[157] 9.1. Inner Solar System
9.1.1. Mercury
In spite of Mariner 10's exploration, many important geologic questions remain unanswered. Mariner 10 imaged only about onehalf of the planet (much of this coverage was at high sun angles and at resolutions poorer than 1 km) and did not provide any direct information on the composition of the planet's surface. Among the major geologic problems that remain to be resolved are (1) the origin of the plains deposits and (2) the nature of the global tectonics. Whether the plains are largely volcanic or whether they were produced by impact ejecta remains unresolved; the answer has profound implications for the geologic and thermal history of the planet. The global distribution of tectonic structures is also poorly known. The observed structures appear to result from compressive stresses, but their distribution and orientations are poorly determined, making it difficult to deduce the time of onset, duration, and probable cause of this compression. These and related important geologic questions could be addressed if global coverage of Mercury at a resolution of better than I km were obtained.
Other important unknowns about Mercury concern the composition of its surface and the present state of its interior. Models of planetary formation suggest that, due to its proximity to the Sun, the planet is enriched in iron and refractories and depleted in volatile materials relative to other Earthlike planets. Mercury's high mean density and intrinsic magnetic field suggest that the planet does have a large metallic core, but little if any precise information is available about the composition of the outer portions and surface [158] of the planet. There is also no information about the current physical state of the planet's interior. The extent to which Mercury's large metallic core remains molten today has important implications for the planet's thermal history as well as for models of how Mercury's magnetic field is maintained.
9.1.2. Venus
Geologic studies of Venus remain fundamentally limited by our lack of information concerning the planet's surface. From more than a decade of spacecraft exploration (including Pioneer Venus altimetry and gravity measurements) and Earth-based radar observations, we have learned that Venus is a differentiated planet that is probably volcanically active today. Fundamental questions about Venus' geologic evolution persist:
Many of the continuing debates still associated with these and related questions would be resolved if global imaging of the planet's surface at a resolution of at least I km were available (this resolution is approximately equivalent to that of Mariner 9's coverage of Mars). Such coverage could be readily obtained by a mapping radar in orbit about the planet.
The evolution of Venus' atmosphere is also of great interest to planetary geology, not only because it determined the history of many important surface processes on the planet itself, but also because of the clues it provides to the nature, timing, and degree of outgassing of other terrestrial planets, including the Earth. It is essential to resolve the remaining discrepancies in the measurements and interpretations of the rare gas abundances in Venus' atmosphere. Some Pioneer Venus measurements have been interpreted to mean that Venus, like Earth, outgassed an ocean of water. Such models suggest that this water escaped primarily through [159] photodissociation, implying that large amounts of oxygen were somehow used up in weathering surface rocks. Such suggestions must be tested by detailed modeling and laboratory studies. In addition, laboratory studies of the chemical and weathering processes that might be occurring on the surface of Venus today are needed.
As the exploration of Venus continues, we will need to learn more about the surface and interior of the planet from in situ measurements. A concentrated effort is required to develop instruments that can operate sufficiently long in the hostile surface environment of Venus to return the desired measurements. Although chemical analyses can be made in a relatively short time, seismic experiments, our only means of learning directly about the internal state of Venus, will demand extended operational lifetimes. The technical problems to be solved are formidable; if operational Venus seismometers are to be available by the end of the century, development work must begin now.
9.1.3. Moon
The Voyager investigations of the Galilean satellites have underscored the fact that much can be learned about the evolution of planets and satellites from the study of lunar-sized bodies. As the best studied object of this important class, and the only one for which we have an absolute chronology as well as returned samples that can be studied in detail in our laboratories, our Moon will continue to provide an essential reference point not only for our understanding of the smaller terrestrial planets but also for our work on other satellites and small bodies. The comparative geologic study of the Moon and of lunar-sized satellites can be expected to be a symbiotic process. For example, studies of our Moon have alerted researchers to the possibility that accretional energy may have been an important early heat source in the case of the Galilean satellites; on the other hand, the apparent efficacy of tidal heating in the case of lo has rekindled interest in this energy source during the early evolution of the Moon, when, according to most theories, our satellite was much nearer to Earth than it is today.
The Moon will also continue to be the focus of vigorous research in its own right. Among the many questions concerning the Moon that still need to be answered in detail are the following:
9.1.4. Mars
Of all the objects in the solar system, Mars is perhaps the most similar to Earth in terms of many geologic surface processes. Although much has been learned as a result of the Mariner 9 and Viking missions of the 1970s, many fundamental questions remain to be answered. Some of these key questions concern the evolution of the martian surface and can in fact be addressed substantially using Viking data, which are available but which to date have not been fully reduced or analyzed. Thus, in the case of Mars, a major step in our understanding may be possible even before the next mission to the planet by a continuation of the steady, systematic analysis of accumulated data. Among the important geologic questions that such a systematic program of data analysis can address are:
Cause of Hemispherical Dichotomy. Most terrestrial planets show global heterogeneities, the patterns of which are roughly hemispherical. On Mars, the southern hemisphere consists predominantly of ancient cratered uplands, whereas the northern hemisphere is mostly younger, less cratered, lowland plains. What is the origin of this dichotomy, and when did it first develop? Did the resurfacing of the northern hemisphere involve only volcanic materials, or did sedimentary debris play an important role as well? In spite of its fundamental importance, no clear understanding of the reasons for the hemispherical dichotomy on Mars (or on other planets) is at hand.
Volatile History. Surface processes on Mars cannot be fully understood without clear definition of the present and past water cycles on the planet. How much water (and other volatiles) did Mars [161] outgas? When did this outgassing happen? Where is this water stored? At what rates can it be, and has it been, cycled among the likely reservoirs (ground, atmosphere, polar deposits)? How has the history of past water cycles affected valley development and regional denudation? Understanding the water cycles on Mars throughout the planet's history is fundamental to understanding the climatic, and consequently the history, of geologically important surface processes.
Origin of the Canyon System. Even though more than ten years have passed since Mariner 9 discovered Valles Marineris, we still lack good models of how this and related canyon systems formed. When did canyon formation begin on Mars? Was it a single continuous event, or was it a series of episodic phenomena? How closely is canyon formation related to volcanic and tectonic events? How was canyon development related to the complex of large outflow channels surrounding the Chryse basin? Answers to such questions are linked to models of the evolution of the interior of Mars and can be addressed using accumulated Viking data (images, topographic data, and IRTM * measurements), as well as Earth-based radar altimetry.
History of Polar Deposits. It is widely believed that substantial amounts of volatiles are stored in the polar regions of Mars. Yet quantitative estimates, essential to models of climatic history and volatile evolution, are lacking. Further geologic studies of the complex laminated terrains and investigations of the causes of the evident asymmetry of the polar deposits between the south and north should lead to better models of the processes and cycles that were involved in producing the polar terrains. Such models should lead to quantitative estimates of the amounts of volatiles that are currently stored in the polar areas.
Importance of Explosive Volcanism. More than half the surface of Mars appears to be covered by volcanic materials, mostly lavas resulting from flood-type basaltic eruptions. Given the presence of water on Mars, it is reasonable also to expect explosive volcanism and attendant ash deposits. This may well be the origin of some of the extensive mantling plains units. Diagnostic criteria to distinguish ash deposits using imaging and other remote sensing data must be developed for application to Mars. The reliable identification of ash [162] flows could provide important clues to understanding the volcanic history of Mars.
Sedimentary History. There is ample evidence that weathering and fluvial and eolian processes have been important throughout much of Mars' history. Accordingly, extensive sedimentary deposits (other than volcanic ash deposits) should exist. Given the evidence for climatic changes on Mars, how have sedimentary processes changed throughout the history of the planet? How have they affected the preservation of the cratering record? How can the sedimentary record be related to the denudational history of Mars? What remote sensing characteristics can be used to identify and map sedimentary deposits ?
Style of Tectonism. Several areas of Mars, most prominently Tharsis and Elysium, show strong evidence of tectonic deformation. Despite several years of intensive study and mapping, fundamental questions regarding the stress fields involved and their time histories remain unresolved.
The questions outlined above provide examples of problems that can be addressed significantly, if not completely solved, with data available from previous missions and continuing Earth-based observations. There are other fundamental geologic questions about Mars, however, that will require future investigations by spacecraft. Most important among these are:
Absolute Ages of Key Units. Radiometric dating of rocks from key geologic units on Mars is essential to establish an absolute chronology for the planet.
Current State of the Planet's Interior. Direct determinations of the characteristics of the crust, mantle, and core are essential to models of the thermal, tectonic, and volcanic evolution of the planet. Improved models of this type should better constrain ideas about the history of volatiles on the planet.
Composition and Mineralogy of Rocks in Key Units. Remote sensing and Viking Lander measurements provide only indirect information on the composition and mineralogy of rock types on Mars. In part, this situation results from the limitations of the techniques used so far, but more fundamentally it derives from the fact that surficial measurements are sensitive only to the more or less ubiquitous weathering rind that appears to cover the very surface of the planet. Important questions include the nature of the unweathered materials [163] in various areas, as well as the rates, mechanisms, and products of weathering.
9.2. Outer Solar System
Our discussion is divided into two parts, the first dealing with the satellites of Jupiter and the second with the satellites of Saturn. We expect that Voyager's investigations of the satellites of Uranus in 1986 and of Triton and perhaps other satellites of Neptune in 1989 will reveal many important geologic problems that will need thorough investigation. We have chosen to exclude any specific mention of planetary rings, since most of the investigations dealing with them have traditionally been supported by NASA programs other than Planetary Geology.
9.2.1. Satellites of Jupiter
Based on the diversity of geologic features and processes observed by Voyager, our recommendations for future studies of the Galilean satellites can be summarized as follows:
Some of these recommendations have emphasized theoretical analog, and modeling studies rather than detailed investigations of Voyager images. Although the Voyager mission established the general geologic style of each of the satellites, image resolution falls short of that needed for systematic geologic analysis, in which stratigraphic relations are determined and the history of action of different geologic processes is traced. Such data have been acquired for the Moon and Mars, but in the case of the Galilean satellites, they will become available only with future missions such as Galileo.
In addition to the Galilean satellites, Jupiter is accompanied by about a dozen smaller objects. The largest of these, Amalthea (Jupiter V) has a spectrophotometrically unusual surface and possibly an involved evolutionary history. Four small satellites (including Amalthea) are now known to exist inside the orbit of lo. All have ubiquitous low albedos and very red colors, probably resulting from contamination of the surfaces by sulfur derived from lo. There is also recent evidence that part of the orange color of Europa is due to sulfur contamination from lo. Theoretical studies of how material from Io is transferred throughout the inner satellite system are needed, as are laboratory simulations of the resulting implantation and regolith contamination processes.
9.2.2. Saturn Satellites
Many of the geologic problems that should be addressed for the Saturn satellites system using the available spacecraft data are similar to those for the Galilean satellites. Our recommendations for future investigations include:
Of all satellites in the Saturn system, Titan is almost certainly the most complex. Although Voyager obtained fundamental data on Titan's atmosphere, the nature of the satellite's surface remains unknown. In this context, high priority should be given to radar investigations of Titan's surface from spacecraft.
9.3. Small Bodies
Due to their diversity and large numbers, the investigation of the small bodies presents special problems. Yet the thorough study of these objects is essential to an understanding of the processes that have shaped the evolution of our solar system. It is highly likely [166] that during the next ten years NASA will fly an exploratory mission to the small bodies; vigorous efforts must continue to develop appropriate instruments to maximize the data return from such a mission. Special attention must be given to instruments capable of determining the composition and mineralogy of surfaces remotely, as well as to techniques of automated sample collection and analysis. Methods of accurately mapping irregular objects and of determining precisely their masses and volumes also must be refined.
Since we cannot hope to explore most small bodies directly remote sensing of such objects from Earth or Earth orbit will always remain an important part of planetary studies. Such investigations are needed to Improve our knowledge of the physical characteristics of these bodies, as well as of their orbits and populations. These data will be needed to select the most attractive targets for future exploratory missions, as well as to improve our knowledge of the Impact rates m different parts of the solar system.
9.4. Geodesy and Cartography
High-quality cartographic products and geologic maps should be prepared for all bodies for which adequate imaging is available. This effort is well under way but will require strong and continued financial support. With about twenty planetary bodies now imaged, the present cartographic backlog could extend for many years.
Improved radar images of Venus should be acquired to establish a control net of the planet and prepare a planetwide series of maps at a scale of approximately 1:5 000 000. The spacecraft should carry a radar altimeter for vertical control and interpretation of gravity field measurements.
In spite of numerous past missions to Mars, we still lack accurate altimetry of this important planet. We strongly urge that the next mission to Mars include a radar altimeter and that, if possible the spacecraft be placed in a polar orbit. Altimetry and additional imaging of Mercury would also be of high interest to complete our reconnaissance of the Earthlike planets.
Given the likelihood of an exploration mission to one or more small bodies during the 1980s, the problems associated with mapping such generally irregular objects and precisely determining their sizes, shapes, and masses must be worked out in detail.
[167] 9.5. Laboratory Studies and Instrumental Techniques
Throughout this report we have emphasized that in order to take full advantage of future spacecraft missions, the continued development of appropriate instruments must be supported. Areas of major concern to planetary geology include remote sensing instruments (imaging systems, spectral mappers, mapping radars, etc.), as well as instruments required for in situ measurements (chemical and mineralogic analysis, age dating, seismic and thermal heat flow measurements). Particular attention must be paid to the development of automated sample collection techniques not only on Earthlike planets, but also on small bodies such as asteroids and comets. Also needed is the development of instruments that will function in very hostile environments such as at the surface of Io or Venus.
We expect that during the coming decade multispectral measurements will continue to provide much of the basic "compositional" information about surfaces of solar system bodies. In order that such data be interpreted as precisely as possible, a vigorous program of complementary laboratory studies should be supported. This program should investigate how such spectra are determined not only by the mineral composition of the surface, but also by particle size, surface texture, and roughness, and especially the effects of mixing. Additional complications that must be understood include the effects of photometric geometry and, in the case of outer planet satellites, the effects of low temperature and intense charged particle radiation.
It is abundantly clear that the nature and rates of weathering processes on a planet are critically dependent on the temperature, pressure, and composition of the planet's atmosphere. Careful laboratory studies of the chemical and physical weathering processes to be expected on Mars, Venus, Io, Titan, etc., should be performed Not only do we need to know the nature of the weathering processes and their end products, but, most importantly, we need information on their rates under the conditions that apply to the planets in question. Such "environmental chamber" studies can also be used to determine the rates of various other atmosphere/regolith interactions, for example, the ability of the martian regolith to store vole tiles and to buffer atmospheric pressures.
[168] Because of the obvious significance of eolian processes on Mars and their probable importance on Venus, wind tunnel investigations of sediment transport and erosion rates on these two planets should continue.
Past laboratory studies have been especially successful in elucidating the mechanics of impact cratering in silicate rocks. With the recent Voyager exploration of many low-density, icy bodies, it is essential to extend laboratory cratering studies to ice and other low-density targets (e.g., ice-silicate regolith mixtures). Concurrent with these impact studies, accurate determinations of the rheologic and general mechanical properties of various ices are needed as a function of temperature. In the context of understanding the cratering and tectonic histories of the icy satellites, such studies should pay special attention to the properties of mixtures of ices and ices contaminated with silicates.
Investigations of available extraterrestrial samples (Moon, rocks, meteorites, and cosmic dust) should be vigorously pursued.
9.6. Analog Studies
As a first step in the identification of different geologic provinces on extraterrestrial bodies, one relies on morphologic appearance. Cleverly designed terrestrial analog studies will go a long way toward revealing the dominant processes responsible for the evolution of various multicomponent land features and also in providing quantitative estimates of the magnitudes, frequencies, and intensities of the events responsible. A broad spectrum of analog studies should continue to be supported both on subaerial features and on some submarine features on the continental slopes. In a few key areas, such as possible analogs of sulfur and water volcanism on outer planet satellites and those of valley networks on Mars, efforts should be accelerated.
9.7. Remote Sensing from Earth
In spite of occasional spacecraft missions, we expect that remote sensing observations from Earth and Earth orbit will continue to play a fundamental role in planetary science during the coming decade. In some cases (volcanic activity on Io, dust storms on Mars), such observations will provide essential extended coverage of time-variable phenomena. In other cases (for example, high spatial resolution spectral observations of the Moon), they will [169] continue to yield basic data needed to support ongoing research. In particular, they will continue to provide the bulk of our information on the fluxes and photometric and orbital characteristics of the multitude of small bodies in our solar system.
Earth-based radar observations, both at Arecibo and Goldstone, can be expected to continue producing important data on the topography and roughness characteristics of the terrestrial planets. In addition, radar studies of asteroids and comet nuclei should begin yielding new information on the surface characteristics (e.g., metal/ silicate ratio in the case of asteroids) of these little-known bodies.
9.8. Remote Sensing of Earth
Observations of the surface of Earth from orbiting satellites, Skylab, and the Space Shuttle provide a superb data base for a multitude of applications in planetary geology. Among the many possible investigations of high interest to planetary geology are the following:
9.9. Archiving and Dissemination of Planetary Data
Continuing attention must be paid to the problem of archiving and disseminating data obtained from past planetary missions. In certain cases, some of these data remain to be thoroughly analyzed (for example, many of the Viking Orbiter images of Mars). In other [170] cases, such past data will provide valuable comparisons with information expected in the future (for example, the comparison of Voyager images of Io with those to be obtained by Galileo). During the past five years, the network of Regional Libraries set up by the Planetary Geology Program across the United States has played a crucial role in preserving and making available to investigators the imaging products (and related data sets) from past planetary missions. The PGWG considers the Regional Libraries an essential part of the Planetary Geology Program and urges continued support and, if possible, expansion of the system.
9.10. Continuity and Future of the Planetary Geology Program
During the past two decades, the Planetary Geology Program has made impressive contributions to our understanding of our solar system. There is every reason to expect that these contributions will continue during the next decade, provided that NASA plans adequately for the maintenance and healthy modest growth of the enterprise. Efforts must be increased to ensure that adequate data analysis funds are available to support recent and future planetary exploration. Steady and predictable support is needed to ensure that planetary geologists can- use state-of-the-art instrumentation and techniques to carry out their tasks. Short-term fluctuations in the level of support, such as those witnessed during the early 1980s, could be avoided by long-range planning, given NASA's continuing commitment to the solar system exploration program in general, and the Planetary Geology Program in particular.
NASA's plans for the future exploration of the solar system include several missions of high interest to planetary geology. Efforts must continue to ensure that adequate support is available to design and fabricate the best state-of-the-art instruments and data handling systems for these endeavors.