and is much needed for our understanding of the formation of Good topographic maps were produced from the Apollo multiple rings around major lunar basins. Moreover, a series metric camera photography (the LTO and LM series of the of contiguous, preferably stereographic, photos could be Defense Mapping Agency) for only about 10 percent of the acquired at the interesting Humboldtianum basin, which lies Moon. Topography derived from Earth-based radar [e.g., predominantiy between 40" and 70" north latitude. The Zisk , 197 8 1 is available for most of the lunar nearside, but northern portion of the Imbrium basin could also be acquired these data have limited vertical resolution (rrtr 100 m). Simi- and the detailed mapping of this major lunar feature could be larly, the old LAC topographic series, derived principally completed. Other sites of geological interest in this zone are from shadow measurements, are highly unreliable for most the Van De Graaf/lngenii region (a geochemically and quantitative studies. It is clear that the lunar topographic data geophysically anomalous zone), transitional crater-basin base is largely inadequate to meet the needs of the second- features such as Compton, mare deposits of the South Pole- generation, advanced geological studies that will elucidate Aitken basin and the unusual Apollo basin on the lunar the nature of complex lunar processes and for lunar resource farside. mapping and lunar base sites reconnaissance. Spot coverage within the lunar equatorial zone (I~I30' lat.) could be utilized in a mode similar to that done during the last Lunar Orbiter flight (Orbiter V). This involved setting up a C.2 IMAGING AND ALTIMETRU ON LGO grid of targets of opportunity at which contiguous, high- resolution images were taken. A similar strategy on LGO Complete global, high-resolution photographic coverage would yield photographs of many important geological sites, of the Moon from LGO may be outside the scope of NASA's including Mare Smythii under optimum lighting conditions, LGO AND LUNAR SCJENCE the geochemically anomalous Balmer and Herlzspring spacecraft for the LGO mission might be an abundance of basins, and possibly young mare volcanic features in south- propulsion fuel. This might enable a change from the nominal eastem Tranquillitatis and the crater Taurantius. LGO 100 km circular orbit to other configurations once the Concurrent with these imaging data, the topography primary mission was completed. From the geological stand- provided by the Radar Altimeter (ALT) would provide us point, an exciting possibility is to move the LC0 spacecraft with an almost completely new topogaphic data base. This into a very high (- 1000km) orbit. From this vantage point, information would greatly improve upon current methods of the spacecraft could take a series of full disk lunar photo- estimating crater and basin volumes, scarp heights and mare gaphs, from a variety of positions, under a variety of lighting thicknesses. It will also shed light on the nature of the lunar conditions. The imporkance of this series is well illustrated by hemispheric dichotomy by testing the ""Pocellamm basin" the acquisition of limited photography of this type from both hypothesis Whitaker,1981;Wilhelms,19851. Topographic the Lunar Orbiter 1%' and the Apollo spacecraft. These information is extremely impoaant in placing the results of photographs have been widely used to deternine regional XGRS and VlMS data in the context of the depth within the geological relationships and discover old basins whose lunar crust from which obsewed surface materials derive. presence is not evident in lowaltitude photography [e.g., Al- Although aspects of a possible extended mission brLGO Khwarlzmi-King basin; El-Baz , 19731. The unique perspec- are not considered here, one point regarding imaging is wodh tive of getting full disk photographs of the western limb and mentioning. One consequence of using an Observer-class farside would most likely lead to other significant discoveries. Appendix D: A Twin Lens CCD Camera for Improving the Geodetic Contro on the Moon

IS.1 GEODETIC CONTROL ON THE MOON network is the center of mass with the equator defined by the axis of rotation and the zero longitude in the mean Earth direction (in contrast to the principal axes coordinate Geodetic control is used to position surface features on a system). In general, it will not be possible to improve on these map, and is accomplished by computing the coordinates coordinates with data from the Lunar Geoscience Observer (latitude, longitude) of selected control points observed and (LGO). measured on pictures of the surface. On the Moon it is important to have base maps with accurate positional infor- mation for scientific data bases, surface navigation, and landing site selection and cerlification. D.2 A TWIN LENS GCD CAMEM FOR THE At this time, there is not a single, unified control network of LUNAR GEOSCIENCE OBSEWER the Moon. There are many control networks of the nearside based on telescopic pictures. There are three control net- The data rate of the LC8mission will be low by design and works based on the Apollo mapping camera pictures. And will, to a great extent, limit the value of an imaging there is a control network of the farside based on Lunar experiment. The twin lens CGD camera (GSSI) will permit Orbiter pictures. These networks are not tied together, and extending the Apollo control network to cover the entire lunar they even have different origins. Currently, positional errors surface and to obtain vedical pictures of the surface at a vary from a few hundred meters in the region of the Apollo 15, resolution (two pixels) of 36 to 108 m. These vedical pictures 16,17 landing sites to perhaps 15 km in regions of the farside. will be valuable for geological analyses and in extending The polar regions have positional errors of a few kilometers. medium resolution lunar coverage. Star pictures will be taken Work is now ongoing on a method to merge these data sets for calibration. into a unified control network with a common coordinate The twin lens CCD camera is designed with a SO mm focal system. length lens looking vertically and a 100 mm focal length lens The coverage of the Apollo control network coincides with looking forward at a 50" angle. In the focal plane of each lens groundtrack coverage of the Apollo 15, 16 and 17 missions is a 1024x1024, 18p pixel CCD. The camera has an (see Figure C.l). In the region of this control, positional electronic shutter in which only one-half of the CCD is errors can be reduced to a few tens of meters in the areas ofthe exposed and the picture is immediately transferred and stored Apollo 15, 16, 17 landing sites and to a few hundreds of on the other half of the CGD. Both CCDs are shuttered meters on the farside of the Moon. The origin of this control simultaneously and the pictures can be transferred to the tape LC0 AND LUNAR SCIENCE recorder or transmitted to Earth. From an altitude of 100 km, and in most lines there is overlap with the Apollo coverage, the two-pixel resolution of both CCDs is about 72 m (see giving strength to the triangulation. When the camera is Figure D. I ). In order to use the pictures for control computa- shuttered, the two pictures are separated by about 4" of tions, the same area on the surface must be photographed by latitude (at 100 km altitude), so it should take about 50 twin each GCD, thus giving good stereo coverage. frames to take pictures along a longitude line from pole to The sequence is carried out by taking overlapping pairs of pole. If pictures are taken every 15" of longitude, a sparse but pictures along approximate lines of longitude going from pole useful and accurate control net would be computed; this to pole. There is a great deal of overlap in the polar regions would require about 1200 twin frames.

CAMERA o /I ATTI TUDE = 100 KM

1 ./// \ CAMERA A

LUNAR SURFArr 130.8 119.2 108.7 9.2 0 9.2 Dl RECTION OF TRAVEL

FOOT P R INT FOOT P RINT FOR CAMERA B FOR CAMERA A

Figure D.1: GSSI camera geometry for LGO altitude of 100 km (all distances in kilometers). Appendix E: Heat F ow Mapping by Microwave Radiometry

E.1 INTRODUCTION where ax is the microwave absorption coefficient per unit length, and unit surface emissivity is assumed (which in The microwave techniques for measuring heat flow from reality is 0.931-0.01 and independent of wavelength in the orbit basically consist of measuring the emitted blackbody region of interest). Thus, since in the ideal case ax = a,/A, radiation averaged over a lunation for a given site at two or a, a constant, more microwave wavelengths. The heat flow, q, out of the Moon is related to the physical temperature increase with q=ka,dT~- . depth such that q = k dT/dz, where k is the thermal dA conductivity. The microwave flux density or its equivalent blackbody brightness temperature will increase with in- creasing wavelength because the microwave absorption Operationally, the heat flow is obtained from the measured coefficient decreases with increasing wavelength in the lunar spectral gradient times the thermal conductivity and the regolith. As absorption coefficient in a homogeneous amor- microwave absorption scale factor. Although our knowledge phous solid decreases as the reciprocal wavelength, the ofthe lunar microwave absorption coefficient and the regolith effective emitting layer is deeper at longer wavelengths and thermal conductivity is good for the Earth-facing hemisphere, the brightness temperature higher. The microwave experi- the use of these values would yield somewhat biased q values. ment will measure this increase in brightness temperature The microwave brightness measurements will have to be with wavelen@h, which will be exactly linear if the lunar averaged over lunations for each site, since the instantaneous regolith is homogeneous to depths of more than about 10 bri&tness is a function of the local sun angle. At a given wavelengths. In such a homogeneous regolith the thermal latitude the brightness temperature will oscillate with an conductivity would be constant (except for a small radiative amplitude decreasing with wavelength. The amplitude of the term) and the physical temperature due to heat flow from lunation oscillation is a simple function of the ratio of the deep sources would be linear with depth, i.e., thermal skin depth to the microwave skin depth at wavelength A, This ratio is (physical temperature),

where T, is the surface temperature. Under these ideal circumstances, the equivalent microwave blackbody tem- perature as a function of wavelength would be where, TB(X)= Ts + (brightness temperature), kax LGO AND LUNAR SCIENCE

(o,being the lunar angular rotation rate with respect to the temperatures over a lunation for each site, while dominated Sun, p is the regolith density, and c, the specific heat of the by the surface value of I, can be modeled for the subsurface regolith material). R, can be accurately determined for each values of the thermal parmeters. This is possible because the site from a series of measurements over a range of sun angles surface temperatures during sunrise and sunset (and during for the site (see below for an error analysis). The operational the night) "remember" the subsurface temperature structure, expression for q is thus i-e., two surfaces with the same upper centimeter of low density dust cool (and heat up) differently if they overlie sublayers of different densities and thermal conductivities. In spite of the long history of the thermal measurements of the Moon since the 1930'~~no one has systematically modeled small, smooth regions over lunations to demonstrate this effect which is quite obvious theoretically. Langseth et al. 119761 did so model Apollo in situ surface temperature measurements, which clearly demonstrates the effective determination of k at meter depths. A factor of 2 improvement over the accuracy of that of the present value of I should be achieved, and if the hypothesis that the Apollo sites are representative is incorrect, I can be directly "measured" from The collection of parameters (kpcp)s is the thermal inertia, the modeling. I, of the regolith but not that of the surface which, in the Before discussing the achievable accuracy of a microwave other-than-ideal case, is nearly a factor of 10 smaller than experiment, goals other than that of measuring a global that in the region of the regolith where the microwave energy average value should be considered. It is "reasonable" to is liberated. bound the range of local q's for the Moon between 1pW/cm2 The last expression above for q can be used to discuss a and 10 pW/cm2. Molten magma nearer than 100 km to the practical microwave experiment for the Moon. The real surface could create higher values, but they would be easy to situation deviates from the ideal case in a number of ways. measure. Using values of k = 0.8~ W/cm/K and ax = However, the expression for q holds in a nonhomogeneous lllOX (A in cm), this range of heat flow yields brightness regolith if we interpret each of the terms I, R,, and dT,/dX temperature increases from A= 4 cm to A= 25 cm of 2.6 K (q as the mean or effective value averaged over the weighting = 1 pW/cm2) and 26 K (q = 10 pW/cm2). Thus, measure- function for the microwave emission. It is known from both ments of the spectral temperature within t.2.6 K over the Earth-based measurements and the in situ lunar measure- wavelength range 4 to 25 cm would yield local values of q ments'that k, a,, and p are functions of depth in the regolith accurate to il pW/cm2 (all other errors being zero!). and that, for the most part, the variations in k and ax are Microwave measurements of this accuracy are certainly caused by the variation in p. Temperature variations in the achievable in lunar orbit. parameters are well understood and are easily modeled for a The suggested goals are a measurement accuracy of lt1 real experiment. The probe measurements at the Apollo sites pW/crn2 for each 100 km surface element with an intrinsic show that p and k increase rapidly in the upper 5 cm of the bias no larger than 0.4 pWlcm2 (20% of the expected global regolith but, due to compaction, increase very slowly with value of 2 pW/cm2) or 1 K bias in the spectral temperature depth beyond 5 cm. Measurements in the wavelength range 4 change from 4 to 25 cm wavelength. to 25 cm will be essentially unaffected by the upper 5 cm. The mean densities and thermal conductivity found at the two sites are p = 1.5lt0.2,2.0+0.2 p/cm3and k= 0.8-tO.4~lop4 E.2 ACCUMCY AND PWCISION FOR W/cm/K, respectively, averaged over the 2 meters below the A MICROWAVE EXPENMENT 5 cm low-density zone. If we make the reasonable assump- tion that the spread in the results of the two Apollo sites is E.2.1 Limitations Caused by Local Properties representative of that for the entire Moon (save fresh craters, of the Lunar Regolith etc.), the value of the thermal inertia at meter depth would be +25% about a value I = 10.9~10-~W/cm2/K. (By Most of the above discussion deals with an ideal lunar convention, I is usually expressed in calories, i.e., I = regolith, i-e., homogeneous, amorphous materials in the 2.6~10-3 and I-' = 385 at meter depth.) The important point depth range from 5 cm to a few meters. Only then is the is that the thermal conductivity enters the experiment as the increase of TB with A linear. Scattering of the microwave square root, so the present knowledge of this parameter may energy by contrasts in dielec~cconstant (rock, clumps) can be sugcient for a meaningful heat flow experiment. How- impart a spectral shape over the wavelength range of the ever, the experiment can be greatly improved by having an IR instmment. However, dispersive effects due to scattering in thermal radiometer (X 20 to 50 pm) collimated with the . the lunar regolith have not been seen at wavelengths as long as microwave beams. The measurement of the surface IR 2 cm in Earth-based observations. Using surface rock and Heat Flow Mapping clump count statistics from Surveyor and Apollo sites, one time, the random error (noise) in a brightness temperature invariably finds a power law distribution of sizes which measurement would be about zt0.02 K. Random noise is not a strongly homogenizes the scattering effects over the wave- problem. Because of this very low noise for a single channel, length range of interest, i.e., the scattering is nearly non- the lunation variation of TB in a given channel can be dispersive and merely increases the effective absorption accurately measured, yielding accurate RA's. coefficient, an effect which is measured to first order in the The requirement of measuring the spectral change in AT, determination of RAfrom the lunation microwave measure- of k1 K over the range X = 4 to 25 cm is more difficult to ments. Scattering of wavelenghs shorter than 2 or 3 cm obtain. This requires a channel-to-channel calibration ac- wavelength is a worry and wavelengths this short should not curacy bias of less than about 10.7 K. Random fluctuations be used. in calibrations somewhat larger than this are acceptable, but A second important deviation from the ideal regolith is a they must not be biased at levels worse than this. This can be shallow regolith. A channel at X = 25 cm is sensitive to the achieved, in principle, by using temperature-monitored cali- temperature distribution down to about 3 meters. In regions bration loads at temperatures such as 100 K, 200 IS,300 K, where a coherent layer occurs within this depth, the flux and 400 K that have been precisely temperature calibrated density at the long wavelength will be anomalously low, since before launch. radiation emitted below this layer would be partially reflected A second source of instrument spectral bias is the antenna back downward. Earth-based measurements of the mean system. It, too, must be calibrated to k0.7 K for each channel lunar temperature invert and decrease with increasing wave- before launch and must remain stable over the mission time. length beyond about 40-50 cm, almost certainly due to this The required aperture size is determined by the longest effect. Thus, the longest wavelength ofthe instrument must be wavelength and the desired beam spot size. An aperture of 1 limited. If measurements are made at 3 or more wavelengths, meter at A= 25 cm has a beam of -0.25 rad and a surface spot regions with regoliths shallower than 3 meters would exhibit size of -25 km dimeter at an altitude of 100 krn. The beams this effect with an inversion at the longest wavelength. at the shorter wavelengths must be similar. The antenna Interesting information about such regions will be obtained, system could be a frequency multiplexed, phased array with a but is best not used for heat flow determinations. mass under 5 kg. The far-field gain at each wavelength would As long as the regions selected for heat flow analysis are have to be determined before launch to better than 2% and limited to ""sooth" areas with regolith depths greater than 3 remain stable during the mission. or 4 meters, the physical properties of the Moon will not In conclusion, it appears that current technology is sufi- prevent the attainment of the experiment goals. Regions with cient to develop a microwave heat flow instrument which complex topography, shallow regoliths, fresh craters, could measure heat flow to an accuracy of better than zt 1.0 etc., can be easily identified in the microwave and IR ,uW/cm2 for individual sites on a scale of 100 km. These sites data and treated separately. It remains to consider the must have regoliths deeper than 3 meters and subsurface rock bias and noise source intrinsic to the instrumentation. and clump distributions similar to (or cleaner than) the landing sites studied with Suweyor and Apollo. Such regions would need to be moderately flat, such that they would require only minor topographic corrections over the center of E.2.2 Instrument Limitations the site. The instrument must be free of bias to a level equivalent to 0.25 pWlcm2 in order to yield a meaningful, Using 1985 technology, a channel at a wavelength from 4 globally averaged heat flow value. to 25 cm can be built with roughly the following charac- Significant but achievable instrument developments are teristics: mass = 2 kg, power = 5 Watts, system temperature required to produce a broadband antenna system and a 200 K, and bandwidth 500 MHz. In a 1 second integration calibration system to achieve the low bias level.

Appendix F: Lunar Geoscience Observer Mission Description

F,1 INTRODUCTION AND PROJECT be more compatible with a duplicate MO spacecraft and CHARACTERISTICS could be used to provide a project cost estimate. The objectives ofthe FY85 study were to further refinethe This appendix summarizes the results of the JPL FY85 mission design and to determine if an LC0 mission con- study in terms of the science, mission, system aod cost strained to the MO spacecraft capabilities would be scien- requirements consistent with the LGO mission as a near-tern tifically viable and fit within the $64 million per year Planetary Observer. This appendix is a modified version of Planetary Observer cost ceiling. If both of these conditions information found in JPL Document No. 642-901 (15 were met, then LC0 could be considered as a candidate for November 1985), prepared by the JPL Planetary Observer the second Planetary Observer mission. Two science work- Planning Team. shops were held to refine the science objectives, instrument The Lunar Geoscience Observer (LGO) mission is one of characteristics and data requirements, and to solicit the the Planetary Observer series, identified by the Solar System opinion of the scientific community on the viability and value Exploration Committee (SSEG), in which low cost space- of the constrained mission. Earlier sections of the present craft derived from Earth orbiters are utilized to investigate report detail the results of these workshops which, in general, focused, high priority scientific objectives. The Mars Obser- concluded that the mission is viable in the sense that it would ver (MO) mission, to be launched in 1990, is the first in the make significant contributions to the resolution of some of the Planetary Observer series. About two years after the MO fundamental questions in lunar science. The cost constraint is launch, the next Planetary Observer mission would be another matter, however. The program costs may exceed the launched. A prime candidate for the next mission is the LGO. $64 million per year limit if LGO were launched two years The mission would deliver a single spacecraft to the Moon for after MO as planned and if an attempt was made to fulfill all of an extended orbital study of scientific questions about the the scientific objectives identified at the workshops. lunar surface and interior. The LC0 mission will deliver a single spacecraft to lunar The history of LGO may be traced from lunar missions orbit. The spacecraft can be launched at almost any time and studied in the early to mid-1 970's. A more recent study was inserted into a 100 km circular polar orbit (see F.3 below). initiated by JPL and NASA in the early 1980's, which This will be followed by a one-year observation period with resulted in the redefinition of a lunar orbiter mission as data collected on daily repetitive cycles. Except during defined by the SSEC in the context ofthe Planetary Observer maneuvers, the spacecraft remains nadir-oriented and points program. The current study began in early FY 1984 to the instruments to the groundtrack. A11 of the instruments are establish the LGO mission and system requirements af- body-fixed or boom-mounted. Data is recorded on the fecting the Mars Observer WP. The second part of the IFU spacecraft at a single data rate (6000 bps) over a 24-hour 1984 study identified a mission and system design that would period. Once a day the recorded data is played back to the LGO AND LUNAR SCIENCE ground over a ten hour 34 m DS S pass at 3 2 kbps.During the global co~xposit~on,and its internal structure and geophysical station pass, the spacecraft is in view for a minimum of 4.5 conditic~n.Four subareas were addressed: geology, geo- hours (68 minutes per orbit). Data collection would continue chemistrgr, petrology, and geophysics. The major science in a simultaneous record and playback mode. All engineering questions addressed included those related to ( I) the lateral and science data will be assennbled on the ground in a digital variations and average composition of the crust, (2)the deep data base. interior composition, structure and state, especially the The strawman mission discussed in this appendix assumes existence ancl size of a central iron-rich core, and (3) the that the LGO spacecraft is a near-duplicate of the MgB geological history of the surface. The workshop also con- spacecraft, with minimal changes to the operational concept, sidered the viability of an LGO mission in resource evalua- team structure, and software design.'Table F.1 contains a tion and lunar base site selection. high level summary of the characteristics of an &GO project The primary measurements discussed by the workshop as currently planned. Example project chronological events were in relation to determining the (1) surface composition, as well as mission characteristics are included in the table. both elemental and mineraiogical, lateral and vertical crustal variations therein, and whether or not there are deposits of corsdensed volatiles at the poles, (2) surface geology in terms of distribution and stmctural relations of surface rock units, F.2 EGO SCIENCE WORKSHOP SUMMARY (3) gravity and magnetic fields and their spatial variations, (4)induced zemporal variations in global" magnetic fields due F.2.1 Introduction to insteraction with interplanetary and geomagnetic tail fields, and (5)subsurface thermal gadient, its global lateral varia- Previous science working group studies and mission design tions and correlations with surface structure and rock units. studies have been conducted to establish o87jectives Birr an The detailed worksl~opresults are the subject of the presel~t unmanned Lunar Orbiter Missjorn. Tlre Lunar Polar Orbiter report. (LPO) study and mission desipwere carried out in B 976-7'7. The 1983 SSEC report identified LGO as a mission in its core program. The present science rationale for a LGO mission was fomulated by a science workshop group that was convened at JPL in 1985to take a fresh look at the EGO science. F.2.3 Strawman Payload The hilowing objectives were identified by the SSEC and used as a starrting point for the workshop deliberations: The lFatlswing experiments were identified by the SSEC (1) Measurement of the global elemental and mineral- and summarize the expected experiment results for the ogical surface composition. nominal SSEC payload. Additional experiments have been (2) Assessment of global resources, including a search for identified by the SSEC as ""ehancements" if compatible frozen voiatiles at the poles. with the project resources and are also discussed below: (3) Measurement of global Effuse and surface topography. (1) Visual and Infrared Mapping Spectrometer (VIMS): (4) Measurement of lunar pavity and magnetic fields. Global map of surface mineralogy including feldspars, The LGO science workshops convened i3 scientists from pyroxenes, olivines and condensed volatiles. various institutions in 1985to interact with the JPL Planetargi (2) X-ray/Cmma-Ray Spectrometers (XGRS): Surface Observer Planning Team ($OPT). The objectives of the elemental composition including Si, 81. Mg, Fe, Ti, Ca, Na, workshop, complementary to the SSEC study, were to (1) V, Th, U, and K: address questions of existence of frozen make a full review and re-look at the L60concept, and (2) volatiles at the poles, contribute to global resource survey. make recommendations regarding the scientific viability and (3) Radar Altimeter (ALT): Global figure and surface mission design aspects of an kG0 mission. The approach topography mapping, and surface dielectric properrties. taken involved three steps: (I) to determine the major lunar (4) Radio Science: Nearside Doppler trackiisg for gravity science questions today (2) dekminme; what measurements fielid anomalies. can be made from l~rnarorbit that can address the major (5) Magnetometer (MAG) and Electron ReRectometer qluestions, and (3) to derive the subset of measuremerlts and (ER) (enhancement): Map local magnetic fields and surface an instmment payload for an Obsewer-class lunar mission. anomalies and their relationship to geologic stmcture: Qe- termination of iron-rich core. (6) Imaging System (SSI) (khancement): Support the understanding of lunar geologic processes and the histotqr of F.2.2 Preliminary results of workshop lunar evolution. In addition, support intewretatton of VIM$ and XGRS data. The major lunar science issues addressed centered around In addition to the SSEC recommended experiments delemining the origin and evolutio~aaryhistosy of the Moon above, the 8985 LGO science workshop considered the by examining its present state in terms of its surface and following instn~mentsas candidates for an LGO payload: Mission Description

TABLE F. l Example LC0 Project Characteristics Mr"ssrcrt2: Lunar Geoscience Observer (LGO) T~Y~PL:Moon Glass~~caticzn:Planetary Observer Major Project mile sf ones: Project start FY '88 System test FY '92 Launch Dec. '92 Arrival--LO1 Dec. '92 End of mission Dee. '93 Cruise duration 115 hrs. Encounter duration I year Mission duration 1 year Major Mksion Characteristics: Launch Enera (h2/s2) -2.1 to -1.9 Orbit insertion delta V (mls) 880 All other delta V Cm/s) 2-70 S/C-Sun range. AU, (min/max) r .s/r .0 SIC-Earth range, AU, (minimax) .csoz/.0o2 Orbit type Circular Orbit altitude (km) 100 Sun-synchronous'? No Orbit inclination (degrees) 96.4 Orbit period (minutes) an8 Operations-Characteristies: Record sates (kbps) 1.5, 3, 6, 12, (nominal 6) Playback rates (kbps) 8, 4 6, 32, (nominal 32) Real time data rates (kbps) 1.5, 3, 6, 12, 3% Command loadslweek 2 Tracking dish diameter (m) 34 Tracking passes/day I

(7) Microwave Radiomekr (MIWm)(candidate): surface selection. The flight time to the Moon is 115 hours for a heat flow. minimum energy transfer. (8) Spacecraft Gravity System (SGS) (candidate): Pdrside Upon arrival at the Moon, the spacecraft is inserled into gravity; higher resolution gravity. orbit with one burn at closest approach. The initial orbit is (9) Thermal Infrared Mapping Spwtrorneter (TIMS) nearly polar and nearly circular with an apolune of 109 km (candidate): s~arfacerock type and textural mapping. and perilune of 91 krn altitude. In this study an initial node is The cost implications of these additional experiments were selected to give a lighting angle of 5Tas measured from the not identified when these experiments were introduced at the Sun to the Moon to the spacecraft at the equator. For stability workshop. the orbit is inclined 96.4" from the equator which allows The? workshop did not attempt to prioritize the ex- several. months before perilune reaches the surface if no orbit periments (5) through (9). corrections are made (with the current gravity model). An exactly polar orbit with an ilrrclination of 90", is desired by most experiments but is only stable for a few weeks. This orbit F.3 LGO MISSION DESIGN could be accommodated in several ways. The spacecraft could be initially inserked into a polar orbit then inclined to The spacecraft will be launched by the Space Shuttle and 96.4" after a few weeks; it could be inserled at 96.4" and made a commercial upper stage such as the Transfer Orbit Stage. polar near the end of the mission; or, it could be inserted polar Considering only injection energy, the launch can occur on and remain polar despite the hi&er fuel requirements to my day of any month. Initid Iibting mgles and timing maintain circulaity. In the cases where a plane change is relative to lunar eclipses may govern the final launch date required the velocity increment is f 80 meters per second. LGO AND LUNAR SClEiVCt;:

Table F.2 summarizes the initial orbital elements and F.4 SPACECRAFT GENERAL CHARACTERISTICS other related values. The Planetary Observer program utilizes lower cost spacecrafi derived fri-c~rnEarth orbiters to investigate focused, high priority scientific objectives. A particular science in- TABLE F.2 LGO Orbital Elements and Characteristics strument is included in the payload depending on the spacecraf's ability to accommodate the instrument's re- Semimajor axis 1838 krn quirements for mass, power, pointing, data rate and other Eccentricity .005 limited spacecraft resources. Various alternative scenarios Inclination 96.4" exist for the EGO mission. For this study, it has been1 Node w.r.t. Sun line 57" assumed that &GOwill inherit the Mars Obsewer spacecraft Argument of periapsis 147" design and upper stage design. LG0 faces limitations in Mean anomaly TBD mass, power, data rate, configuration and other parameters Orbit period 117.89 minutes imposed by the Mars Observer hardware in addition to the Orbit speed 1.6 km/s basic cost constraint imposed by the low cost Planetary Initial perilune altitude 91 km Observer philosophy. While the exact nature of the hardware Equatorial groundtrack spacing 32.5 i 1.0 km limitations will only be known after the Mars Obsener Sidereal rotation period 27.322 days spacecraft contractor is selected in early 1986,a preliminaq Node precession rate 0.12"Iday understanding of the spacecrafi imposed constraints, impor- tant to early recopition of mission critical areas, was Orbital repeat period 27.57 days established by the 1985 study. Appendix G: LGO Instrument Descriptions, Requirements, and Characteristics

This appendix is modified from JPL Document No. 642- will be no scan platform on the spacecraft and those 901 (15 November 1985), prepared by the JPL Planetary instruments requiring pointing will be hard mounted and Observer Planning Team. The instruments described below boresighted in the nadir direction. Thus, any pointing away will reside on a nadir pointed spacecraft and will be either from the nadir must be implemented within the instrument. body-fixed (ALT, VIMS, SSI, MRAD, ER, GSSI, SGS and Tables G. 1through G. 11 describe instruments typical of the TIMS) or boom-mounted (XRS, GRS and MAG). There type expected to be flown on LGO. LC0 AND LUNAR SCIENCE

TABLE C,1 Visual and Infrared Mapping Spectrometer (VIMS) Characteristics

Heritage Galileo (NIMS), MO (VIIMS) Mass (kg) 23 (Includes Eleetronies and Radiator) Power (W) 12 average, 13 peak Size (crn) Optics: 83~37x39;Electronics: 20x25~13 Mounting Bus Axis orientation(s) Optic: Nadir; Cooler: Space Field of View Optic: 120 mr x 1 mr; Cooler: 150" Exclusion Angle (View) 30" Cooling Passive Radiator (120 K Sensor) SIC Environmental Restrictions Sensitive to gas and parliculate contarninants on optics and thermal control surfaces (instrument is a 13QK cold trap) Environmental Output None Mechanisms, etc. Optic drives, covers (2), purge heaters Calibrations Required Periodic view of two reference targets, one reflective, one active thermal Modes Several internal Duty Cycle Collect data on daylight side of body only, until full surface mapped Data [rate(s) (kbps)] 1.5, 3, 6, 12 commmdable, internal buffer Data Volume (bitslday) up to 0.5 x 109 Pointing Accuracy (kmrad) 10 Pointing Knowledge (tmrad) 5 Pointing Stability (mrad7sec) 10 Typical Proponent(s) Instrument Development Science Team (VIM$ Facility Team) Instrument Requirements

TABLE G.2 Gamma Ray Spectrometer (GRS) Characteristics

Heritage Apollo 15, 16 plus developments Mass (kg) 15 (Includes Electronics and Cooler) Power (W) 10 Size (cm) 29x32~50 Mounting Full instmment on boom, >1 S/C diameter. Prefer extendable rather than articulated Axis Orientation(s) Detector: Nadir; Cooler: Space Field of View Detector: 180" (2~ ster); Cooler: 150" Exclusion Angle (View) 180" Cooling Passive radiator 100-1 10 K SIC Environmental Restrictions No radioisotopes (e.g., RTGs, thoriated alloys, potassium paints, etc.). No strong magnetic field (> 1 G) Environmental Output Some weak magnetic field from photomultiplier tubes Mechanisms, etc. Cooler shield Calibrations Required 1. At >10 planetary radii 2. Before, mid, and after boom extension 3. During cruise Modes Single (on-off) Duty Cycle Continuous, day and night, full mission time Data [rate(s>(kbps)] 1.5 Data Volume (bitslday) 1.3 x 108 Pointing Accuracy (tmrad) 50 Pointing Knowledge (tmrad) 50 Pointing Stability (mradlmin) - Typical Proponent(s) A. Metzger, JPL; J. Arnold, UC San Diego LGO AND LUNAR SCIENCE

TABLE 6.3 Radar Altimeter (ALT) Characteristics

Heritage Pioneer Venus (BRAD),MO Mass (kg) 17 (Includes Antenna & Electronics) Power (W) 28 Size (cm) Antenea: 1100 (Dia. of Dish); Electronics: 120x60~10 Mounting Bus Axis Orientation(s) Antenna: Nqdir Field of View 0.5" Beamwidth Exclusion Angle (View) 30" Cooling Passive thermal control S/C Environmental Restrictions None Environmental Output None other than radar beam Mechanisms, etc. No moving parts (except possible deployment of antenna) Calibrations Required Internal, automatic, or on command Modes Several antenna Duty Cycle Continuous until full surface mapped Data [rate(s) (kbps)] 1.0 (maximum) Data Volume (bits/day) 7 x 107 Pointing Accuracy (kmrad) 4 Pointing Knowledge (kmrad) 2 Pointing Stability (mradlsec) 1 Typical Proponents(s) R. Phillips, SMU; C. Elachi, JPL; D Smith, GSFC TABLE G.4 Magnetometer (MAG) Characteristics

Heritage Voyager, Pioneers, ISPM, Galileo Mass (kg) Sensor(s) 1; Electronics 2 Power (W) 3 Size (cm) Sensor: 8x5~5;Electronics: 22x 11x 15 Mounting Sensor(s) on boom 3 SIC diameter, deployed during cruise. Electronics on bus Axis Brientation(s) Orthogonal sensors Field of View NA Exclusion Angle (View) NA Cooling None SIC Environmental Restrictions No SIC magnetic fields 0.01 nT at sensor location Environmental Output Radiates weak sweep magnetic fields (100 nT) in sensor coil haechanisms, etc. No moving parts Calibrations Required Internal, on command Modes Several internal Duty Cycle Continuous Data [rate(s) (kbps)] 0.4 Data Volume (bitslday) 3.2 x 107 Pointing Accuracy (+mad) Pointing Knowledge (rtmrad) Pointing Stability (mradlmin) Typical Proponent(s) L. Hood, University of Arizona; C. Russell, UCLA LGO AND I,[JNAR SCIENCE

TABLE 6.5 Electron Reflectometer (ER) Characteristics

Heritage Apollo 15 and 16 subsatellite charged particle experiment Mass (kg) 5 Power (W) 5 Size (cm) 20x 20x 20 Mounting Boom preferred. Bus OK but away from non-conducting SIC surfaces Axis Orientation(s) Nadirlzenith plane1 Field of View 5" fan x 360" revolution in nadirlzenith plane Exclusion Angle (View) Same as field of view Cooling None SIC Environmental Restrictions 1. Must be flown with magnetometer 2. SIC mapetic fields to satisfy magnetometer 3. Sensitive to SIC electrostatic charging Environmental Output None outside instrument package Mechanisms, etc. No moving parts Calibrations Required Internal Modes Several internal Duty Cycle Continuous Data [rate(s) (kbps)] 0.3 Data Volume (bitslday) 2.6 x 107 Pointing Accuracy (kmrad) 30 Pointing Knowledge (kmrad) 30 Pointing Stability (mradlmin) 100 Typical Proponent(s) K. Anderson, UC Berkeley; Pa. Lin, UC Berkeley

- 'Understood to be parallel (or roughly parallel) to orbit plane. Instrument Requirements

TABLE 6.6 X-Ray Spectrometer (XRS) Characteristics

Heritage Apollo 15, 16 plus developments Mass (kg) 11 Power (W) 10 Size (cm) 20x20~40 Mounting Body Axis Orientation(s) Detector: Nadir; Cooler (if used): Space; Solar Monitor: Anti-Nadir Field of View Detector: 20"; Solar Monitor: 20"; Cooler (if used): 150" Exclusion Angle (View) 20" Cooling TBD 1150 K if Si(Li) Detector used] SIC Environmental Restrictions No solar radiations from S/C components Environmental Output None Mechanisms Cooler Shield Calibrations Required Internal; Reference detector solar illuminated Modes Single (on-off); possible solar burst and flare mode Duty Cycle Daylight side only until global coverage Data Rate(s) (kbps) 0.3 Data Volume (bitslday) 3 x lo7 Pointing Accuracy (~mrad) 9 Pointing Knowledge (kmrad) 9 Pointing Stability (mrad/min) 100 Typical Proponent(s) J. Trombka, GSFC; J. Arnold, UCSD LGO AND LWAR SCIENCE

TABLE 6.7 Geodesic Imager (GSSI) Characteristics

Heritage Various Planetary Imaging Systems, SOT Heads Mass (kg) I1 Power (W) 10 (camera), 5 (heater) Size (em) Camera: 20x20~12; Electronics: 25x 18x5 Mounting Bus Axis Orientation(s) Nadir and 50" forward of nadir Field of View (degrees) Nadir: 42x2 1; Fonvard: 35x 1 1 Exclusion Angle (degrees) 45 Cooling 173 K Passive SIC Environmental Restrictions None Environmental Output None Mechanisms, etc. No moving parts Calibrations Required Star Pictures, temporary SIC Modes Several internal, on command Duty Cycle 1200 frames during I year Data Rate (kbps) 5 Data Volume (bits/day) 8 x 106bits per frame Pointing Accuracy (+mrad) 30 Pointing Knowledge (kmrad) 30 Pointing Stability (mradlmin) 100 Typical Proponent(s) M. Davies, Rand Corporation; J. Head, Brown University Inslrument Requirements

TABLE 6.8 Microwave Radiometer (MRAD) Characteristics

Heritage LPO design Mass (kg) BO (including antenna) Power (W) 10 Size (em) Antenna: 11 00x60~20;Electronics: 1Ox 30x40 Mounting Bus Axis Orientation Nadir Field of View 30" Exclusion Angle 30" Cooling None SIC Environmental Restrictions None Environmental Output None Mechanisms, etc. None Calibrations Required Internal Reference Modes Internal, Automatic Selection Duty Cycle Minimum 3 data points per lunation; Midnight and noon data for each point Data Wate(s) (kbps) Data Volume (bitslday) Pointing Accuracy (cmrad) 17 Pointing Knowledge (+mad) ? Pointing Stability (mradlmin) ? Typical Proponent(s) D. Muhleman, CIT LC0 AND LUNAR SCIENCE

TABLE 6.9 Thermal Infrared Mapping Spectrometer (TIMS) Characteristics

Heritage Aircraft Scanner (TIMS) Mass (kg) 16 Power (W) 12 Size (cm) 60x50~30 Mounting Bus Axis Orientation Nadir Field of View 30" Exclusion Angle 40" Cooling 100 K Passive SIC Environmental Restrictions Gas and dust contaminants on optics Environmental Output None Mechanisms Optic drives, covers, purge heaters Calibrations Black body reference source Modes Several internal Duty Cycle Collect data on dayside, nightside, and during eclipse Data Rate (kbps) 10 Data Volume (bitslday) ? Pointing Accuracy (+mrad) TBD Pointing Knowledge (kmrad) TBD Pointing Stability (+mrad) TBD Typical Proponent(s) D. Pieri, JPL; A. Kahle, JPI, Instrument Requirernerlts

TABLE 6.10 High Resolution Solid State Imager (HRSSI) Characteristics Heritage CaIileo, SOT, CWF Mass (kg) 15 Power (W) 20 Size (cm) TBD Mounting Bus Axis of Orientation Optics: Nadir; Cooler: Space Field of View 4" Detector CCD, 18 mlpixel, 1024x 1024 Hybrid Optics Refractor, light-weight, 250-mm FL Drive Electronics Hardwire Signal Chain 8 bits per pixel-no compressor Exclusion Angle TBD Cooling Passive to 250 K S/C Environmental Restrictions TBD Environmental Output TBD Mechanisms Shutter: focal plane, 6-ms blade: 6-8 position filter wheel Calibrations Required ? Modes ? Duty Cycle ? Data Rates (kbps) 10 Data Storage f Frame RAM Data Volume (bitslday) TBD Pointing Accuracy (tmrad) Control: 7; Knowledge: 7 Pointing Stability (stmradlsec) 0.1 Typical Proponent(s) Instrument Development Science Team (Imaging Facility Team) LGO AND LUNAR SCIENCE

TABLE G.11 Spacecraft Gravity System (SGS) Characteristics

Heritage Proposed LPO design Mass Antenna + Switch Electronics (in addition to shared altimeter electronics) Corner Reflectors

Power (W) Size Antenna (61 ern dia. x 13 cm) 37992 cm3 Electronics (30x 30x 12 cm) 10800 Comer ReRectors (40x30~20cm) 24000 72792 em3 Mounting and Orientation The SGS antenna horn is mounted behind the altimeter antenna and views along the spacecraft velocity vector. The comer reflec- tors are released along the velocity vector and the spacecrafi ma- neuvers away from them. Field of View There should be approximately a 5"-10" field of view from the SGS antenna and the corner reflector. Detector The Doppler shift between the transmitted and reflected frequency is counted in the SGS and stored for a later telemetry dump. Cooling No special temperature controls are needed. Environmental Output Radio frequency at 8 GHz. Mechanisms Release of 3 corner reflectors at 3 different times. Calibrations Required Oscillator stability. Modes Continuous operation for 14 days, then again for 14 days a month or so later etc. Duty Cycles (When operating in the 14 days mode) Doppler count every 5 seconds continuously over 1 orbit then skip 2orbits, then repeat. Data Rates (kbps) 0. 1 Data Storage (kb/orbit) 100 Data Volume (kb/day) 1600 Pointing Accuracy 1" or -10 mrad Pointing Stability Doppler unaEected to 0.1 rn~n/sec Typical Proponent(s) W. Sjogren, JPk; R. Phillips, %MU Appendix H: References

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