sign in sign up
<<
Home , Galileo (spacecraft), Gordon Pettengill, Harold Masursky, Magellan (spacecraft), Pioneer Venus Orbiter, Planetary series

rEESti42 INTEBESZLC IN ?HE EXEFCLAXION CE , NC. 8, 24 EAhCE 1EE6 Eagfllan update (Jet EroFulsicn Lab.) 14 F CSCL 0313 unc 1d s G3/91 440QO t (Formerly VRM) Update according to the previously agreed to Warren W. James schedule, but has taken action to start the procurement of the long-lead items needed Challenger Accident Impact on Magellan to replace this hardware so that will not be left without its spares. The launch accident which destroyed the Challenger on January 28, A New Name for VRM 1986 is having major adverse impacts on all of the payloads that were scheduled to fly Following a two year on the Space Shuttle during this decade, decision-making process at NASA including Magellan (which was formerly Headquarters, a new name has been selected called Venus Mapper). Launch delays for the Venus Radar Mapper (VRM) project. for the projects that were to precede us, The new name is Magellan and we will use i and a reduced number of Space Shuttles, MGN as its abbreviation. This name is .-.a*, WIII itsuii in increasea competition for consistent with NASA’s general plan of Shuttle launch services. naming major planetary missions after We do not presently know if we will famous historical persons; such ns be able to obtain a place on the Space scientists, astronomers, and explorers. i Shuttle manifest for our planned launch Magellan was deemed an appropriate name date in 1988. This is a question that for VRM because it connotes exploration, cannot be answered until the cause of the discovery and circumnavigation. Challenger accident has been determined Concurrent with the announcement and corrective solutions found to prevent of the new name, the project is selecting a this kind of accident from happening again. logo. Various designs for this have been Once that has been accomplished, NASA submitted and a panel is evaluating them. will revise its Shuttle payload manifest and The selection will be announced soon. we will know if it is possible to launch Magellan on our desired launch date. A Magellan Progress preliminary revision to the Space Shuttle manifest shows Magellan slipping to the The Magellan Project has been going 1989 launch , but the Magellan through a series of reviews as it progresses project continues to work to its present toward its 1988 launch date. The Final schedule. Mission Design Review was held on A second impact from the Challenger February 19, 1986 and no major problems accident involves the use of residual Galileo were uncovered, according to John hardware by the Magellan project. This Gerpheide, the Project Manager. The problem has arisen because the launch of Multi-mission Image Processing Laboratory Galileo has now slipped from May, 1986. to (MIPL) Baseline I1 Critical Design Review a later date. Magellan needs the use of (CDR) was also recently held and this this hardware now. The Magellan project system was judged to be in good shape for expects to receive the residual hardware handling our needs. Martin-Marietta has CW POOR QUALlrv

V-gram successfully completed the CDRs on all of scientist's research activities so that, in its flight sub-systems, with the exception of sum, they answer the major questions about one. Everything is looking good for the the geology, geophysics, and spacecraft. geological history of Venus, as described i However, the Radar Sensor and Alta below. reviews have revealed some design problems The Magellan RADIG was formed out which have been given special attention. of the Synthetic Aperture Radar (SAR) and i This has resulted in a tight schedule for Altimetry Investigation Groups of the building the Flight Model of the radar. An progenitor Venus Orbiting Imaging Radar important interface test for the radar and (VOIR) Mission, spacecraft is to start about May. scientists and The CDRs for the Missfon Omations System (MOS) and Radaf Data Pt&g 'i System are both planned for the second quarter of 1986 (although the MOS CDR has reconfirmed for Magellan. Of these, five I now slipped a month from its original are foreign residents and are supported by I schedule). The MOS flight operations phase their own governments. A list of the organization has been formed. A "tiger RADIG members and their affiliations is team" for evaluating the telemetry given in Table 1. Because of the large size processing subsystem requirements has of the RADIG membership, an executive defined both the high-rate and low-rate subset (identified in Table 1) has been requirements. chosen to represent it in the Project Significant progress has been made in Science Group (PSG), which meets defining and approving the data products quarterly during the years prior to the needed by the scientific investigators. launch of the spacecraft in April, 1988. To accomplish its assignment, the RADIG is organized around seven task THE MAGELLAN RADAR groups, each responsible for a general task INVESTIGATION GROUP as shown in Figure 1. The task groups themselves cluster into two categories, depending on whether they are primarily Overview concerned with some aspect of Mission design, or with the scientific interpretation The Magellan (MGN) Radar of the data when received. We will be Investigation Group (RADIG) has the major hearing more about the plans of these task responsibility for the radar science of the groups, from their leaders, in future issues Magellan Mission. To do its job, the RADIG of the V-gram. must participate in the choice of parameters Most of the MGN data processing will for the radar system, in the design of be carried out at JPL, using the calibration and test procedures, in planning Multi-mission SAR Processing Laboratory the in-flight operations that relate to the (MSPL) and the Multi-mission Image radar system and, finally, in laying out the Processing Laboratory (MIPL). Some 1 processing and presentation of the data. specialized data processing will be done at The RADIG must make sure that all aspects other institutions, however, and we will of the Magellan mission are capable of hear more about these activities in future 1 producing results that satisfy the scientific V-Grams. I objectives of the Project. One of its most In order to compare the results from important tasks, of course, is to interpret the various experiments, it is necessary to the data resulting from the mission and to establish an accurate geodetic control publish the results of its studies. network for Venus, and to present the The RADIG is a collection of various types of radar observations in a individual scientists, each having his own cartographic style that is useful for perspective and field of expertise. The interpretation. The detailed specifications RADIG will attempt to coordinate each for the imaged and mapped products are ORIGINAL PAM 1s OF #K)R QUALm V-Gram Page 3 just now being finalized. The principal temperature and emissivity. data product that will receive wide In addition to being presented as distribution is a set of 62 maps in the standard cartographic maps and standard 1:5M U.S. Geological Survey photomosaics, the radar data from Magellan (USGS) planetary series. These maps will will also be made available in annotated display the SAR data at approximately 1-km digital form, thus preserving its full spatial resolution, and will contain altitude resolution and dynamic range. Careful contours. Beyond these, a set of about 200 documentation and archiving of the photomosaics will show the entire mapped accumulated results from MGN form an area of the planet at 225m resolution, while important part of data management for this about 250 photomosaics using the highest mission. Not only must the essential resolution SAR data (on the order of 100m) results be protected against deterioration will be produced for selected parts of the over time, but the relevant engineering planet. These maps and photomosaics will information needed to understand how these be used as the basemaps when presenting data were obtained must be stored so that data from the other experiments. future researchers can adequately interpret Complementary data products will include a these data. Also, the system used to store topographic map at about 10-km surface these data must provide easy access for resolution with a height accuracy of better users in the general planetary science than 50 m, and special products displaying community over a period of many years. surface roughness, reflectivity, brightness

Table 1. RADIG Members

Team Member Affiliation

Raymond E. Arvidson (PSG) Washington University Victor R. Baker University of Arizona Joseph H. Binsack Massachusetts Institute of Technology Donald B. Campbell National Astronomy and Center (NAIC), Cornel1 University Merton E. Davies (PSG) The Rand Corporation Charles Elachi (PSG) Jet Propulsion Laboratory John E. Guest University of London James W. Head, I11 (PSG) Brown University William M. Kaula University of California, Los Angeles Kurt L. Lambeck Australian National University Franz W. Leberl Independent Consultant Harold C. MacDonald University of Arkansas (PSG) U.S. Geological Survey, Flagstaff Daniel P. McKenzie Cambridge University Barry E. Parsons Oxford University Gordon H. Pettengill (PSG) Massachusetts Ins titu te of Technology Roger J. Phillips (PSG) Southern Methodist University R. Keith Raney (PSG) Canada Centre for R. Stephen Saunders (PSG) Jet Propulsion Laboratory Gerald G. Schaber U.S. Geological Survey, Flagstaff Gerald S. Schubert University of California, Los Angeles Laurence A. Soderblom (PSG) U.S. Geological Survey, Flagstaff Sean C. Solomon (PSG) Massachusetts Institute of Technology H. Ray Stanley NASA, Wallops Island Manik Talwani Gulf Research and Development Co. G. Leonard Tyler (PSG) Stanford University John A. Wood Smithsonian Astrophysical Observatory It . Page 4 V-gram I

U ORIGINAL PAQE IS V-Gram OF POOR,QUALcrV Page 5

RADIG Science Investigations the RADIG will develop hypotheses to explain the observed correlations, as well as The scientific tasks planned for the methods for testing them. MGN Radar Investigation Group (RADIG) All of the data from the above are summarized in the following paragraphs. investigations will be synthesized and used (For more background on the scientific in an attempt to infer the likely geological questions behind the Magellan mission and geophysical evolution of Venus as a please refer to the article entitled, "VRM planet. This synthesis will be used to Science Background," which was pu b 1ish ed compare the evolution of Venus to the in V-gram Number 6, July 1985.) other planets of the so that Data from the SAR, altimeter and we may better understand the history of radiometer experiments will be used to the solar system and the physical processes identify the major surface units on Venus occurring on the planets over geologic time. and to describe, as quantitatively as The following paragraphs discuss some possible, their radar scattering behavior, of the specific science questions that will radiothermal emissivity and altitude. This be addressed by the activities of the description will include not only the mean RADIG. values of the observed properties, but also their variations within a given surface unit. Volcanic and Tectonic Processes The members of the RADIG will identify the types of geological processes -based and 15/ 16 radar that produced the observed surface units on images of the surface of Venus have Venus. This characterization will be done demonstrated widespread evidence for based upon experience with the Earth and volcanic activity. A goal of the RADIG is the other planets, as well as by to provide a detailed global characterization comparisons with laboratory experiments of volcanic landforms on Venus and an and computer simulations. This will involve understanding of the mechanics of doing a geological mapping of a limited volcanism in the Venus context. Of number of areas and a comparison of particular interest is the role of volcanism geological inferences with measurements of in transporting heat through the lithosphere vstiniJr ~(r~pe-ti~g~f the s~rfc;zt~f x/caca. Uc 'v'enus. wniie mucn of this goal will be The geologic evolution of Venus will accomplished by a careful analysis of be studied by formulating hypotheses images of volcanic features and of the concerning the evolution of geological geological relationships of these features to features in selected study areas, and then tectonic and impact structures, an essential testing these hypotheses for consistency aspect of characterization will be an with the available data concerning the integration of image data with altimetry surface of Venus. These studies will be and other measurements of surface based upon traditional methods of properties. For instance, since volcanism is stratigraphy using superposition relations highly dependent on lithostatic pressure, it amongst the surface features to deduce should be possible to determine if variations their relative ages. in load resulting from differences in relief The members of the RADIG will have led to differing styles of volcanism. identify any correlations between One possibility is that flood volcanism inhomogeneities in the gravitational field of occurs only in lowland regions, and that Venus and the long-wavelength component shield volcanism is found only in higher of its topography using data from the SAR areas, with pyroclastic deposits violating and altimeter experiments, which will this rule. Such ideas will be tested using identify specific features on the surface of altimetry data combined with identification Venus, and the gravity experiment, which of volcanic constructs and flows from the will provide a measurement of the SAR images. Measurements of relief and its gravitational field of Venus. This effort will associated volume (the areal sum of relief), be undertaken jointly with the Gravity when combined with knowledge of the local Investigation Group (GRAVIG), with whom geopotential field, may reveal the i geomorphological aspect of magma dynamics. Erosional, Chemical and Depositional Measurements of longitudinal and transverse Processes slope, flow margin relief, and flow surface relief will also provide powerful constraints The nature of erosional and on mechanisms and rheological and material depositional processes on Venus is poorly properties of lava flows, as embodied in known, primarily because the diagnostic viscous fluid flow models. landforms typically occur at a scale too A parallel goal is the global small to have been resolved in Earth-based characterization of tectonic features on or and 16 radar images. MGN Venus and an appreciation of the tectonic images will be carefully examined for evolution of the planet. This goal evidence of such processes, including addresses issues on several scales. On the wind-eroded terrains, landforms produced by scale of individual tectonic features we are deposition (e.g., dune ), landslides and t interested in the mechanical nature of the other downslope movements, as well as faulting process, the documentation of aeolian features such as radar bright or geometry and sense of fault slip, and the dark streaks "downwind" from prominent relationship between mechanical and thermal topographic anomalies. One measure of properties of the lithosphere. On a weathering, erosion and deposition is somewhat broader scale, we are interested provided by the extent to which soil (e.g., in linking groups of features to specific porous material as implied by its relatively processes (e.g., uplift, orogeny, gravity low value of bulk dielectric constant) sliding, flexure, compression or extension of covers the surface. The existence of such the lithosphere) and in testing quantitative material, and its dependence on elevation models for these processes against SAR and geological setting, provides important images and supporting topographic, into the interactions that have gravitational and surface compositional data. taken place between the atmosphere and On a global scale, we are interested in the lithosphere. whether spatially coherent, large-scale The ratio of to hydrogen in patterns in tectonic behavior are the current is discernible, patterns that might be related consistent with the hypothesis that Venus to an organized system of plates or to had relatively large amounts of water at mantle convective flow. some time in its geologic past. Because of this possibility, the SAR images from Impact Processes Magellan will be searched for evidence of past episodes of fluvial activity (e.g., The final physical form of an impact drainage systems) and for lakebeds and crater has meaning only when the effects coastal signatures (e.g., strandlines and of the cratering event and any subsequent ancient river deltas). deformational modification of the crater can The existence of a thick and cloudy be distinguished. To this end, a careful atmosphere precludes , visual, search of the SAR images will attempt to , x-ray or -ray observation locate and document both relatively pristine of the surface of Venus from orbit. Thus, and degraded impact craters, together with it is impossible to obtain information on a their ejecta deposits, in each size range, as global basis about the surface composition well as to distinguish impact craters from or mineralogy using standard remote-sensing those of volcanic origin. The topographic techniques. Measurements of the surface measures of depth to diameter ratio, composition at the Venera sites is stratigraphic relationships (e.g., a graben possible, but this provides only highly that cuts through an older volcano), localized information that is of limited geomorphological evidence of degradation utility when investigating global and counts of crater number-size density all compositional trends. provide information through which the Radar and observations of Venus relative temporal evolution of large areas of from the Earth, and from the Pioneer the surface of Venus can be reconstructed. Venus and Venera 15/16 orbiters, have ORffilNAL PAQE IS Page V-gram OF POOR QUALfW 7 disclosed that very often the tops of topography alone. Spectrum here means the elevated regions possess both anomalously spatial wavelength dependence, Le., the high normal-incidence radar reflectivity (as spherical harmonic degree and order on a high as 0.44) and anomalously low radio global basis and Fourier wavenumber (or emissivity (as low as 0.56). harmonic), on a local or regional basis. In the absence of liquid water, which The simplest theoretical (model) admittance is known from a variety of evidence not to functions are those associated with isostatic be present today on Venus, it is necessary compensation. True mechanical models to assume an unusual (in terrestrial involve flexure and dynamic compensation experience) surf ace composition to explain against buoyancy flows (Le., convection) in the large values of dielectric constant the interior. implied by these observations. The most Modeling of the interior density using acceptable of the current hypotheses gravity data is, of course, non-unique. requires significant amounts of electrically Meaningful interpretation rests on conducting materials in the surface. If integrating other data sets and/or these are iron sulfides, as some chemical incorporating specific mechanical models of inference suggests, they may possibly result the interior. For example, a single density from volcanic activity. The good spatial structure underlying the known topography resolution of the MGN instrumentation, both cannot be uniquely identified; rather, a in determining the surface reflectivity from multitude of structures can all be modeled. the altimetric observations and in measuring Each of these structures could potentially the emissivity from radiometric correctly produce potential fields matching observations, promises to outline the the one observed by the gravity experiment. structure of these regions in far greater The interface can be at any depth; the detail than is now available and may shed greater the depth, the larger the density light on their origin. These results will be contrast. Thus, professional scientific applied to testing hypotheses for regional judgement and correlative data from other and global buffering of atmospheric experiments will be needed to eliminate the composition by reactions with crustal implausible models for density striirtiirty materials. The thickness of the elastic lithosphere of Venus, Le., that outer region Iscstatic ani! Concective Processes ol' the pianet which behaves eiasricaiiy over geologically long periods of time, is of Topography and gravity are intimately special interest. The base of this zone is and inextricably related, and must be likely to be defined by a specific isotherm jointly examined when undertaking whose location depends on the particular geophysical investigations of the interior of temperature-dependent flow or creep a planet, where isostatic and convective properties of the material underneath. If processes dominate. Topography provides a this isotherm can be mapped in space and surface boundary condition for modeling the time, then models for the thermal evolution interior density of Venus. of the planet can be developed. The key In the simplest approach, the to determining lithospheric thickness gravitational effect of topography is variations in space and time is through removed from the observed gravity (loosely, flexure studies. If a mass load (e.g., a the free-air gravity), with the resultant shield volcano or a mascon) is placed on Bouguer gravity anomaly becoming a the planetary surface, then the elastic function of the interior density distribution lithosphere will flex under the load. The only (assuming a valid density is used in controlling parameter is the flexural rigidity removing the effects related to topography). which is dependent on the elastic constants A second approach to interrelating and lithospheric thickness. gravity and topography involves the Crucial to applying estimates of dimensionless admittance function, which is flexural rigidity to the task of unraveling the ratio of the spectrum of the free-air the thermal history is an estimate of when gravity to that resulting from the the load was emplaced. Thus, age I Page 8 V-gram 1 determinations derived by various geologic from the users’ point of view and will techniques are essential to this scheme, attempt to make sure that the calibration despite the known inadequacies of current and performance of the radar meet the !I dating procedures. needs of the scientists. The SAR Data Processing Task Group RADIG Team Organization (L. Soderblom, chairman) is charged specifically with monitoring the design of I A synopsis of the areas of the algorithms and data flow involved in responsibility assigned to each of the seven the Multimission SAR Processing Facility’s Task Groups is given below. handling of Magellan SAR data. The Cartography and Geodesy Task The Altimeter and Radiometer Data Group (U Davies, chairman) will require Processing Task Group (R. Phillips, extensive data sets where the radar chairman) monitors the algorithms and data observations frequently overlap at high flow from the altimeter and radiometer latitudes, in order to set up a preliminary instruments through the various processing geodetic control network for the planet on steps to a final product. which the USGS maps may be laid, and Finally, the Mission Operations and from which improved estimates of pole Sequence Planning Task Group (H. position and rotation rate may be derived. Masursky, chairman) is charged with closely The Surface Electrical Properties Task monitoring the Project’s plans for Group (G. L. Tyler, chairman) will work scheduling and operating the spacecraft with the radar scattering intensities once in orbit, in order to ensure maximum obtained from both the SAR and altimetry surface coverage and scientific return from operating modes, as well as with the the radar component of the mission. thermal emission data obtained from the While not formally under the RADIG’s radiometry mode, in order to determine the control, the Data Products Working Group electrical properties of the surface. (DPWG), reporting directly to the PSG, The Geology and Geophysics Task plays a very important role that interlocks Group (S. Solomon, chairman) is charged strongly with the RADIG Task Groups. The with meeting the scientific objectives which DPWG coordinates the data needs of both constitute the core of, and have largely the RADIG and the Gravity Investigation justified, the Magellan Mission. With 19 Group (GRAVIG) and makes members, it is by far the largest of the recommendations to the relevant Project task groups. Because of its size, it has facilities as to the form and distribution of been broken down into four subgroups: a) the final digital and photographic products. Volcanic and Tectonic Processes; b) Impact The function and decisions of this Processes; c) Erosional, Depositional and important Group will be described in later Chemical Processes; and d) Isostatic and issues of the V-Gram. Convectional Processes. While these specialized processes provide a convenient Biography framework, it is recognized that many, if not most, studies of the surface and Dr. Gordon H. Pettengill, Principal interior will embrace more than one of Investigator for the radar experiments them. Participation in these subgroups, and carried on the Magellan Project, is probably the consequent division of effort among the best known for his discovery in 1965 of the task group’s membership, will be fluid. 3/2 spin-orbit resonance of , using The System Calibration and Test Task radar astronomical techniques. In fact, his Group (R. K. Raney, chairman) is name has also been closely linked to much responsible for monitoring the radar test of the development of procedures and associated support equipment since its early years in the late fifties. established by the Project and its Beginning with the first application of contractors. While not carrying the prime coherent earth-based radar to studies of responsibility fur the performance of the the in 1959, his observations have radar system, this Group will provide advice embraced Mercury, Venus, , several ORIGINAL PAN IS V-gram OF POOR QUALW Page 9

and comets, the Galilean satellites The Magellan Gravity of and the rings of . He was Investigation Group the Principal Investigator for the Radar William Sjogren Mapper Experiment carried out on the Warren W. James from 1978 through 1981. Overview Dr. Pettengill received a B.S. in physics from MIT in 1948, and a Ph.D. in The Magellan Gravity Investigation physics from U.C. Berkeley, in 1955. Since Group (GRAVIG) is 2 group of 9 then he has been affiliated primarily with investigators who are responsible for the MIT, first with Lincoln Laboratory and determination of the gravity field of Venus then, since 1970, as a Professor in the MIT using the Magellan spacecraft. They will Dcpt. of Earth, Atmosphere and Planetary do this by analyzing data from the Sciences. Leaves of absence enabled him to Earth-based Doppler radio tracking of the I serve as Associate Director, 1963 - 1965, Magellan spacecraft during the orbital phase l and as Director, 1968 - 1970, of the of its mission. This group will work in Arccibo Observatory in . Since conjunction with the Radar Investigation I 1984 he has been Director of the MIT Group (RADIG) to develop models of the Center for Space Research. internal structure of Venus that fit the Dr. Pettengill is a member of both the gravity observations. These models will American and National Academies of also incorporate measurements of the Science, and currently lives with his family surface topography and structure of Venus in Concord, Massachusetts. as measured hy thc ?/I.gc!!an SAP, azd altimeter and wi!! be constrained by deductions based on gcological and geomorphological studies of the Synthetic Aperture Radar (SAR) images. These models, in turn, will constrain scenarios for planetary formation and thermal history and will serve as an important source of information for comoarative olanetnlosy *+..A:,.^ .* '. .' 1 .- 5. The GRAVIG is composed of two teams of American and French scientists who will use different software and data reduction techniques to analyze the gravity data. The use of independent analysis techniques will provide important cross checks on the calculations and improve our confidence in the estimated gravity field paramctcrs and modeling results. Unlike the members of the RADIG, who will receive their primary data during Magellan's nominal mission, the members of the GRAVIG will have to wait for the extended mission before they can obtain their primary data. Magellan's nominal mission will be the first 251 days following Venus Orbit Insertion (VOI). This will provide eight days to check out the spacecraft following its arrival at Venus and 243 days to map the entire planet. The extended mission will be that portion of the mission starting 252 days after VOI. The Page 10 V-gram reason for this is explained by the way that be possible to obtain good gravity data the gravity data is obtained. during the extended mission. Starting 390 In order to obtain any gravity data days after VOI there will be a period of the spacecraft must be continuously 260 days during which the orbital geometry tracked, with its high-gain antenna pointed will allow the acquisition of high quality toward the Earth so that it can serve as a tracking data. Additionally, by that time tracking transponder. The highest the primary objective of the mission, i.e., resolution gravity data will be obtained mapping over 70% of the surface of Venus when the spacecraft is at its closest range at better than 1 km resolution, will have to Venus, i.e., when the spacecraft is near been completed. It will then be possible to the periapsis of its orbit. However, during use the periapsis parts of the spacecraft the nominal mission, the time around orbit to obtain gravity data instead of SAR periapsis will be dedicated to obtaining SAR imaging. However, the acquisition of images of the surface of the Venus and the gravity data will not require the use of all high gain antenna will be pointed toward of the spacecraft orbits, so it will be Venus so that it can be used to obtain possible to mix in the acquisition of those images. Since the high gain antenna supplemental SAR imaging observations with cannot be pointed toward the Earth for the acquisition of the gravity data, i.e., on tracking purposes during the near periapsis each day one orbit could be used to obtain part of the spacecraft orbit during the gravity data and seven orbits could be used nominal mission it will be impossible to to obtain SAR images. This period of time obtain high resolution gravity data during will be sufficiently long to allow the that phase of the mission. However, during acquisition of high resolution gravity data the nominal mission there will be a period through 360° of Venus longitude. It is of two hours on each orbit where higher during this time that the members of the altitude gravity data will be acquired. GRAVIG will obtain their primary data. These data will be obtained as a part of the normal spacecraft tracking activities GRAVIG Scientific Objectives and will be used in conjunction with Pioneer Venus Orbiter data to produce a Gravity and altimetry data obtained revised model for the global gravity field of from the Pioneer Venus Orbiter Venus. The acquisition of high resolution (1978-1982) have laid the present foundation gravity data must, however, wait for the for the geophysical modeling of the extended mission. Venusian interior. There is a significant The low resolution gravity map correlation between the gravity field of obtained during the nominal mission will be Venus and its topography. This is very somewhat degraded because of two different from the situation on the Earth, constraints related to the orbital geometry the Moon or Mars. All of these planets of the Magellan mission. First, during the have different amounts of correlation first 100 days of the mission the spacecraft between their topography and the large will be occulted for short periods during scale structure of their gravitaZiona1 fields. the times when the gravity data can be This fundamental difference suggests that obtained. This will cause the loss of there are major differences between the modest amounts of tracking data, which will processes acting within Venus and these in turn cause a degradation of the gravity other objects. The amplitude of the gravity data. Second, toward the end of the anomalies on Venus are comparable in primary mission the will be between amplitude to those observed on the Earth, Venus and the Earth (this is called solar although they are much smaller than those conjunction) and radio frequency observed on the Moon and Mars. interference from the sun will degrade the The objectives of the GRAVIG will be tracking data significantly and lower its to enhance the existing gravity data set so quality. that more detailed modeling can be However, the nominal mission will end performed. These models will be used in an following solar conjunction and it will then attempt to answer such scientific questions ORIGINAL PAGE IS V-gram OF POOR QUALITY Page 11 as: made on the planet's structure and evolution. 1) What is the nature of mantle convection in Venus? GRAVIG Data Requirements

2) To what extent is the continental The data required for the gravity sized feature known as Aphrodite in experiment consists primarily of isostatic equilibrium? Earth-based, 2-way coherent Doppler radio tracking of the orbiting spacecraft at an 3) How does the thickness and flexural X-Band frequency. During the nominal rigidity of the lithosphere vary with mission these data will be obtained when location and geologic province? the spacecraft is not near the periapsis part of its orbit; they will be used to 4) What is the deep structure of Beta determine the orbit of the spacecraft and Regio and Alta Regio that causes their to produce a low resolution gravity map of large gravity anomalies? the planet. If X-Band data cannot be obtained, S-Band data will be used. The analysis of gravity data alone Additional data will be needed in order (Le., without correlation with other data to reduce the raw tracking data. This data types) can provide a basic picture of the will involve accurate mass and cross section mass distribution within Venus, including measurements of the spacecraft, descriptions the orientation of its moments of inertia of the spacecraft attitude as a function of and the roughness and power spectrum of time (these are needed in order to properly its gravity field. However, to go beyond account for solar pressure and atmospheric that basic picture will require the combined drag perturbations on the spacecraft orbit), efforts of both the GRAVIG and RADIG data concerning any spacecraft propulsive teams. Working together they will produce maneuvers (so that changes in the realistic geophysical models that are spacecraft orbit caused by those maneuvers consistent with observations from all can be separated from perturbations caused - re:evaiii instruments. kor example, SAR by the gravitational field of the planet), imaging of specific structures will provide information relating to the performance of important constraints on the interpretation the r~rti~system, 2nd ZCCE~~:~desciii;tioiis of the associated gravity anomalies. The of the locations of the Deep Space Tracking altimeter will supply topographic data, but Network (DSN) antennas (these are needed assumptions must be made concerning the to properly convert the tracking data into a density of the topography to estimate the measurement of the acceleration of the loads on the lithosphere. However, the spacecraft). gravity experiment will provide data that During the extended mission it will be can constrain those assumptions. The possible to track the spacecraft during the response of the elastic lithosphere of Venus near-periapsis portion of its orbit and thus to those applied loads will be reflected in obtain high resolution gravity data. The the signature of the gravity data and will spatial resolution of the gravity data has allow geophysicists to model the properties approximately the same scale as the altitude of the interior of Venus. of the spacecraft during the time when it Since the models used to explain the is being tracked. Thus, when tracking the gravity data are intrinsically non-unique spacecraft during periapsis, it will be (Le., it is always possible to generate more possible to obtain gravity data with a than one model that can explain any set of spatial resolution of approximately 250 km. gravity data), the members of the GRAVIG The frequency of the near-periapsis will rely heavily on the other experiments trackings is controlled by the time that it to help constrain the model parameters to takes for the spacecraft ground track to realistic values. This will rule out some move approximately 250 km so that new models and add relevance to others, thus parts of the planet's gravitational field can permitting more credible inferences to be be sampled. Since the spacecraft will shift Page 13 accelerations will then be compared to the Data Products accelerations that would be produced by various models for the density structure of The data products that will be the Venusian interior. These models will produced by the GRAVIG using the incorporate the topography data produced processed Doppler tracking data will include by the altimeter experiment and other the following items: constraints and assumptions as agreed upon by the GRAVIG and RADIG working groups. 1.) Spherical harmonic coefficients that Although the above three approaches will describe the geoid of Venus. The will be emphasized during the primary coefficients and their uncertainties will be mission, other analyses combining the tabulated on hard copy and on magnetic existing Pioneer Venus Orbiter data with tape. the Magellan data will also be performed. Whereas the American team will be 2.) Line-of-sight accelerations on each estimating new parameters for the gravity orbit of periapsis data. It is anticipated field of Venus using just the data from the that no data will be acquired during the Magellan mission, the French team will primary mission, but at least one orbit per combine the previous Pioneer Venus Orbiter day will be acquired after VOI + 390 days. data with the Magellan data so that they These will be plotted on hard copy and can produce a gravity field estimate for magnetic tape (see figure 1). Venus. Their model will use spherical harmonic coefficients to describe the field. 3.) Gravity field power spectrum (see Since all of the French software is figure 2). independent of the American software, any similarity in the results from the two teams 4.) Gravity anomaly map (in milligals). will significantly increase the confidence in the results. This is similar to activities 5.) Geoid map (in meters) (see figure 3). that were done previously with the analysis of the data relating to the Martian Products that will result from gravitational field obtained by the Mariner combining gravity and topography data will r\ - . .* x7.r * 7 SLIIU vi~iiigspacecrart. inciucie: In the extended mission, when high resolution data from periapsis are obtained. 1.) Bouguer gravity maps. solutions for spherical harmonic coefficients of degree and order 18 to 36 will be 2.) Isostatic anomaly maps. attempted using additional data from the Pioneer Venus Orbiter. Once the gravity 3.) Admittance function curves. field parameters (i.e., the spherical harmonic coefficients for the Venus gravity 4.) Parameter values for proposed internal field) are established, a combined effort of structure models. the French-American GRAVIG team members and the RADIG geologists and geophysicists Organization of the GRAVIG will produce models for "best-fit" parameters to gravity, topography, and The American component of the other geophysical constraints. GRAVIG is lead by Mr. William Sjogren of In addition to being scientifically the Jet Propulsion Laboratory (JPL). Mr. interesting in their own right, the refined Sjogren is the Principal Investigator for the gravity field models may make it possible to American portion of Gravity Investigation, reprocess and improve some of the along with Dr. Mohan Ananda as a altimeter and SAR data using more accurate Co-Investigator. Two additional team estimates of the spacecraft trajectory. This members will be selected during the could significantly improve the quality of operations phase of the mission. One will those other observations. be a navigation analyst who will coordinate the interface between data processing and Page 14 V-gram

I I I VENUS AP HRO D I T E LINE-OF-SI GHT GRAVITY PROF1 LES /340 KM ALTlTUDl

20

*J 10 4 (3 =!z - 10

- 20 ' NIOBE 234 KM ALTITUDE

36" 1to -5" LAT ITU0 E, DEG

Figure 1. Acceleration profiles from Pioneer Venus Orbiter

10-4 F4 VENU S 1

TOPOGRAPHY BILLS & ROBR

1 Y bC

GRAV ITY MOTTI NGER ET AL. ( 1984)

I 1 I ,I I ,,I I 2 5 10 20 HARMONIC DEGREE

Figure 2. Power spectra from Pioneer Venus Orbiter OF~~C(NALPAGE is

V-gram OF POOR OUALrrY * Page 15 navigation and the other will be a One of the ncw GRAVIG members will geophysicist who will help with the be responsible for the overall software dcfinition of internal structure parameters. chcckout to assure the possibility of Mr. Sjogrcn will lead the activities for exchanging the inputs and outputs bctwccn the Gravity Investigation and will thc various software packages used by the coordinate the gravity experiment between GR A VIG. This will be needed to the GRAVIG and the project. He will also cross-check their results. This will be done monitor the X-Band downconverter system for both thc and the French to assure that quality data are received for software. This person will also be involved the experimcnt. He will inform the French with the correlation between the gravity team of current status, problems, etc. so and altimetry data, as well as providing that cf fective communications are geophysical interpretation for all gravity maintained between the two elements of the reports. GRAVIG. He will review all aspects of the The other new GRAVIG member will American effort and be most heavily coordinate the activities between the involved in the detailed correlation of GRAVIG and the navigation team. During gravity and altimetry. He will be real time opcrations he will monitor data responsiblc for monitoring all reports acquisition and quality. Ee will select the produced by the GRAVIG. data set for the 6th degree and order The work of Dr. Ananda will primarily spherical harmonic field reduction, and be involve the study of the gravity field of rcsponsiblc for the final report on this Venus by the analysis of the variations in solution. the orbital elemcntq of the ypcccrnft nrbit. The FFCRC~p~rtiort of thc GRAKG is Thereforc he wil! be responsib!e fer the !cc! by Er. Michel Lefebvrc, who is the software development, chcckout and data Principal Invcstigator for the French group. reduction results for this effort. He is He has threc Co-Investigators: Dr. Georges presently associated with the Aerospace Balmino. Dr. Nicole Borderics and Mr. B. Corporation. Moynot, as well as a technical aide, Mrs. N.

30

w n 3 k +o 4

-30

-60

240 300 0 60 120 180 LONGITUDE

Figure 3. Geoid map as determined from Pioneer Venus Orbitcr data (10th dcgrcc + order) ORIGINAL PAGE E3<+ Page 16 OF POOR QUALW - V-gran

~~

Vales. They all reside in Toulouse, France subsequently in 1968 he and a JPL colleague and work for CNES, the French space discovered the lunar mascons. Since then agency. All costs of the French team (Le., he has been continually involvcd with salaries, travel, data analysis, etc.) are paid planetary data analysis, being the Principal for by the French government. Investigator for the gravity experiments on Dr. Lefebvre will provide coordination Apollos 14- 17, Vi king, Pioneer Venus between the geophysical investigations of Orbiter and Magellan. He has twice the French and U.S. Teams. He will assist received the NASA Scientific Achievement in developing functional requirements for Award and was the recipient of the the grnvity investigation, perform data Magellanic Premium of the American analysis and interpretation for the Gravity Philosophical Society. He has been on Investigation and serve on the Project numerous NASA working groups and is now Science Working Group. on the NASA Planetary Cartography Dr. Balmino will be responsible for Working Group. He is a member of the producing the French results on the IAU, AAS, AGU and Sigma Xi. He is spherical harmonic coefficients of the oresently the supcrvisnr nf the So!~r gravity field and will participate in the System Dynamics Group in the JPL geophysical analysis of the data. Navigation Systems Section. Dr. Borderies will participate in

NASA National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology Pasadena, California

JPL 410-11-3 5/86