[85] Within the planetary context, geochemistry is concerned with seeking answers to some very fundamental questions. Most concern the past and present states of the solar system and especially the processes that have been active during the system's evolution over the past 4.6 billion years. Examples of general questions considered by planetary geochemistry include the following:
To the planetary geologist, geochemistry can provide important information of two basic types. First, there is information on the present bulk and surface composition of a planet or satellite. Such fundamental data constitute important constraints in many geologic discussions concerning the evolution of a particular object. Second, there is essential information about the types, rates, and chronology of various processes that have molded the planet's surface into its present state.
Ideally, to provide the most reliable information, the geochemist will need for analysis a well-selected suite of rock samples representative of the planet's surface (and perhaps a sample of its atmosphere). Only for Earth and the Moon are such samples available. True, numerous meteorites have been studied in detail, but [86] here the problem is that we are not quite sure of the sampling processes or of the precise parent bodies.
Given a sample of a surface, determinations of the mineralogic and petrologic relations, along with a detailed chemical analysis in terms of major. minor, and trace elements, can be used to deduce the processes that produced the various rock types and formed the major rock units. At the same time isotope techniques can be used to date some of the major events involved. Such a comprehensive understanding of the geochemical and petrogenic processes that have operated on a planet is required if we are to develop a reliable model of the object's evolution. So far such an ambitious undertaking has been realized only for Earth and the Moon and to a much lesser extent for the various parent bodies of our meteorites. It is hoped that at least one other planet (Mars?) will be added to the list before the end of the twentieth century.
4.1. Information from Sample Analysis
Accurate compositional data are essential to fully understand the evolution of planetary bodies. Studies of the mineralogy, mineral chemistry, texture, and bulk chemical composition of rocks are necessary to define their physical and chemical histories. Evidence for processes ranging from crustal formation to chemical weathering at the surface can be investigated through such studies, which are especially important when rocks and soils have experienced sequences of complex events.
Trace element chemistry can be used to determine a wide variety of signatures of specific geochemical processes. Analyses of siderophile, chalcophile, lithophile, and volatile elements in groups or m pairs yield evidence on, among other things, the nature of the bulk starting material for planetary differentiation, the degree of differentiation, the planetary heat sources, the temperatures and pressures of infernal processes, and the nature of meteoritic material impacting the planet. Such data, interpreted in conjunction with petrologic data, can unravel the complex evolutionary history of planetary surfaces.
Precise isotope analyses, which to date can only be accomplished in terrestrial laboratories, can be used to study a wide variety of chronologic and geochemical problems. Long-lived radioactive species (Il-Th-Pb, K-Ar, Rb-Sr, Nd-Sm) can be used to obtain isotopic ages for rocks and to establish an absolute chronology for a [87] planet (Kirsten, 1978). The stable isotopes (O, Si, C, S, N, H) provide geochemical tracers that can be used in conjunction with chronologic data to give information on past states of the interior of the planet as well as more recent surface processes and atmospheric modifications (e.g., Holland, 1978). Anomalies left by the decay of extinct short-lived radioactive isotopes can provide evidence for preaccretion conditions and time scales (Podosek, 1970).
Analyses of the rare gases (He, Ne, Ar, Kr, Xe) and their isotopes provide information on the differentiation history of the planet, its interaction with cosmic radiation, and the evolution of the atmosphere. Xenon isotopes are perhaps the most versatile, as the isotopic patterns may be affected by extinct short-lived isotopes, fission of long-lived and extinct isotopes of U and Pu, as well as mixing effects with various reservoirs of gas. In situ isotopic studies of atmospheric gases can yield important clues to the evolution of a planet's atmosphere. Examples include measurements of the N14/N15 ratio in the atmosphere of Mars (McElroy et al., 1976) and of the H/D ratio in the atmosphere of Venus (Donahue et al., 1982).
Rocks can also be examined for evidence of remanent magnetization, a clue to the history of the planet's magnetic field, and for a variety of physical properties, such as density, porosity, thermal conductivity, and seismic wave velocities, which are essential to complement the geophysical measurements that can be made from orbit and from surface stations. The physical properties of surface materials must also be understood in order to determine their capacity to adsorb and release gases and to quantify the rates of gas interchange with the atmosphere (Fanale and Cannon, 1974).
The study of returned samples provides an excellent opportunity for the detection of evidence of past or present life. In order to preserve the integrity of information contained in the returned sample, sterilization (by whatever means) should be avoided.
4.2. Information from Atmospheres
Planetary atmospheres are mixtures of gases and aerosols retained by the planet's gravitational field. Within the context of planetary geoscience, atmospheres are studied for information about the composition, origin, and evolution of a planetary body. They provide information about the planet's chemical and isotopic composition and about the extent and chronology of outgassing. Relevant questions include the following:
Planetary atmospheres vary considerably. The massive atmospheres of the giant planets have changed little since the formation of the solar system. The terrestrial planets have weaker gravitational fields and cannot retain light gases. Even if these planets once had atmospheres of primitive solar nebula composition, the light gases would have been lost quickly (Hunter, 1973; Pollack and Yung, 1980).
The nature of the initial atmospheres of the terrestrial planets is related to the mode and timing of planetary formation and differentiation (Turekian and Clark, 1975; Pollack and Yung, 1980). If the planets formed cold before the solar nebula dissipated, then their primitive atmospheres would have consisted of the noncondensed solar nebula minus the light gases. If the planets formed hot, then the products of outgassing from the condensed solar nebula materials must be added to the primitive atmosphere. If the planet formed after the solar nebula was dissipated, then the initial atmosphere would consist only of the products of outgassing from the condensed solar nebula materials minus the light gases. If the early Sun went through a T-Tauri phase, then a part of the primitive atmosphere may have been lost from the terrestrial planets.
Processes that change the composition of a planetary atmosphere include both those which add material and those which remove It. The former includes capture from the primitive solar nebula, capture from the solar wind, additions during late-stage accretion, additions during later collisions with comets and meteorites, accumulation from outgassing of the planetary interior, accumulation from biologic activity, and accumulation from chemical reactions with surface materials. Processes that subtract material Include escape to space (by blow-off or thermal escape), consumption by chemical reactions with surface materials, and escape by ionization and subsequent electromagnetic acceleration by the solar wind. (In the presence of a sufficiently strong planetary magnetic [89] field, the upper atmosphere is protected from ionization and subsequent removal by the charged particles of the normal solar wind.) Most of these removal processes were probably very active during the early histories of the terrestrial planets and eliminated virtually all traces of the primitive atmospheres.
What factors determine the composition and evolution of the atmosphere of a planet such as Earth? Certainly one of the most important factors is the volatile content of the planet. This, in turn, is thought to be partly a function of the position (temperature) within the preplanetary nebula at which the material comprising the object condensed (Lewis, 1974). However, it is possible that volatile-rich objects such as comets have altered some volatile inventories (Anders and Owen, 1977; Chang and Kerridge, 1982) A second factor is the thermal history of the object. It determines whether volatiles present in bulk have been transferred to the surface. The object's thermal history is. in turn, mostly determined by the amounts of accretional (impact) energy added to the growing planetary nucleus, as well as energy from both short- and long-lived radionucleides, and in special cases, even tidal energy. In the most general terms it is the size of an object that determines its energy history; large objects accumulate more gravitational heat and retain energy longer than small ones. Of course, a small object can have an active degassing and differentiation history under special circumstances. A general principle, however, is that, except for the tidal heating, continuing or increasing degassing after billions of years is more likely for large objects than for small ones. General discussions of thermal evolution are given by Toksoz et al. (1978) for rocky objects, and by Parmentier and Head (1979b) for icy ones. A third factor is that once degassed, planetary volatiles can react chemically with primary igneous rocks to form alteration products such as clays and carbonates. Additionally, such volatiles can be incorporated physically into a planetary regolith (usually in a frozen or adsorbed state). A fourth major factor affecting inventories of volatiles is loss to space. Loss to space can be very selective by mass; preferential loss of hydrogen can convert a reducing atmosphere to an oxidizing one. Planetary escape is a complex process (e.g., Hunten, 1973; Pollack and Yung, 1980) and may involve other mechanisms than blow-off or simple deans escape.
Differences in the surface volatile inventories of solid bodies in the solar system can be understood largely in terms of these four [90] factors: initial bulk volatile content, degree of outgassing, reaction of atmospheric products with surface material, and escape to space. As far as bulk volatile content is concerned, it is likely that solar system-wide compositional gradients exist. For example, in one hypothesis, the center of the solar system represents a heat source that prevented condensation of volatile and semivolatile elements and compounds in its inner portions prior to gas dispersal. Thus, a single compositional gradient radial to the Sun could describe the resulting array of solid planetary bodies: the outer objects being more volatile rich. Specific compositional predictions have been published, most notably by Lewis (1974), Grossman (1972), Grossman and Larimer (1974), and others.
Although remote sensing of planetary atmospheres is a well-developed science, many of the characteristics of atmospheres are determined most reliably from analyses done in situ. The atmospheres of Earth, Venus, and Mars have been sampled directly, and Galileo is expected to sample the atmosphere of Jupiter in the late 1980s. Once the detailed composition of an atmosphere is known, studies of surface samples can provide valuable constraints on the history of atmosphere/crust interactions. Martian surface materials could provide evidence for a previous "wet" regime on Mars; samples of the venusian surface could provide information on the importance of such reactions as CaMgSi2O6 + CO2 -> MgSiO3 + CaC03 + SiO2, which might be buffering the partial pressure of CO2 in the atmosphere to some extent.
4.3. Geochemical Clues to Surface Processes
The many processes that affect the surfaces of planetary bodies are outlined in chapter 2. Such processes are commonly characterized as either internal (endogenic) or external (exogenic). For planets with atmospheres, there is a special class of exogenic processes (eolian, fluvial, and chemical weathering) that depend on the interaction of the surface with the fluid atmosphere/hydrosphere.
Some surface processes, such as weathering, erosion, transportation, deposition, and lithification, are universal to all solid-surface planetary bodies. However, their relative intensity varies depending on local conditions, and in simplest terms one can define the following two broad categories: (1) surface processes on planets with atmospheres and (2) surface processes on bodies with no atmospheres.
[91] 4.3.1. Surface Processes on Planets with Atmospheres
For this class of objects, which includes Earth, Mars, Venus, Titan, and perhaps Pluto, the essential questions are:
Weathering of surface materials may result from both chemical and physical interactions with the gases and liquids of the atmosphere and, in the case of Earth, the hydrosphere and biosphere. The net result of chemical weathering is a modification of the original surface and the production of secondary materials. Physical weathering modifies the original surface material primarily by fragmentation processes that increase the fraction of fine-grained relative to coarse-grained material. The interaction of chemical and physical processes is complex; for example, fragmentation processes increase the surface-to-volume ratio of en assemblage of particles, rendering them more susceptible to chemical reaction; physical separation of light from heavy minerals may allow the preferential chemical or physical destruction of one in favor of the other. The physical and chemical processes operating on a given planetary surface can be understood and evaluated only if the composition, pressure, temperature, and dynamic processes of the atmosphere, as well as the composition of the surface materials and the crustal processes acting on these materials, are well determined.
When products of weathering, erosion, transportation, deposition, and lithification are preserved, they contain a historical record of a planet's climate, atmosphere, volcanism, and tectonism. Before these records can be deciphered, an adequate description and understanding of surface processes is essential (see chapter 2).
All planetary objects are continuously bombarded by electromagnetic radiation, solar wind electrons and ions, and highly energetic galactic cosmic rays. Solar electromagnetic radiation heats the atmosphere and is the major driving force for atmospheric circulation producing erosion by wind. The products of the interaction of [92] solar and galactic particles with planetary atmospheres and surfaces may also be utilized to characterize various planetary processes ranging from atmospheric mixing to deposition rates of surface materials.
All planetary bodies are subject to meteoritic bombardment. The presence of an atmosphere places a lower limit on the size of a meteorite that can impact the surface. In general, the lower limit varies inversely with atmospheric density, which in turn may vary with time (Kahn, 1982). For present atmospheric densities, for example, a meteorite initially weighing 1.0 kg will not reach the surface of Earth but can impact the surface of Mars.
4.3.2. Surface Processes on Bodies with No Atmospheres
This category includes the Moon, Mercury, most satellites, and asteroids. The key questions include:
On planetary bodies with virtually no atmosphere, the surface layer composed of impact comminuted debris, is referred to as regolith. So far the only regolith studied in detail is that of the Moon. Since the evolution of a regolith depends on numerous factors, we can expect that significant differences among planetary, satellite, and asteroid regoliths will exist. For objects with virtually no atmosphere, electromagnetic radiation, solar wind ions, and highly energetic galactic cosmic rays penetrate to the solid surface (unless some of the charged particles are deflected by a magnetic field). This bombardment produces changes in the lattice structure and composition of regolith grains, effects which have been studied extensively in lunar soils. Some of the effects of this irradiation are preserved in grain surfaces and may hold a record of past variations of solar output (Crozoz, 1977). They have also been used to date lunar craters and to determine rates of regolith turnover (Langevin and Arnold, 1977). Similar investigations could be carried out for other airless bodies once samples of their regoliths become available for study.
From the study of the Moon, meteorite impacts are known to be important agents of weathering, erosion, transportation, deposition, and lithification on a body with virtually no atmosphere. Meteorite impacts excavate and transport materials (chapter 2, section 3) and can affect profoundly the physical, chemical, and petrographic characteristics of surface materials. Moreover, large-scale cratering events may trigger igneous activity or even the breakup of crustal plates.
4.4. Examples of Key Issues and Problems
From the point of view of planetary geology, one of the essential geochemical tasks is to test the extent to which current models of the formation and evolution of the solar system and of particular planets are supported by actual observations. For example, one popular series of models, that of J. S. Lewis, L. Grossman, and others, is based on the premise that a strong radial temperature gradient existed in the preplanetary nebula. Such models predict large differences in the bulk volatile content of the Earth-like planets, with Mars being more volatile rich than Earth, and Venus more volatile poor. In some extreme versions of such models, Venus accretes at most minor amounts of water. However, recent measurements of rare gas abundances in Venus' atmosphere by the Pioneer Venus spacecraft shed some doubt on such a simple picture. The measurements indicate that Venus accreted unexpectedly large amounts of 36Ar. One explanation that has been offered is that the early nebula was much more isothermal (Pollack and Black, 1982) than traditional models allow. Such a conclusion, if confirmed, would affect current estimates of the initial volatile content of the Earth-like planets and of the bulk chemical composition of objects as a function of distance from the Sun.
The satellites of Saturn provide another excellent example of why it is important for planetary geology to know the initial chemical composition of solar system objects. At the present time it is not clear whether small satellites such as Enceladus, Tethys, Dione, etc., contain any ices other than water ice. The issue is essential to models of their thermal evolutions, since the presence of ammonia [94] ice would lead to a significant lowering of melting temperatures, allowing much more vigorous tectonic histories for the same amount of heat energy. If the small Saturn satellites accreted ammonia ice (probably in the form of ammonia clathrate) in addition to water ice, then the geologic evidence of internal activity still preserved on the surfaces of Tethys, Dione, and especially Enceladus would be easier to understand (Poirier, 1982; Squyres et al., 1983).
A closely related problem involves the evolution of Titan's atmosphere. If it is true that the nitrogen that makes up the bulk of Titan's atmosphere comes from the photodissociation of ammonia, and the methane either from outgassing or sublimation from the surface, then clearly Titan accreted not only water ice, but also ammonia and methane. Whether this evidence can be used to infer that a satellite such as Enceladus accreted ices other than water ice depends critically on the temperature gradient in the vicinity of Saturn at the time of satellite formation (Pollack et al., 1976). Following Voyager, enough is known about the composition of Titan's atmosphere and the bulk composition of the satellite itself that geochemical modeling aimed at determining the state of the satellite's surface becomes possible. Available suggestions vary widely: in some schemes Titan's surface is icy; in others, it is covered at least in part by liquid hydrocarbons; still another scheme has the surface covered to great depths by complex organic material ultimately derived from Titan's photochemical clouds. Since attempts will be made to study the surface of Titan by spacecraft during the next two decades, it is essential to start thinking about the likely geologic characteristics of Titan's surface. Geochemical modeling of interior, surface, and atmospheric processes can provide valuable constraints on such speculations.
It is a truism that one cannot proceed far in geologic investigations of the surfaces of outer planet satellites without running into questions that involve geochemistry. Perhaps the most extreme example involves the surface of lo. Traditionally, the attempt has been to interpret the surficial colors, albedos, and morphology in terms of sulfur and sulfur dioxide frost, although numerous geologic and spectrophotometric difficulties with such a simple scheme are now known (Schaber, 1982; Sill and Clark, 1982). An important question that still has not been addressed adequately is the following: given that sulfur and/or sulfur dioxide are erupted onto the surface, what compounds will be preserved given realistic modeling of lo's bizarre...
....environment? More important, how likely is it that the volcanic products would be pure sulfur or sulfur dioxide? What about explosive eruptions of silicate materials? Far from being a simple mixture of various forms of sulfur and sulfur dioxide frost, the surface of lo probably consists of entire suites of sulfur compounds, silicate ash, and perhaps even basaltic eruption products. The geochemical problems involved must be carefully studied, both in terms of the likely composition of erupted materials and in terms of short-term and long-term chemical alterations on the surface.
A related problem involves the contamination of neighboring satellites by materials from lo. Currently the best explanation of the red colors of Amalthea and the other small inner satellites of Jupiter and of the reddish tinge of Europa is that these surfaces are [96] contaminated by sulfur from lo (Thomas and Veverka, 1982; Eviatar et al., 1981). Adequate measurements of what ions exist in the vicinity of lo are available, but precise studies of how these materials would modify the regoliths of neighboring satellites are lacking.
In the inner solar system, among the most pressing geochemical problems of importance to planetary geology are those associated with the evolutions of the atmospheres of the Earth-like planets. Did Venus really outgas an ocean of water? Was the atmosphere of Mars always as thin and cold and dry as it is today? The importance of precise answers to such questions in understanding the history of weathering and other surface processes on these planets is evident.
4.4.1. Venus
We have already noted that equilibrium condensation schemes that involve a preplanetary nebula with a strong temperature gradient suggest that Venus accreted from relatively water-poor materials (e.g., Lewis, 1974), a conclusion that can be viewed as consistent with the planet's present low atmospheric water content. Yet it is equally possible that Venus lost a great deal of water through a series of processes involving dissociation, exospheric escape of hydrogen, and oxidation of the Venusian surface. In fact, if the recent models inspired by the high 36Ar content of the atmosphere measured by Pioneer Venus are correct, then Venus somehow lost the equivalent of an ocean's worth of water (Donahue et al., 1982). Given projected rates of water dissociation and hydrogen escape, the process could have taken as little as 100 million years. Exposing enough fresh rock to use up the associated oxygen this quickly is difficult but perhaps possible. No matter which scheme is correct, it is evident that the atmospheric environment of Venus has changed through time and therefore so have weathering and other surface processes.
Three major scenarios for the evolution of Venus' atmosphere seem plausible at present. The first possibility is that Venus outgassed only a small amount of water and that this water was quickly eliminated by the processes outlined above. The second possibility is that Venus quickly outgassed as much water as Earth and that its atmosphere rapidly reached its current greenhouse state. According to this scenario, atmospheric conditions on Venus have been similar to the present ones over most of geologic time. A third possibility is that, although outgassing of water was substantial and rapid, Venus [97] entry into the runaway greenhouse mode was substantially delayed, perhaps because the Sun was initially some 30 percent dimmer than it is today. Recent speculations (Pollack and Yung, 1980; Phillips et al., 1981) and interpretations of Pioneer Venus rare gas data and D/H measurements (Donahue et al., 1982) seem to favor models that involve losses of substantial amounts of water by the planet.
The implications for geology are profound. If Venus were cool enough at the beginning for liquid water to exist on the surface, huge carbonate deposits might have been formed. Certainly, any massive loss of water must have been accompanied by a thorough oxidation of the surface materials. If the loss of water were accompanied by a dehydration of the lithosphere, it could have led to a sudden stiffening of the lithosphere, profoundly affecting tectonic processes. Whether evidence of such a stiffening (if it occurred) is preserved in the surface morphology depends in part on when it happened and on the rate of weathering. One would expect that under present conditions (high pressure, temperature, and acidity) weathering would be rapid. However, detailed investigations of current and past weathering processes and rates are needed.
4.4.2. Mars
Mars is generally considered to be richer in volatiles than Earth, an idea consistent with the planet's lower mean density and with ubiquitous geomorphological evidence of subsurface ice (Rossbacher and Judson, 1981). Channels and valley systems (chapter 2 and Baker, 1982) provide strong evidence of climatic change, although the timing, severity, and causes of these changes remain largely unresolved. Based on interpretations of rare gas abundances in the atmosphere measured by the Viking spacecraft, it is commonly argued that Mars outgassed less material (per unit mass) than Earth; estimates vary, ranging from I to 10 percent of that of Earth (Pollack and Yung, 1980). Measurements of N15 enrichment relative to N14 have been interpreted (e.g., McElroy et al., 1977) to suggest upper limits on the total atmospheric content at any time of about 200 millibars.
No matter which estimate of outgassing is used, it appears that the bulk of the volatiles outgassed by the planet is not in the atmosphere. Fanale and co-workers argue persuasively that most of these volatiles are stored in various forms in regolith and not in the polar caps (e.g., Fanale, 1976; Fanale and Cannon, 1979). For....
....example, water exists as ground ice, as adsorbed water, and as chemically bound water. Thus, one expects that atmosphere-regolith interactions have been complex on Mars, yet this complexity must be understood if we are to unravel the past history of Mars' environment. We must understand how the various volatiles are stored in the regolith and how they can be cycled in and out, and on what time scales.
Viking Lander analyses and remote sensing from Earth strongly suggest that water is chemically bound in martian regolith materials. The relatively high water contents (1 to 1.9 percent) measured by the Viking GCMS* are unlikely to be attributable entirely to water [99] adsorbed on free surfaces. Chemically bound water or interlayer water in clays must be involved, as well as possibly water in hydrated salts that appear to be present in the "duricrusts" sampled by Viking (Clark, 1978; Gibson et al., 19X0). The existence of hydrated minerals, including clays, on the surface of Mars had been revealed by a variety of remote sensing data prior to Viking. I he interpretation is consistent with the results of Viking's X-ray fluorescence experiment, which provided partial chemical analyses compatible with the presence of a major clay component in the martian regolith (Clark, 1978). There is strong evidence from the Viking measurements that other volatiles, most notably sulfur, are stored chemically in the regolith. Although more C02 is stored in the regolith than is present in the atmosphere or in the polar caps, at most a minor amount is in the form of carbonates (Fanale and Cannon, 1979). The Viking GCMS observed a C02 release pattern in heated Mars soils that is said to be matched better by the desorption of C02 from clays than by release from thermal-labile carbonates.
Clearly, the regolith of Mars is chemically extremely complex. The weathering processes in it are very poorly understood, even under current environmental conditions. If at some past epoch liquid water existed in the surface layer, then the chemical incorporation of water, C02, and other volatiles into the regolith would be relatively easy to understand. In the absence of liquid water, one might have to rely on ultraviolet-simulated chemical weathering, a process about which considerable debate remains. Fortunately, ultraviolet-simulated vapor-solid weathering is not the only plausible mechanism for fixing volatiles into the regolith of Mars. When lavas flow over, or intrusions are emplaced into, hard frozen soil on Earth, extensive interaction between preexisting soil, igneous rock, and remobilized volatiles occurs. Such interaction sometimes produces a volatile-rich melange soil called palagonite. Conditions on Mars seem favorable for similar processes, and terrestrial palagonites have optical properties quite similar to those of martian soils.
Thus, a first-order problem in the geologic study of Mars is to understand the current weathering processes and the nature, extent, and accessibility of various volatile reservoirs. Such an understanding is essential if we are to trace the likely history of the martian atmosphere. Only with such a history in hand can we determine the effectiveness of various regolith and surface processes throughout the age of the planet. The fundamental problem of understanding....
[101] ....weathering and regolith-atmosphere interactions under present conditions can be tackled by thorough laboratory investigations. Special attention should be paid to detailed studies of analog terrestrial soils (e.g., from the Dry Valleys of Antarctica) and to terrestrial palagonites.
4.5. Summary
In a fundamental sense planetary geology relies on the progress of geochemical studies for the framework of its investigations. Developments that improve our knowledge of the processes in the early solar system provide firmer initial conditions for studying the geologic evolutions of planets-initial bulk compositions, volatile content, radioactive content, which is closely related to internal heating and degree of outgassing, etc. Absolute chronologies, which can only be obtained by geochemical studies of isotopes, are also essential for our studies. In the broadest sense, the fundamental task of geochemistry-the analysis of planetary samples- provides essential constraints on the nature and timing of events that affected the sampled bodies.
In the inner solar system there are important fundamental problems associated with the evolutions of the atmospheres of the Earth-like planets that can only be addressed through geochemical techniques. A key question in this context remains the initial distribution of rare gases in the materials that accreted at various distances from the Sun and its connection to the total volatile content. It has become the practice to infer the total volatile content and outgassing histories of terrestrial planets from measurements of rare gas abundances. The assumptions implicit in such techniques must be thoroughly understood and their validity ascertained. Obviously, in terms of surface geology it does make a significant difference whether or not Venus outgassed the equivalent of an ocean of water, or whether or not the atmospheric pressure on Mars has always been close to its current low value.
Many geologic processes involve the transport of weathering products. Thus, the rates and processes of past and present weathering are of key importance. For planets with atmospheres this problem is complicated by chemistry. The weathering processes (both past and present) on Mars and Venus must be thoroughly investigated. Relevant laboratory studies and, in the case of Mars, appropriate analog studies, should be pursued vigorously. [102] We also need a better understanding of the processes involved in the development of regoliths in general, especially on icy bodies and on objects such as Amalthea and Europa, and perhaps ever lapetus, which may be subject to significant fluxes of foreign contaminating material.
The most effective strategy for the study of a planet's surface processes and composition is to proceed systematically with the mineralogic, petrologic, and chemical characterizations of its components. However, it is insufficient merely to analyze chemically the surface rocks, soils, or atmospheres; some assessment must be made of the geologic history of the samples. Determination of the mineralogy, texture, Ethology, and other properties of the rock that might be relevant to origin is required. Mineralogic composition is particularly important in this context, since mineral assemblages reflect both major element chemistry and conditions of formation; it is therefore far more diagnostic of rock types and formative processes than chemical composition alone. Indeed, if a choice must be made between a mineralogic and a chemical analysis, most geologists would opt for the mineralogic analysis because it provides much more than just chemical information. For example, the X-ray analysis of the martian surface soils obtained by the Viking landers is incomplete because we do not know what mineral components and alteration products made up the samples analyzed.
Future exploration strategy for the planets, satellites, and asteroids should be more comprehensive than that carried out for the Moon in the past. In the case of the Moon, difficult experiments were deferred until samples were available for analysis on Earth. Because the acquisition of samples from other bodies will be far more costly and technologically difficult, selected detailed investigations should precede sample return. On Mars, for example, experiments such as age determinations, petrologic characterization, and isotopic and trace element studies need not be delayed until samples are returned to Earth. Instruments necessary for performing such measurements remotely should be developed and tested.
The success of any geochemical exploration program will depend largely on how sampling sites are chosen. Information from geochemical orbiters and mappers, along with Earth-based instruments, are crucial in selecting sites for detailed study. The usefulness of most remote sensing techniques depends not so much on the precision with which they can determine surface chemistry, but [103] on their ability to detect differences. Careful preparatory study is required to determine the variability of surface materials, the areal extent of geologic units, as well as their likely modes of origin. The maximum scientific return from the investigation of any extraterrestrial sample will be realized only if the detailed geologic context of the sample is established.
* Gas chromatograph mass spectrometer.
