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. Material in this document may be copied without restraint for library, abstract service, educational or personal research purposes. Republication of any portion should be accompanied by the appropriate acknowledgment of this document. For further information, contact: 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