The term geology literally means the study of Earth, but the discipline normally concentrates on Earth's surface and near surface because these are the most accessible parts of the planet. Through the examination of the nature and disposition of surface rocks we attempt to reconstruct how Earth formed and how it has evolved to its present state. Since the beginning of space exploration two decades ago, the term geology has assumed a broader meaning and includes the study of solid planets, as well as the satellites, asteroids, and comets of the solar system. The emphasis is still on surface studies, since, as in the case of Earth, the surfaces are the only parts accessible to direct observation. Planetary geology attempts to reconstruct the evolution of planetary bodies, largely from the fragmentary records that have survived on their surfaces.
The evolution of any planet or satellite is in large part controlled by initial conditions such as size, bulk chemistry, and distance from the Sun. The object's surface is subject to a variety of processes which fall into three broad categories:
Surfaces in the solar system tend to have complex configurations, reflecting intricate histories of the interaction of a variety of processes. Not only do different processes dominate on different....
...bodies, but their relative importance on a particular planet or satellite may have changed with time. The Moon provides a familiar example: impacts dominated until some 3.9 billion years ago, from which point until 3.2 billion years ago, volcanic processes responsible for the formation of the mare prevailed. Since that time, as volcanism declined, impact processes again became dominant on the Moon. On Mars, geologic evolution was more complex. There it seems that volcanism, tectonism, impact cratering, eolian and fluvial activity, as well as other processes, were significant throughout the planet's history. The geologist's task is to reconstruct from the  fragmentary record that is preserved what processes have shaped the planet's surface and what their relative roles have been as a function of time.
In the sections that follow, important geologic processes are discussed individually, and specific recommendations are made that will lead to a better understanding of their roles in the evolution of planetary and satellite surfaces. The different studies are clearly interrelated. Silicate magmas may react with ground water, impacts may induce volcanism, tectonic features are subject to mass wasting, etc. In addition, different processes can produce similar landforms; for example, both volcanism and fluvial processes can produce channels. Only by studying the whole range of geologic processes and phenomena that might affect a planet can we hope to arrive at a secure interpretation
2.1. Structural Geology and Tectonics
Unraveling the deformational history of planetary bodies involves the application of the methods of both structural geology and tectonics. The two fields ask related questions but differ in the scale of phenomena they treat. Structural geology deals with local deformational features such as faults and folds, the local stress fields which caused them, the timing and sequence of events within these local structures, and the mechanisms by which they operated. As the size of the region expands from the subcontinental to global, the term structural geology gives way to tectonics. The word tectonics springs from Greek roots meaning framework and denotes the concern of that discipline for the broader framework of deformational and geologic history into which individual local events are to be fitted. This tectonic overview of a planet combines structural data with those of geophysics, geochemistry, and a variety of types of age determination to build a conceptual model of the global planetary machine in terms of geometry, mechanics, and energy sources that have powered it through its various evolutionary stages.
2.1.1. Basic Questions
The kinds of local questions structural geology might ask include:
Tectonics may ask many of the same questions but in a more global context. Most of these questions are closely intertwined with data and problems dealt with in geophysics and geochemistry:
Thus, tectonics seeks to determine the models that can explain the sequence and timing of development of the first-order features of a planetary body, including the local details derived from structural geology. This is the ultimate goal of tectonics: to determine an overall model of the way a planet has evolved.
2.1.2. Geologic Maps
More than most other subdisciplines of the geosciences structural geology and tectonics involve relating surface observations to depth and time,. Among the most important tools for making these links is the geologic map, a graphic representation of the distribution of rock units and structural features with indications of their  vertical and age relationships. It is through the careful production of such maps that the details of the geologic history of a body are worked out. Their production requires much more shall the mere tracing of areas of similar landforms onto a map. The three-dimensional relationships, as well as the time sequence of events, must be linked into a logical sequence. Surficial processes must be understood in enough detail to recognize how they create the landforms, redistribute materials, and mark surfaces of different ages. Various types of multispectral images can provide variable information on distribution of rock and mineral types. Geophysical observations can give clues of subsurface boundaries and characteristics.
Ultimately, the subsurface of a planet must be linked with its surface characteristics either by geophysical techniques or by some surface phenomena that connect the two levels. The most common of these links are volcanic deposits and structural features. By means of cratering, degradation of topography, or superposition, the volcanic deposits are usually dated more easily than fault or fold structures; on the other hand, the structural features give more unique indications of stress type and orientation. Both types of information are embedded in the geologic maps and represent an outline of events that the tectonic model must explain. The geologic maps constitute the base on which the bulk of geologic observations about a planet must be placed; they must be upgraded as geologic understanding of a planet increases. At any point in time the best current geologic map represents the major detailed data base the tectonic model seeks to explain.
2.1.3. Experimental Stress/Strain Studies
Much of the focus of structural geology and tectonics is on the magnitude and character of present and past stress fields. Unfortunately, the present stress field is essentially unobservable except by the strains it produces; past stress fields are even more enigmatic. Thus, the process becomes one of recording or mapping the complex strains that have been imposed on the rocks and interpreting or deducing the stresses that caused them. Furthermore, many of these strains were produced at depths and temperatures at which the mechanical characteristics of even Earth materials are poorly known, adding a complication to the interpretation.
Over the last half century a massive body of experimental data on the mechanical properties of most ordinary rocks and minerals  has been accumulated. This includes stress to strain relationships under many conditions, including those in the deep interior of Earth and in the presence of a variety of volatiles. The internal glide and recrystallization mechanisms utilized by many minerals are reasonably well known. The yield and breaking strengths of many rocks are well documented, as are their effective viscous behaviors under long-term loading.
The recent recognition and better definition of volatile-rich solid planetary bodies in the outer solar system necessitate an improved data base for relating stress to strain on such objects. Although we can handle many of the deformation problems of ordinary rocks, improving the structural/tectonic analyses of the more exotic bodies of the outer solar system requires a significant expansion of the rock mechanics data base to include volatile-rich materials as well as compositions far different from the traditional silicates that dominate so much of the chemistry of the inner solar system.
2.1.4. Stress Field Indicators
On Earth, the modern or present-day stress fields are determined by detailed observations and measurements of their effects. Precise measurements across active fault or fold zones give indications of both horizontal and vertical displacements produced by the stress. The pattern of seismic energy release in a quake can be related to the magnitude of stress drop in the fault that produced that event. Compressional or extensional displacement patterns of seismographs by first motions of an earthquake define the orientation of the stress field causing the quake. Strain gauges used in drilling operations record the expansion of rocks as they are removed from their confining stress and hence indicate the magnitude and orientation of that stress. Fracture operations associated with oil fields break rocks by excess pressures in drill holes, the magnitude of overpressure required for breakage being an indication of the stress at depth and the fracture orientation being related to the stress orientation. These methods, although highly useful on Earth, cannot be applied at present to other planets.
On Earth, fault systems are widely used as paleostress field indicators. As noted by Anderson (1951), the free surface of Earth is a plane of no shear; hence, one of the three principal stresses should be approximately vertical at any location on the surface of our planet. If the maximum compressive stress is vertical, a system  of normal faults should result, with the strike of the faults parallel to the intermediate principal stress. If the minimum compressive stress is vertical, a system of thrust faults should result, with the faults striking parallel to the intermediate principal stress. If the intermediate principal stress is vertical, a paired system of strike slip faults should develop, with the maximum compressive stress as their acute bisector. Anderson further expanded his model to point out that dikes or other swarms of fracture filling should open perpendicular to the least compressive stress direction.
Many other indications of paleostress orientation or magnitude are available on our planet. Most surficial fold systems indicate compression approximately perpendicular to the trend of the folds; the wavelengths of the folds are related to viscosity contrasts in the folding materials. A host of outcrop and microscopic-scale strain indicators are utilized for terrestrial deformation analyses, including mineral twins, kink zones, veins, and planes of recrystallization. At the present stage of exploration, such indicators have little applicability to other planets.
For other bodies, the most valuable stress/strain indicators are fault systems viewed in the context of Anderson's ideas (e.g., Plescia and Saunders, 1982; McKinnon and Melosh, 1980). Strain markers in the form of originally straight-line or originally circular crater outlines can be utilized to measure the magnitude of strain. Other patterns of fractures of radial or concentric nature may indicate stress concentrations about some center. Arrays of fractures in formation or in echelon to each other can indicate lateral displacement along buried fracture zones. These surficial indicators of deeper stress must be used with caution because preexisting weaknesses or earlier fracture systems may influence the orientations of younger fractures.
Among the most widespread and least understood of all paleostress indicators are large-scale fracture traces or lineaments etched by erosion into the topography of almost all areas of Earth and readily visible on Landsat and Seasat images (Wise and Allison, 1981). Similar linear features appear on almost all solid bodies of the solar system imaged to date and have been the basis of proposed "lunar grids," "martian grids," and "mercurian grids." It is generally agreed that these represent some type of fracture traces related in some way to stress systems, but the exact relationships remain hotly debated. As a ubiquitous topographic element, these linear features have considerable potential for determining the....
.....stress history of planetary surfaces. Unfortunately, at present this potential cannot be realized. The reproducibility of the lines drawn by different observers is questionable. Furthermore, there is no agreement on the statistical methodology that should be used to treat such data, and the origin of the linear features on Earth is not clear.
2.1.5. Tectonic Energy and the Generation of Stress Fields
Identification of the timing and character of past or present stress fields is an intermediate step in understanding the geologic  history of a planet (e.g., Wise et al., 1979). Ultimately, the causes of the stress fields must be related to an overall geologic and tectonic model and to the sources of energy that power that model.
On Earth, the driving forces of plate tectonics and the associated stress fields are believed to involve the sinking of ocean plates as they cool and grow denser at subduction zones, as well as the sliding of plates off upraised, less dense, hotter ridges of oceanic mantle material. To the extent that the density contrasts are thermally driven, this process is a form of convection, even though the mantle is a relatively passive element in the system. Other ideas of the driving process include large, thick cells of deep mantle actively overturning and convective mantle plumes dragging passive surface materials along on their tops. The relative importance of these various processes on Earth today is uncertain; their importance at times past, especially on other planets, is even more uncertain.
Many other processes can generate stress fields on regional or planetary scales. Impacting bodies create intense but short-lived stress fields. Internal temperature changes associated with the decay of radioactive elements, core formation, etc., can lead to expansion or contraction at differing depths. Volumetric changes associated with permafrost regions of Mars probably account for many local fractures. Changes in surface or subsurface loading can be produced by volcanic piles, polar deposits, erosion at surficial or subcrustal depths, or lateral magmatic movements. Gravitational sliding of surface materials from topographic highs creates many local stresses. Any load not supported by isostatic equilibrium must be supported by stresses maintained by the strength of the planet; for example, the lunar mascons create strong local stress fields in their immediate vicinity. Even in isostatic equilibrium, differences in density at the same depths in adjacent columns will cause stress differences. Tidal torques can stress surfaces and change rotation rates. "Despinning" of planets alters their oblatenesses, expanding their polar crustal regions, compressing the equatorial regions, and deforming the middle latitudes by shear.
The development of structural and tectonic models of planetary bodies progresses through several stages. Initially, the level of tectonic and surface activity for any newly imaged planetary body is estimated from the relative abundance of surviving craters versus  structurally or erosionally produced features. Basic planimetric and topographic maps are produced. At this stage, the major topographic and structural features are classified and located in a general way, and tentative mechanisms for their origin are proposed. Examples might include fault troughs, volcanic features, lineaments, wrinkle ridges, etc. In addition, the first-order geologic provinces are defined and crudely mapped based on the distribution of these features. Subsequent stages of the exploration involve detailed geologic mapping closely linking surface characteristics, topography, structural and geomorphic features, unconformities, and age indicators into a relative sequence of events. The mapping is supplemented by many types of geochemical, geophysical, and remote sensing data. More detailed relative age distinctions are made based on crater densities; age correlations related to these densities also may be attempted from region to region. Based on models of the cratering history in the solar system, these cratering ages may be linked to the more traditional time scales derived from radioactive decay (chapter 3).
From this set of data, the broad outline of the planet's history is developed, including the cratering history, stress history, and volcanic history. If possible, limits on the atmospheric and volatile evolution are determined. Some idea of the density structure and layering may be deduced from the figure of the planet, isostatic considerations, and crustal flexures. In general, these multiple approaches to the planet's history are mutually supportive, constraining each other and suggesting which geologic processes and mechanisms are most important. Usually the result is a second generation of improved geologic mapping, more sophisticated geologic process studies, better stress orientation determinations, and improved timing of events. At this stage, it becomes possible to formulate working tectonic models, to test the physical plausibility of these models, and to judge their implications for creating the details of the observed geologic surface of the planet and its associated internal characteristics.
A major long-term goal of our planetary exploration program should be the development of tectonic models of all the solid bodies of the solar system. This task would involve working out the detailed structural and geologic history of the bodies, combining  these with geophysical and geochemical observations to provide tectonic constraints as the basis for each model. From these models, general principles of planetary behavior and evolution should become much clearer and illuminate much more fully the history and mechanisms of Earth's evolution.
Standard geologic mapping is a vital foundation to understanding the nature and evolution of any planetary body. The use of multispectral and other remote sensing maps to supplement the standard geologic maps should be exploited more fully.
Intriguing linear patterns exist on all the solid planets imaged to date. It is likely that they constitute important hints to the past stress history of the bodies, but our ability to understand and interpret them is limited at best. There is ample room for much more sophisticated work in this area.
The mechanics of icy planets are understood very poorly. Even the basic physical and deformational properties of many candidate materials for these planets are precisely known. A major effort is needed in these areas if the results of exploration of the outer solar system are to be interpreted properly and fully.
Volcanic landforms provide some of the clearest evidence for how a planet has evolved. They give clues concerning thermal conditions in the interior, the composition of the mantle, and the structure of the lithosphere; if the landforms can be dated, then changes in these characteristics can be traced through time. The landforms also provide clues to the style of volcanism and to the composition and physical properties of the materials involved. Because of the applicability to Mars and the Moon, studies of planetary volcanism during the 1970s tended to focus on activity involving the eruption of large amounts of fluid lava (Carr, 1973; Carr et al., 1977; Greeley, 1976; Carr and Greeley, 1980). However, with acquisition of broader photographic coverage of Mars and following the Voyager encounters with Jupiter and Saturn, we have been confronted with a wide array of volcanic features, the products of various types of volcanism. Io appears to erupt lavas of sulfur (Schaber, 1980), the surfaces of Europa and Ganymede appear to have been modified by the eruption of ice-rich materials (Smith et al., 1979b), and interaction of silicate magmas with ice and ground water may have been important in producing various landforms on Mars (Carr, 1981). Planetary volcanic studies must therefore be  broadened considerably in scope if we are to achieve a better understanding of these bodies. Inevitably, the shift in emphasis will bring a change in the style in which the study of planetary volcanism is pursued. In the past, considerable reliance has been placed on comparisons among the planets, particularly with Earth, where some volcanic processes are reasonably well understood. These comparisons have been largely geomorphologic. As more exotic types of volcanism are encountered, simple comparisons become less appropriate and we expect greater reliance on theoretical studies and experimental work to extrapolate from our terrestrial experience while still acknowledging the need for analog studies where possible to test the modeling work.
2.2.1. Geomorphology of Volcanic Landforms
Geomorphology is an important basis for almost all planetary volcanic studies; observation of landforms leads directly to examination of different volcanic mechanisms; and all the theoretical and experimental studies must ultimately be reconciled with what is actually observed. The most straightforward geomorphologic studies involve comparison of landforms seen on other planets with those formed on Earth by known processes. Leveed channels, lobate flow fronts, and lines of "skylights" over a lava tube, for example, all suggest eruption of fluid lavas, whereas steeply sloping cones are more indicative of explosive activity. Such comparisons have proven extremely useful in leading to a better understanding of planetary volcanism and in fact are the basis for much of our present knowledge of other planets.
To facilitate comparative studies, extensive documentation is needed of volcanic landforms on Earth and other bodies. We need a comprehensive collection of satellite images of Earth's volcanic fields at scales similar to those obtained from the other planets. Moreover, we need quantitative measures of volcano heights, diameters of vents and edifices, depths of calderas, flow thicknesses, and so forth. Such data not only would allow more precise comparisons to be made (taking into account the different planetary environments), but are essential for testing theoretical models of various volcanic processes and for assessment of the physical properties of the materials involved.
Although volcanic processes occur on other planets under conditions vastly different from those on Earth, and with magmas of radically different compositions, comparison of extraterrestrial and....
 ....terrestrial volcanic features must still be an essential element in any program of planetary studies. If nothing else, analog studies demonstrate the complexity of volcanic processes and emphasize the enormous gap between idealized models and reality. Often analog studies also suggest possible explanations for features seen on other planets and provide a means of testing theory. Studies should include a broad array of volcanic features such as those formed by subglacial eruptions, by phreatic phenomena, or by magmas of exotic compositions. Detailed investigations of how volcanic landforms evolve should be included, for often the ultimate form is the result of a complex sequence of events that is not obvious from a cursory examination of the final configuration. The growth of Mauna Ulu, Hawaii, for example, consisted of a complicated sequence of events involving the repeated formation and drainage of underground lava reservoirs, development of ancillary vents and lava tube systems, and repeated growth and partial filling of the main vent.
The icy satellites of Jupiter and Saturn present an unusual challenge. Europa, Ganymede, Enceladus, and possibly others appear to have been resurfaced by eruptions of ice from the interior (Smith et al., 1979b, 1981). Whether this process should be called volcanism could be debated, but it does involve the eruption of a relatively warm and mobile fluid from the interior of the body onto a cooler surface. Terrestrial analogs for such features should be sought.
2.2.2. Modeling of Volcanic Processes
Because volcanism on other planets take place under conditions vastly different from those on Earth, an understanding of these processes is unlikely to follow from simple comparison of the resulting landforms; theoretical work is needed to determine how the processes are affected by the different environments. For example, low gravity and the absence of an atmosphere might result in different disposition of volcanic products around a vent; the dimension of lava flows may vary in a predictable way with rheologic properties and eruption rates, and volcano heights may give an indication of lithosphere thickness. Evaluation of such effects requires that the processes be modeled and that they be described in some analytical way so that the effects of changing different variables can be assessed.  It is inappropriate here to suggest a comprehensive list of specific topics for research; these will become evident as the field advances and as specific gaps in our understanding begin to hinder further progress. However, the types of theoretical studies that certainly should be pursued include:
 These suggestions provide examples of the types of theoretical work that would be useful for a better understanding of volcanic processes on other planets. It should be reemphasized that such work must be done in close conjunction with studies of terrestrial volcanoes so that the theory can be tested. Experimental data may also be needed to support the theory in many cases, for example, in relating flow morphology to rheologic properties.
2.2.3. Experimental Petrology
Perhaps one of the largest gaps in our understanding of planetary volcanism is in the area of petrogenesis. Most of the experimental work on silicate melts has been done on systems that are appropriate for terrestrial conditions. Almost no work, for example, has been done on extremely sulfur-rich or ice-rich systems that might be more appropriate for the satellites of the outer planets.
Experimental petrology is the laboratory investigation of chemical systems thought appropriate to the understanding of petrogenesis. This effort involves the study of chemical systems much simplified over natural ones as well as direct experimentation on representative natural samples. The goal of experimental petrology is generally elucidation of (1) the physiochemical conditions under which a rock formed, these being temperature, total pressure, fugacities of the various participating gaseous species, and activities of other components in the system, and (2) the specific reactions and reaction paths actually followed in rock formation. This goal is accomplished by duplicating, in heated pressurized vessels, the assemblage of minerals found significant in a particular natural setting. The value of experimental petrology thus lies precisely in the fact that it quantifies petrology.
Important contributions provided by experimental petrology to understanding the planet Earth include:
In principle, all the points outlined are applicable to the terrestrial planets and to rocky satellites of the Jovian planets. The crucial question is, to what extent can information from laboratory studies of chemical systems be applied to other planets? This question involves estimations of both the relevant bulk compositions and the appropriate physical conditions. It seems clear that much additional work will be necessary simply because evidence is abundant that bulk compositions are different from one planetary body to the next. Highly oxidizing conditions have rarely been investigated for Earth materials at high temperatures and pressures because such conditions were not applicable here. Yet such studies may be appropriate for other planets. In a similar vein, highly reducing conditions were not investigated before the advent of lunar studies, as there was no immediate reason to do so. On the other hand, a substantial body of phase equilibrium data exists for many common  minerals. Wherever these data have been determined for a wide range of physical conditions, they will probably be useful in planetology.
For Mercury, an important question in view of the existence of an internal magnetic field is whether the planet has a liquid core. Bulk geochemistry as inferred from some formation models points toward a sulfur-free, iron-rich core. The pressure at the core-mantle boundary should be about 60 kbar. In the absence of sulfur, temperatures would have to be above 1600°C for the core to melt. However, if sulfur were present, a liquid outer core could be maintained at only ~1000°C. Thus, thermal and differentiation models are critically dependent on the bulk composition chosen. A mantle silicate chemistry rich in Ca and Mg and low in Fe would imply very high melting temperatures.
For Mars, some geochemical models imply a high sulfur content (see also Clark et al., 1976). Thus, a liquid core could be maintained at moderate temperatures. On the other hand, the basalt melting interval may not be appropriate because of the possibility of a higher oxidation state for the planet. If the higher oxidation state hypothesis were true, other consequences follow; Higgins and Gilbert (1973) have argued that nickel will then be released into the silicate system and be housed in the "ferromagnesian" minerals. Significant changes in melting temperatures, element fractionation, and phase stability are then possible. Since it is often postulated that the volatile content of Mars exceeds that of Earth, much of this theory might turn out to be inappropriate, in which case other studies (e.g., Eggler, 1974) might be more pertinent.
During the past decade, planetary volcanic studies have focused on features formed by eruption of fluid basaltic lava because of their importance on the Moon and Mars. Recently, however, we have been confronted with a variety of volcanic features on Mars and the satellites of the outer planets that have been produced by other forms of volcanism. Planetary volcanic studies should accordingly broaden in scope to include the full range of volcanic phenomena encountered on Earth, as well as volcanic processes that might occur on other bodies through eruption of magmas of exotic compositions' such as sulfur and ice. Study of terrestrial analogs,  modeling of volcanic phenomena, and theoretical and experimental work on appropriate chemical systems should all proceed vigorously in concert.
Cratering has been a ubiquitous process in the solar system since its formation. After some initial debate, a consensus has been reached that most of the craters seen on the surfaces of the Moon and the planets were formed by hypervelocity impacts of cosmic debris. Although the earliest work focused on using these features to estimate the relative ages of surface units, more recent studies have concentrated on understanding the cratering process itself. Whereas many of these investigations have dealt with impacts into rocky surfaces, spacecraft investigations of Mars and the satellites of the outer planets have demonstrated the importance of extending such studies to other target materials, including ice-rich regoliths and pure ice.
2.3.1. Impact Cratering: An Introduction
A hypervelocity impact results in two shock fronts, one of which propagates into the target and the other into the projectile. The shock front in the target compresses the material comprising the medium, setting it in motion and, provided the resulting stresses are sufficiently large, causing changes in its physical state (e.g., from solid to liquid or vapor, or combinations of the two). Decompression of the stressed, moving material alters the "flow field" initiated by the shock, inducing ejection of target material from a growing cavity and rebound of the target medium that was compressed more or less in situ. The end results are a hole in the ground (the crater) and a surrounding deposit of ejecta.
The final shape of the crater is determined by a number of factors, among which are the surface gravity and size of the target body, the physical and chemical properties of the target and projectile, the size, velocity, and angle of impact of the projectile, and the ambient atmospheric pressure. These parameters will also govern the characteristics of the crater's ejecta deposit. As the magnitude of the impact increases, elastic properties of deep-seated material and the gravitational field and size of the target body generally become more important in governing the outcome of a cratering event, while the effects of target strength, angle of impact, and.....
....atmospheric pressure decrease. Thus, elastic and gravitational mechanisms modify crater cavities more intensely as larger craters are considered; indeed, the increase in crater complexity with size has been well documented on all cratered surfaces studied to date. Very large craters usually possess morphologies characterized by multiple, concentric rings and, depending on the number of such rings, are called peak-ring or multiring basins. The diameters of these structures range from ~100 km to over 2000 km, implying that their formation affected the target bodies on truly gigantic scales.
The average changes in crater shape and appearance with increasing diameter are firmly established (e.g., Hartmann, 1972b, 1973); the reasons for these trends, however, are not fully understood. A precise assessment of the relative importance of the major  variables mentioned above and of the specific roles they play in determining the observed variation in crater morphology with size remains a fundamental goal of crater research (Roddy et al., 1977).
2.3.2. Methods of Study
Craters can be studied by a variety of methods, but in general the most valuable results are achieved by combinations of complementary techniques. The most important of these include:
Spacecraft Imaging. Cameras carried by spacecraft allow the examination of craters spanning a vast size range and a wide spectrum of degradational states in their natural settings. Such observations provide data from which topographic, morphologic, and other information can be derived. Comparison of craters on different planets permits an assessment of the effects of varying parameters, such as surface gravity, target properties, and average impact velocity.
Field Observations. Detailed descriptions of target stratigraphy coupled with subsurface structural information are perhaps the most valuable results of in situ field work. Petrologic, petrographic, and other geochemical data from impact sites are exceedingly useful m analyzing the propagation and effects of impact-generated shocks. Geophysical measurements-such as those obtained by gravity and magnetic surveys-yield information about deep crater structure and bulk target response to the impact event.
Remote Sensing. At optical, ultraviolet, and infrared wavelengths, spectrophotometric and color-differencing techniques have been used to investigate the spectral properties of solar system objects. Insofar as these methods provide resolutions approaching those of familiar imaging systems, they can produce data directly applicable to individual craters. Specifically, in the lunar case, spectral, albedo, and, in some instances, chemical characteristics of various crater units and deposits have been distinguished, and changes in color and reflectance brought about by impact events have been studied. In addition, the stratigraphy of the target can often be determined from such studies. Spacecraft carrying gamma- and x-ray detectors, magnetometers, and radio transmitters (used in gathering gravity data) have also garnered information valuable to crater studies.
Laboratory Simulations. Small-scale impact and explosion cratering experiments are characterized by stringent control of variables that would otherwise be weakly constrained or unknown during natural, large-scale events. For this reason, laboratory work presents....
....the opportunity to determine the roles played by different factors in the cratering process, and, w hen scaling is correctly employed, direct extrapolation to larger structures can often be made. In addition, high-speed photography, stress and strain gauge measurements, and other techniques provide the luxury of recording the time histories of the phenomena associated with the mechanics of the cratering event. In a number of cases, the scale of these experimental events can be increased through the use of very large explosive charges.
Computer Modeling. The scaling relationships involved in extrapolating from small to large craters are only vaguely understood. This factor, combined with our inability to perform large-scale impact experiments, means that a substantial gap exists between our first-hand knowledge derived from cratering experiments and the information we need to interpret large-scale cratering events. Here we must resort to numerical modeling. Very large events under a variety of initial conditions can be simulated in a computer, and time-dependent phenomena can be followed in smaller time increments than is currently technically possible, even in the case of small events. Computer simulations also permit time to be "accelerated" if degradational or other long-time scale processes are to be investigated. Although such calculations are often expensive and Sometimes depend on poorly characterized material properties, they  provide insights into processes that would otherwise be intractable, in terms of both time and physical magnitude.
2.3.3. Craters in Geologic Studies: Some Established and Emerging Uses
An impact is an event that occurs essentially independently of the physical properties of the target body. The results of the process, however, are influenced by the environment in which the event occurs. As pointed out earlier, a number of environmental factors play significant roles in determining the detailed outcome of a given impact.
Craters can be used in two principal ways to study the planet on which they occur. The traditional method applies statistical techniques to crater populations in order to learn about the age, history, physical properties, or other characteristics of a particular planet, region, or geologic unit. The second procedure, which has become more popular in recent years, uses individual craters or specific groups of craters as probes, markers, and windows in attacking specific problems often related to the stratigraphy of the region in which they occur.
Statistical Studies. One of the first applications of craters in planetary studies was the use of crater statistics to establish the relative ages of the lunar highlands and maria (Opik, 1960). Under the reasonable assumption of an areally isotropic impact flux, old surfaces will present greater areal densities of craters than will younger regions. Refinements of this technique have proved invaluable in interpreting the histories of planets and satellites explored during the past two decades (e.g., Soderblom et al., 1974; Neukum and Wise, 1976; Hartmann, 1977) (chapter 3). In addition, such information provides valuable constraints on the population of projectiles (e.g., Strom and Whitaker, 1976).
To first order, craters of a given size possess similar morphologies immediately after their formation. Any degradational modifications of the crater population (such as disappearance of morphological features due to blanketing, flooding, and other processes) can be treated statistically and used in studies of obliterational mechanisms. Coupled with crater-counting techniques, such degradational studies can aid in interpreting the geologic history of the region in question (e.g., Jones, 1974).
 Growing knowledge of the physics and effects of impact cratering has encouraged further use of craters in more detailed and sophisticated investigations. For example, interplanetary comparisons of crater statistics, are suggesting the possibility that separate and distinct projectile populations were responsible for the bombardment of the solid-surface bodies during the early history of the solar system (Strom and Whitaker, 1976).
Crater-Specific Studies. The shock associated with an impact event fractures the target medium around and under the final cavity; consequently, large craters can serve as localized outlets for the extrusion of lava and as zones of weakness at which tectonic stresses can mobilize crustal material. For example, craters are believed to have served as eruption centers for a substantial fraction of the basalts of the lunar Mare Australe (Whitford-Stark, 1979); on Mercury, craters deformed by large apparent thrust features serve as convenient strain indicators for crustal shortening studies (Strom et al., 1975).
Since the shapes of craters can be predicted approximately (at least in rocky materials), these features have been used as valuable markers in gauging the thicknesses of various geologic units and deposits. Knowing the average interior geometry of a crater permits the depth of any infilling material to be estimated with a reasonable degree of accuracy (e.g., Whitford-Stark, 1979). By the same token, known distribution of rim heights as a function of crater diameter can be used to estimate the thickness of exterior deposits by documenting the degree to which craters of a given size are buried in a particular region (De Hon, 1974). For such purposes, accurate topographic data are necessary to establish the morphometric characteristics of the crater populations.
Classification of crater morphologies with respect to target type is producing information on the properties of the target materials as well as on the effects of these properties on the cratering process itself (e.g., Cintala et al., 1977; Wood et al., 1978). Since large quantities of heat are released in a very short time during an impact event, substantial volumes of the target are melted and vaporized. Under these conditions, parameters such as volatile content appear to play a significant role in influencing the cratering style and the final crater morphology. For example, few (if any) craters with central pits are observed on dry bodies such as the Moon and  Mercury, but their abundance increases from Mars to Ganymede and Callisto-bodies which are believed to have volatile-rich surfaces.
The availability of accurate topographic maps and a better understanding of possible mechanisms involved in the large-scale fracturing of crater floors has permitted the mathematical modeling of such endogenic processes (Hall et al., 1980). On a much larger scale, mathematical analysis of basin structure is leading toward better models of the lunar (Melosh, 1978; Solomon and Head, 1979) and martian interiors (Solomon et al., 1979). Such calculations, however, depend strongly on the values assumed for various material parameters, including the effective crustal and mantle viscosities, most of which are poorly known. Schaber et al. (1977) have attempted to constrain the effective viscosity of the mercurian mantle by analyzing those members of the planet's crater population which are believed to have adjusted isostatically. Thus, craters of all sizes can be useful tools in deciphering the histories of their parent planets.
2.3.4. Persistent Problems and Unanswered Questions
Our knowledge of craters and cratering mechanics has grown remarkably over the last two decades. The first-order problems of origin and mode of formation have been treated in abundant detail, but a large set of more specific questions remains to be answered. Impact cratering has been a ubiquitous geologic process throughout the history of the solar system, and it is likely that crater and basin forming events affected significantly the formation of early planetary crusts, including that of Earth. Craters also provide an essential means of establishing planetary chronologies and clarifying the evolutionary histories of projectile populations. The processes that degrade, erode, and otherwise modify craters must be better understood if chronologies derived from crater counts are to be trusted.
Important specific questions that must be addressed include:
Although many first-order questions concerning the origin, formation, and morphology of craters have been answered satisfactorily as a result of data acquired through a variety of techniques during the past decade, some detailed aspects of the cratering process remain poorly understood. These more detailed questions, involving the sources, fluxes, and characteristics of projectiles, the specific physical mechanisms and sequences of events of both large and small time scales involved in the crater excavation process, as well as the effects of shock on planetary materials, must be pursued vigorously. Since so many aspects of planetary geology are dependent on the use of craters as essential tools, a vigorous program of crater research must be maintained to resolve the remaining unanswered questions.
2.4. Eolian Processes
Many physical and chemical processes modify planetary surfaces. Eolian is defined (Gary et al., 1972) as "pertaining to the wind; esp. said of rocks, soils, and deposits (such as loess, dune sand, and some volcanic tuffs) whose constituents were transported (blown) and laid down by atmospheric currents, or of landforms produced or eroded by the wind, or of sedimentary structures (such as ripple marks) made by the wind, or of geologic processes (such as erosion and deposition) accomplished by the wind."
Thus, any planet or satellite having a dynamic atmosphere and a solid surface is subject to eolian processes. A survey of the solar....
Surface pressure (mb)
9 x 104
Mean temperature (°C)
Composition of atmosphere
Surface gravity ()
Minimum threshold friction speed (cm/sec)
Particle diameter most easily moved by winds (µm)
....system shows that at least Earth, Mars, Venus, and possibly Titan meet these criteria (Table 1). In this section we review our knowledge of eolian features and activity, consider the relevance to planetology of understanding eolian processes, and discuss the means for studying these processes in the planetary context.
2.4.1. Review of Eolian Activity on the Planets
Wind blowing across a planetary surface has the potential for directly eroding material and redistributing it to other areas. Most eolian erosion occurs through abrasion caused by windblown particles of sand size or smaller. Winds transport sediments via three modes: suspension (mostly silt and clay particles, i.e., smaller than about 60 µm), saltation (mostly sand-size particles, 60 to 2000 µm in diameter), and surface creep (particles generally sand size and larger). The ability of wind to move particles is a function of atmospheric density and velocity (low densities require stronger winds than high densities), particle size, shape, and density, and other less important factors. Wind threshold curves define the minimum wind speeds required to initiate movement of different particles for given planetary environments (Bagnold, 1941).
Earth. On Earth, eolian processes occur principally in arid regions, near shorelines, and wherever strong slope or thermal winds occur (e.g., near glaciers). Because vegetation tends to retard near-surface winds and trap windblown sediments, eolian processes are enhanced and in some cases become dominant in vegetation-free regions, which constitute a large percentage of Earth's solid surface. Eolian deposits occur throughout the geologic column and include  ancient sand seas and loess deposits. Given that sands can be important reservoirs for petroleum, knowledge of form and structure of eolian deposits is of practical importance.
Because the extent of desert areas changes with time and climate, there is great economic and social interest in understanding the factors (including eolian processes) that are involved in causing such changes. The term "desertification" is used to describe the process by which areas are converted to deserts. In the past decade much research has focused on desertification, in part coordinated by UNESCO.* Fundamental to this research is an understanding of the mechanics of eolian processes-knowledge that is required also for extrapolations to other planetary environments.
Mars. Windstorms were believed to occur on Mars even before the successful space probes in the early 1970s returned conclusive evidence of eolian activity. Earth-based observations beginning in the nineteenth century showed seasonal albedo patterns that were attributed to a variety of causes, including schemes which involved dust storms and other eolian processes (Briggs et al., 1979).
As our knowledge of the composition and density of the martian atmosphere improved, theoretical predictions of the wind velocities required to set particles in motion were made. Because of the low atmospheric density on Mars, the estimated minimum wind speeds were about an order of magnitude higher than those on Earth (Sagan and Pollack, 1969). Wind tunnel tests conducted under low atmospheric pressure to simulate martian conditions substantiated these early estimates (Greeley et al., 1976, 1980a).
With the arrival of Mariner 9 and the Soviet spacecraft Mars 2 and 3 at Mars in 1971 during a major global dust storm, the speculations concerning the efficacy of martian eolian processes were amply confirmed. After the dust cleared, the Mariner 9 cameras revealed abundant features attributed to eolian activity, including sand dunes, yardangs (wind-sculpted hills), various pits and grooves, and albedo patterns on the surface (termed variable features) that changed in size, shape, and position with time. After the Mariner 9 mission it was suggested by some (e.g., Sagan, 1973) that the rate of eolian erosion on Mars is very high, based on three factors: (1) the high wind speeds required for particle motion on  the planet, (2) the high frequency of dust storms documented by telescopic observations from Earth, and (3) the variety and large number of surface features attributed to wind erosion and deposition. It was reasoned that sand grains, once set into motion by the wind, would be accelerated to high speed and would be very effective in "sandblasting" the surface.
The Viking mission has added substantially to the inventory of martian eolian features, including the discovery of the vast north polar sand sea, equal in size to the Sahara sand sea on Earth. Examination of the highest-resolution Viking Orbiter images (better than about 15 m per line pair) shows that sand dunes occur in most parts of the planet (Thomas, 1982). Viking Landers showed, for the first time, the windswept surface of Mars, including views of eolian deposits and rocks that appear to be wind eroded. The Viking mission also caused a reassessment of wind erosion rates on Mars. Although Viking verified the widespread occurrence of eolian features, several lines of evidence now suggest that the rates of eolian processes may not be as high as previously thought. For example, more than four years of monitoring wind speeds at the two Viking landing sites show that the near-surface winds seldom attain threshold speeds. More importantly, the Viking Orbiters reveal numerous surfaces that have small (~10 m), fresh-appearing impact craters; their presence signals surfaces at least hundreds of millions of years old that have been modified very little by erosion of any type. Thus, there appears to be a conflict between the predicted high rate of eolian erosion on Mars and the constraints posed by the Viking results. Resolving this conflict is important for understanding Mars as a planet, since the answer is related closely to many important questions, such as:
Venus. The dense atmosphere of Venus appears capable of sustaining eolian processes. Venera landers measured wind speeds of about 2 m s-1 at two locations, and extrapolations of measurements obtained by the Pioneer Venus probe at a third location yield values  of about 1 m s-1. Theoretical studies and extrapolations of wind tunnel experiments to the venusian environment suggest that these values are well within the range required to initiate particle movement on the surface (Greeley et al., 1980b).
Venera images of the surface of Venus show a bimodal distribution of particle sizes-coarse fragments of cobble size (several centimeters and bigger) and fine-grained material. It has been suggested that the fine-grained material has been transported by wind and that the coarse fragments have been shaped by wind erosion.
Although the requirements for eolian activity on Venus appear to be met-winds capable of moving grains and a supply of particles in the appropriate size range-the degree to which eolian processes modify the surface at present and how effective they have been in the past remain to be studied.
Titan. Voyager measurements have shown that this icy satellite has a thick nitrogen-rich atmosphere. The surface temperature and pressure are 90 K and 1.6 bars, respectively. Whether the atmosphere is dynamic and whether granular particles exist on the probably icy surface are unanswered questions.
Thus, Earth, Mars, probably Venus, and possibly Titan are all subject to the same basic eolian processes (erosion, transportation, and deposition of windblown material), but the surface environments are strikingly different. These differences afford the opportunity to investigate a basic geologic process in a comparative sense. Because terrestrial processes and features have been studied for many years, Earth remains the primary data base. However, because surface processes are much more complicated on Earth-primarily because of the presence of liquid water and vegetation-many aspects of eolian processes that are difficult to assess on Earth are more easily studied on other planets. For example, on Earth cohesion among particles resulting from moisture is difficult to separate from that resulting from electrostatic charges; in the comparatively dry atmosphere of Mars, effects of water are less important, and the general problem is made simpler.
2.4.2. Relevance of Eolian Studies to Planetary Geology
Eolian processes can redistribute enormous quantities of sediment over a planet's surface and form features large enough to be seen from orbit. On Earth deposits of windblown sediments can be hundreds of meters thick. Any processes capable of effecting such  changes must be relevant to understanding the present and past geologic environment of a planet. Furthermore, because eolian processes involve the interaction of the atmosphere and lithosphere, an understanding of eolian activity sheds light on meteorologic questions. In this context, eolian activity can be examined in terms of large-scale modification, small-scale modification, and observable active processes, such as dust storms and changing surface features.
Large-scale modification of a surface can be defined as involving features on scales that can be observed from orbiting spacecraft. Such features can be either erosional or depositional. By far the most useful eolian feature for interpreting eolian processes is the dune, a depositional landform. A recent document edited by McKee (1979) describes the major sand deposits on Earth and discusses various dune forms; because much of the book is based on spacecraft images, it is particularly relevant to planetology. Both the planimetric shape and cross-sectional profile of dunes can reflect the prevailing winds in a given area. Thus, if certain dune shapes....
 ....and/or slopes can be determined from orbital data, local wind patterns can be inferred. Repetitive viewing of the same dunes as a function of season may reveal seasonal wind patterns. Although dunes seldom change significantly in plan form with season, the slip face on the dune crest can be observed to show the influence of alternating wind directions.
Identification of dunes demonstrates the presence of sand-size (60 to 2000 µm) particles because sands are the only materials known to accumulate in dune deposits on Earth (Bagnold, 1941), and there is nothing in the physics of windblown sediments to suggest that the situation would be different on Mars, Venus, or Titan.
On Earth great quantities of silt and clay are transported in dust storms. This material is eventually deposited as vast sheets or mantling units, and is commonly termed loess. Loess deposits exceeding hundreds of meters in thickness are found throughout the geologic column. Even when relatively young and well exposed, loess deposits are nearly impossible to identify by remote sensing. Yet identification of such deposits could be very important in understanding planetary surfaces. For example, substantial areas of Mars are interpreted to be mantled with ancient eolian sediments. However, other processes could also lead to terrains of similar appearance. Thus, some definitive means of identifying fine-grained eolian sediments by remote sensing is needed.
Large-scale eolian erosional features include (1) pits and hollows (called blowouts) that form by deflation or removal of loose particles, (2) wind-sculptured hills called yardangs, (3) wind-eroded "notches" in ridges, and (4) general, unclassified wind-eroded landforms. Some of these features can be diagnostic of bedrock materials. For example, although yardangs have been reported in crystalline rocks, they form most commonly in friable, nonindurated clastic deposits such as loess.
Impact crater frequency distributions are widely used in planetary geology to obtain relative dates for different surfaces (chapter 3). On planets having active eolian processes, the erasure of craters by erosion or burial by eolian sediments can drastically alter the crater record and invalidate crater-derived ages. Thus, a knowledge of rates of eolian erosion and deposition for a wide range of planetary environments is required in order to assess the possible effects on the impact crater record.
 Small-scale eolian features include ventifacts (wind-shaped rocks)-features that can be observed only directly on the ground. Ventifacts can provide information about local wind directions and the length of time a surface has been exposed. Identification of ventifacts has relevance to other aspects of planetology. For example, rocks at the Viking landing sites that show pitted surfaces have been interpreted as vesicular igneous rocks and form part of the basis for identifying the surrounding plains as volcanic; alternatively, the pitting could be the result of eolian erosion.
Eolian processes can both mix and sort sediments. Deposits consisting of a wide range of particle sizes, such as river sediments or glacial deposits, when subjected to winds may have the coarser particles left behind, leading to "desert pavement" surfaces. Conversely, windblown dust derived from a wide range of rocks may become compositionally "homogenized" in dust storms and settle over extensive areas. In either of these cases (wind-sorted or wind-mixed deposits), remote sensing data could lead to erroneous conclusions about the composition of the areas observed. Knowledge of how eolian processes operate under a wide range of planetary environments and recognition of identifying characteristics are critical to the problem.
Observations of certain eolian features can provide direct information about the atmosphere. For example, crater streaks on Mars are albedo patterns on the surface that show surface wind direction; they occur in great numbers over much of the planet's surface. Repetitive imaging of these and other variable feature patterns has shown that many of them disappear, reappear, or change their size, shape, or position with time. Mapping the orientations of variable features has been used to provide information on the near-surface atmospheric circulation of the martian atmosphere at different seasons (Veverka et al., 1977; Thomas et al., 1979).
Investigation of past eolian regimes on Mars is an important aspect of studying the surface history of the planet. Although the orientations of most eolian landforms have been related to the current atmospheric circulation patterns, the degree of erosion and deposition suggested by the morphology and size of many of these features cannot be explained completely by the contemporary eolian regime. Arguments for major variations in atmospheric conditions due to changes in the obliquity and orbit of Mars suggest that the wind efficiency has fluctuated widely throughout the planet's history...
. Large-scale ancient deposition and possible active deflation are evidenced by features such as the partially exhumed craters and the massive, layered polar deposits. The nonuniformity of wind direction inferred from yardangs, an erosional landform, within a single  region is further evidence of these fluctuations. Since the stratigraphic history of the planet is closely related to the large-scale variations of past eolian regimes, these must be determined as precisely as the preserved record permits.
2.4.3. Suggested Approach for Investigating Eolian Processes
Since eolian processes incorporate elements of geology, meteorology, physics, and chemistry, a unified study requires knowledge in all these areas or a multidisciplinary approach. The approach commonly used is to isolate parts of the eolian process for detailed study, as was done in the pioneering work of Bagnold ( 1941), who analyzed the physics of windblown sand. Once the fundamental principles are understood, it is possible to extrapolate to a wide range of conditions, i.e., other planetary environments. Before this can be done, however, it is necessary to fit the results from the studies of isolated parts of the problem back into the system. For example, Bagnold's work on threshold winds for particle movement was carried out principally in wind tunnel studies; before making generalizations, he field tested the results using natural sands. In the study of planetary eolian processes, we seldom have the opportunity to field test our extraterrestrial ideas. Thus, we must rely on a somewhat different method, which can be outlined as follows:
The benefits of this approach are not only that it provides a logical means for understanding extraterrestrial problems, but that it contributes toward solving eolian problems on Earth as well.
Some of the key questions that should be addressed through ongoing and future research in planetary eolian phenomena include:
How are dust storms initiated? This question applies to both Earth and Mars. Dust storms involve very fine-grained materials (typically a few micrometers in size). The "threshold curve" shows that extremely strong winds are required to move such fine material, making it unlikely that such grains are placed into suspension by direct entrainment. The explanation frequently given is that sandsized grains (relatively easily moved) placed into saltation impact dust particles, dislodging them from the surface and subjecting them to entrainment by the wind. This scenario does not explain frequent dust storms on Earth that begin in areas lacking sand. Other factors may be involved, perhaps related to electrostatic effects, clumping of fine particles into sand-sized aggregates, or other triggering mechanisms. This is a general problem that has received too little attention despite its importance to eolian studies on both Earth and Mars.
What are rates of eolian erosion in various planetary environments? Despite much interest in eolian erosion (both deflation and abrasion), relatively few studies have been done even on Earth that provide quantitative data, much less that are relevant to other planets. There are two parts to the problem: one deals with erosion on a small scale (e.g., individual rocks), and the other concerns erosion of landforms (e.g., crater rims).
What is the evolution of eolian landforms? Many studies have been carried out that define stages of development for various eolian features on Earth. In some cases there is disagreement over both the results and the interpretations. For example, some investigators  consider certain types of longitudinal dunes to have evolved from other dune forms and to represent a mature stage of eolian activity; the apparent absence of longitudinal dunes in the north polar region of Mars would therefore signal a relatively recent development of dune-forming processes. However, before such an interpretation can be made, two important questions must be answered: first, is the basic idea valid, and second, does it apply to Mars? This and other models of the evolution of eolian landforms should be tested on Earth, and studies should be done to determine how to apply them to extraterrestrial environments.
What are the compositions of eolian sediments on various planets? On Earth, most eolian sediments are quartz sand and silts, various clay minerals, plus minor amounts of particles of other compositions (gypsum sands, calcite sands, etc.), and most of the knowledge of the physics of particle motion and eolian features such as dunes deals with these materials. If windblown particles exist on Titan, they may be composed of methane ice or other frozen volatiles. The composition of eolian particles on Venus is completely open to speculation; on Mars, quartz is probably absent or is only a minor constituent because major source rocks for quartz, such as granites, are apparently absent. Given the widespread occurrence of basaltic lava flows, the low albedo of martian dunes, and the prevalence of mafic to ultramafic compositions suggested from spectral studies, it is believed that basaltic sands are common on Mars. How do windblown basaltic sands behave? What is their evolution? How efficient are they as agents of abrasion in the eolian environment? Do they accumulate in deposits having morphologies different from those made of quartz particles? At what rate do they weather into clay-like material under martian conditions? These and other questions are intimately linked with understanding the general sedimentary cycle on Mars. Similar sets of questions can be asked about Venus and possibly Titan.
What are the eolian conditions on Venus? The physics of windflow and particle movement in the high-temperature and high-pressure atmosphere of Venus involves flow regimes not previously investigated. Even such fundamental questions as threshold wind speeds can only be estimated crudely at present.
How has the eolian environment on Mars changed through time? There is ample evidence that changes in Mars' climate have occurred in the past (Carr, 1981; Baker, 1982) and that these changes have  been accompanied by variations in the eolian cycle (e.g., changes in the number and severity of global duststorms [Pollack and Toon, 1982]). Although the program outlined above does not directly address such questions, it should lead to a better understanding of the physics of eolian processes, which in turn should make it possible to begin addressing these more complicated problems during the coming decade.
2.5. Fluvial Processes
On Earth, worldwide surface reservoirs, recirculating ground water, and atmospheric cycles form the so-called hydrologic system. This system, which drives most surface geologic processes and sustains the operations of the biosphere, has operated as long as we can trace geologic history. In 1972 Mariner 9 revealed the widespread existence of "large channels" on Mars, which were generally interpreted as evidence for the presence of significant quantities of liquid water on the planet's surface at some time past. Today Mars has a hydrologic cycle, but one greatly reduced in intensity and vigor from that inferred to have existed in the distant past. There are reservoirs of frozen water in the polar caps and possibly extensive areas of ground ice. The polar caps could be part of a global hydrologic system consisting of a poleward atmospheric flow of water vapor and an equatorward return migration of ground water.
Although water is certainly the most abundant, and perhaps the only, liquid responsible for fluvial processes in the solar system, more exotic possibilities cannot be ruled out. For example, Voyager results suggest that fluvial processes involving liquid hydrocarbons could occur on the surface of Titan. Yet it must be admitted that any discussion of fluvial processes at the present time must concentrate on Earth and Mars as locales, and on liquid water as the agent.
2.5.1. Water as a Geologic Agent
Water is a dominant geologic agent on Earth, operating on scales from the global to the molecular. A true appreciation of the evolution of a planet such as Mars requires that we investigate the role of water in its geologic system as well. The primary questions to be answered are:
This section outlines how such questions might be answered by summarizing what has been learned and what potentially might be learned in four closely connected areas: (1) conceptual studies of sediment dynamics, (2) theoretical and laboratory modeling of fluid flow and sediment transport, (3) mapping of surface geology on Mars, and (4) field studies of analogs on Earth.
2.5.2. Dynamics of Fluvial Sedimentation
Water acts to dislodge sedimentary particles through current shear, local channel bed pressure differentials (Bernoulli lift force), and inertial impacts (raindrops or grains saltating in the fluid). Once initiated, the patterns and intensity of sediment transport are controlled by many factors, including fluid shear stress, turbulence,  viscosity of the water/sediment mixture, size distribution of the particles, and the rate of sediment supply to the transport systems. The interaction of all these variables in terrestrial sediment transport systems is fairly well understood. However, this knowledge has been acquired largely through observation and experiment rather than through basic theoretical physical analysis. Given this semi-empirical basis, we are poorly equipped to transfer the information to other planets, on which the physical parameters may differ from those on Earth. On the other hand, asking questions about sediment dynamics on other planetary bodies forces us to penetrate deeper into the physics of the problem.
Current investigations of general fluvial sediment dynamics on other planetary surfaces (specifically Mars) include studies of (a) the relative role of wash load versus bed material load, (b) the effect of gravity acting on fluids in different environments, (c) the rate of evaporation from the fluid surface, (d) the efficiency of particle entrainment, and (e) the capacity for bedrock erosion by cavitation and macroturbulence.
Sediment transport in a fluvial system is usually separated into bed material load and wash load based on the settling velocities of the particles. Bed material load is that fraction of the total load which is locally derived from the bed. Wash load, on the contrary, is that sediment population which is moved through the river system entirely in suspension.
The distinction is essential because the relative magnitudes of the two populations are controlled by entirely different factors (Shen, 1971). The bed load is governed by the transport capacity of the river. Transport rates can be calculated once the river hydraulics are known. For wash load, on the other hand, the actual amount being transported is governed by rates of supply or production of the drainage basin. The wash load cannot be calculated from river hydraulics. Wash load transport requires little or no expenditures of stream power; therefore, very fine particles may be carried in nearly unlimited quantities.
Because the sediment transport mode is largely a function of the ratio between particle settling velocity and the shear stress acting on the bed, both of which depend on the acceleration of gravity, rivers on Mars will have transport characteristics different from those on Earth. Komar ( 1980) suggests that the floods inferred to have carved the large equatorial channels on Mars could....
.....have transported cobbles in suspension and carried all sand-sized material as wash load. The figure above compares the modes of transport of quartz-density grains on Mars and Earth.
In another investigation of the effects of reduced surface gravity, Komar ( 1979) compared submarine channels on Earth to the large outflow channels on Mars. Due to buoyancy, the effective acceleration of gravity (g) acting on a deep-sea turbidity current is reduced to about 1.4 ms-2, from the normal subaerial value of 9.8 ms-2. On the surface of Mars (g = 3.7 ms-2) water flow would take place in a gravity field intermediate between that obtaining for river flows on Earth and that governing deep-sea turbidity currents. Therefore, deep-sea channels provide important terrestrial analog systems to test the effects of gravity on channel formation.
One unknown parameter in all hydraulics calculations for Mars is the atmospheric environment at the time of active channel flow. If atmospheric conditions were anything like those at present, one would expect rapid evaporation and perhaps the formation of a layer of surface ice or ice fragments. Calculations by Wallace and Sagan ( 1977) suggest rapid formation of a meter-thick ice layer,  with possible continued flow of liquid water underneath for many hundreds of kilometers. This work suggests that the observed channels could have been formed under climatic conditions not very different from those present today.
Nummedal (1977) studied the case of particle entrainment into flow over a noncohesive bed of sediments. Using the Shields entrainment function and scaling for martian gravity, he found that initiation of movement on Mars for particles ranging in size from I cm to 10 m will occur for shear stresses only 25 percent of those required on Earth. In the case of entrainment from a cohesive or lithified channel bed, both fluid and bed characteristics determine the efficiency.
Baker (1978) has argued that the martian surface may provide conditions particularly favorable for bedrock erosion by combined action of cavitation and macroturbulence. There is, however, a question about the effectiveness of cavitation as an erosional process on Mars if channel flow occurred under the present low atmospheric pressure.
In spite of such investigations, it is not possible at present to calculate the erosion rates, predict the characteristic channel bedforms or even the viscosity of the fluid that actually carved the channels. The theoretical work on fluvial sediment dynamics must be continued, and experimental investigations under conditions similar to those existing on Mars aimed at testing directly various theoretical predictions should be initiated.
2.5.3. Channel and Valley Morphology on Mars
Studies of basic sediment dynamics have revealed that, due to differences in the acceleration of gravity and ambient pressure, sediment entrainment and transport processes on Mars should differ significantly from those on Earth. Yet we are a long way from theoretically predicting the ultimate channel form based on hydraulics and sediment loads. Consequently, the approach taken to explain the origin of channels on Mars is to investigate terrestrial analogs, correctly appraise their mode(s) of origin, and then test, in light of differences in flow dynamics on Mars, the applicability of the terrestrial models.
In this discussion a distinction is made between channels and valleys. Channels refer to large sinuous or curvilinear depressions that contain direct evidence of fluid flow, such as linear grooves,  streamlined islands, and other bedforms on their floors. Channels may or may not have tributaries; they may originate at a point or in a diffuse source area. Valleys refer to networks of generally smaller linear depressions. Although no bedforms are visible, valleys carry indirect evidence of being caused by fluid erosion, including integrated network patterns and tributary junction angles consistent with flow down the regional slope.
In the decade since their initial discovery, the channels on Mars have been variously attributed to nearly every conceivable fluid: water, ice, clathrate, wind, lava, and mud. The proliferation of models is in part due to the search for pure morphologic analogs, with little attention being given to dynamic similitude, and in part due to construction of theoretical models unconstrained by terrestrial experience. The current consensus is that wind and lava must be ruled out as a primary agent for the formation of channels on Mars. Therefore, the study of the origin of channels and valleys on Mars becomes synonymous with fluvial studies, taken in its broadest sense to include investigations of ground water and debris flows as well as river systems (Baker, 1982).
The two primary constraints on models for the channels on Mars come from (1) extensive geomorphologic mapping (Baker, 1978, 1982), which has demonstrated that large floods were responsible for the primary formation of many channels, and (2) theoretical studies that indicate that surface flow on Mars might have exhibited a dynamic similarity to terrestrial flows in submarine settings (Komar, 1980).
Subaerial terrestrial analogs are presently studied much more extensively than submarine ones within the Planetary Geology program, although Nummedal and Prior (1981) have argued that the terrestrial submarine environment-specifically the Mississippi Delta front-provides valuable analogs for a wide range of instability features associated with some of the large channels on Mars. Investigations of catastrophic flood features, moderate-discharge braided stream features, mudflows, and investigations of the origin of deeply incised bedrock canyons have all provided analogs to selected channels or valleys on Mars (Baker, 1982). Literature reviews of the morphologic imprints of large pleistocene glaciers in arctic Canada suggest that glacial action could also produce many of the fine-scale features observed within the large martian outflow channels (Lucchitta et al., 1981). Field investigations of these features...
....from the viewpoint of martian analogs are desirable, as is continued work on submarine analogs to martian outflow channels (Numme a and Prior, 1981) .
Among ideas on the origin of the large channels on Mars, the catastrophic flood hypothesis has gained the most widespread acceptance (Masursky, 1973; Baker and Milton, 1974). The channeled scabland of the Columbia Plateau in Washington is generally considered to be the best terrestrial analog (Baker and Nummedal, 1978). The scale of the scabland channels, the anastomosing system, the dry falls and cataracts, the linear channel floor grooves and streamlined hills all have their inferred equivalents on Mars.
Three theories have been put forth to explain possible catastrophic release of water on Mars. Soderblom and Wenner (1978) proposed that headward retreat of a scarp or small channel tributary intercepts a subsurface fluid reservoir, causing sudden dramatic increase in the flow rate and overburden collapse. Carr (1979)....
 ....presented a detailed analysis of a model relating ground collapse and fluid release to the venting of a subsurface overpressure aquifer, and Nummedal (1978) suggested that liquefaction of a metastable subsurface sedimentary unit might be the origin of some of the chaotic terrain. According to this last hypothesis, the mud released would have been the primary channel-forming agent.
Studies of catastrophic flood features in Alaska (Thompson, 1980) and Iceland (Malin, 1980) have generally strengthened the flood hypothesis for the origin of Mars channels. Yet, as demonstrated by Boothroyd (1980) in a comprehensive study of quaternary landscape development on the arctic slope of Alaska, many smaller-discharge braided streams that frequently change their active channels of flow may produce erosional features strikingly similar to the flood-generated streamlined shapes encountered in the channeled scabland. This fact demonstrates that no single geomorphic feature or channel bedform in isolation is diagnostic of a process or origin. Rather, the entire assemblage of related features must be correctly interpreted in modeling the evolution of a landscape.
Perhaps the most important questions regarding the martian valley networks are their mode of origin and what this origin implies for the history of the martian climate. The ramified pattern of these valley systems has provoked comparisons with terrestrial drainage networks, particularly with those formed primarily by runoff associated with rainfall. This analogy has fueled the controversy of whether or not it has ever rained on Mars. Despite the evocative nature of hypotheses for martian rainfall, close scrutiny of valley interiors and network planimetric morphologies reveals features that are strikingly different from those associated with terrestrial rainfall-runoff drainage networks. A closer affinity in morphology appears to exist between the martian valleys and those formed in the terrestrial environmental by basal sapping or seepage-fed runoff (Pier), 1980; Baker, 1982).
Martian valley networks are distinctive in the notable absence of the dendritic pattern so common to terrestrial streams (Pier), 1980). This pattern is characterized by a nearly uniform distribution of tributary directions and filling of the available intranetwork space. Many martian valley systems show pronounced parallelism and lack of tributaries in undissected intervalley terrain, thus appearing to be sparse relative to most terrestrial systems. Differences  of pattern with scale, along with system parallelism, most probably result from the introduction of fluid into the system from a restricted headward source region.
The presence of steep-walled, theater-like valley terminations suggests that headward extension by undermining and wall collapse ("sapping") may be an important process in valley formation on Mars (Pier), 1980; Baker, 1982). Field studies in the Colorado Plateau and Hawaii have identified morphologic characteristics useful in differentiating valley systems cut by sapping from those produced by surface runoff. Sapping mechanics and wall retreat have been examined as functions of bedrock Ethology, surface versus subsurface flow rates, and rock structure (Laity, 1980). Future work should emphasize detailed mapping and interpretation of martian networks and experimental and theoretical studies. The distribution, complex history, and origin of the valleys on Mars should provide a prime focus for continued fluvial studies.
Attempts to define the temporal evolution of the fluvial systems on Mars utilize both the standard terrestrial stratigraphic principles of superposition and a technique unique to extraterrestrial planets: relative age dating based on the frequency of impact craters. Carr (1980) and Masursky et al. (1980) summarize the current knowledge concerning the temporal evolution of the fluvial systems. According to Carr (1980), nearly all Mars valleys are found in the old densely cratered terrain. The valley morphology ranges from barely visible depressions to linear and curving troughs that are quite crisp in appearance. Their exclusive occurrence within densely cratered terrain suggests that they are all quite old. Crater counts on the small valleys themselves cannot be performed with accuracy, but the youngest, extensively dissected intercrater plains have around 3300 craters (>1 km) per 106 km2, suggesting an absolute age of around 3.5 billion years. Plains formed since that time have few valleys or none at all. Conditions on Mars during all but its very early history appear to have been unfavorable for the formation of valley networks.
Channels, on the other hand, dissect the old cratered terrain as well as many younger units. Channel floors are in many cases large enough to permit crater counting. The crater dating suggests that large martian channels range in age through most of geologic history. Some highly degraded channels might be correlative with the  episode of valley formation. Others, for example, the big channels around Chryse Planitia, may date back to mid-martian history. Ares, Vedra, and Tiu Valles could be among the most recent geologic features on Mars.
Stratigraphic studies internal to each channel system are also important because they can reveal whether a channel was formed by one or multiple events. Few such detailed studies have been made so far, yet the basic information is available in the extensive Viking Orbiter data base.
Although the possibility of exotic fluvial processes (e.g., on Titan) must be. kept in mind, to date evidence of fluvial processes has only been found on Earth and Mars. For Mars, fluvial studies have provided important clues to the planet's evolution and to its budget of volatiles. While many questions remain, a few answers are emerging. Large quantities of liquid water did exist at the time of primary channel formation. Yet, since there are no visible shorelines, significant amounts of liquid water never accumulated in low-lying basins. The observed geology is consistent with limited outgassing on Mars, equivalent to an amount that would cover the surface to a depth of some tens of meters at most, and with the conclusion that it never rained on the planet. Yet some mechanism for the recharge of the global ground water system is suggested, although the hydrologic system may be very different from the terrestrial one (e.g., Clifford, 1980). The development of integrated dynamic and time-stratigraphic models for the release, transport, and accumulation of fluid and associated sediment in the major channels should be given high priority. Even more important is the total integration of valley networks and channels into a general understanding of dissection, denudation chronology, paleoclimatology, and tectonic history on Mars. Comprehensive studies are needed to relate Mars' fluvial history to the planet's climatic past and to the evolution of its atmosphere.
2.6. Mass Movement
Mass movement is one of the most universal geologic processes operating on planetary surfaces. Rock falls, landslides, and creep are evident in most crater walls, and debris flows, rock slides, and  slumps have been identified on the Moon and Mars and probably occur on many other planetary bodies. Mass movement may occur on any slope where the force of gravity exceeds the cohesive strength of the surface material. It operates regardless of the presence or absence of water, atmosphere, or tectonic activity. The style of mass movement, however, and the role it plays in shaping the surface features of a planet depend on the nature of surface materials and variables such as the degree of saturation of pore space with fluids, etc. Many of the variables affecting mass movement on Earth (where water is the dominant pore fluid) have been studied thoroughly with reference to slope stability and engineering problems. In order to understand mass movement on other planetary surfaces, the basic factors that control the process must be identified and their relative importance ascertained under specific environmental conditions.
Among the more significant factors affecting mass movement in Earth's system are steepness of slopes, strength of surface materials, presence of pore fluids, weakening of surface material by shock, alternating cycles of freezing and thawing, stratigraphic and structural relationships, and the size and shape of particles that constitute surface material. Some of these factors apply to mass movement on all planets, whereas others may be important only under special conditions that exist on a specific planet. Therefore, it is probable that much can be learned about the mechanics of the process by a comparative study of mass wasting throughout the solar system.
2.6.1. Mass Movement on Earth
On Earth, a variety of types of mass movement are recognized on the basis of (1) rate of motion, (2) kind of motion, and (3) material involved. The more significant types include (cf. Carson and Kirby, 1972) rock falls, rock slides, debris slides, debris flows, creep, block slides, slumps (landslides), solifluction, rock glaciers, and subaqueous sand flows. The presence of water at Earth's surface is perhaps the most important single factor influencing mass movement. In this respect Earth is unique among the planets. Mass movements on Earth are relatively small compared to some that have been observed on Mars (Carr, 1981). Few terrestrial landslides exceed 10-15 km in length, and many are so small that they are difficult to observe even on high-resolution aerial photographs.
 2.6.2. Mass Movement on the Moon and Mercury
The Moon exhibits a variety of types of mass movement (Lindsay, 1976). Slumping of the inner walls of craters displays many of the classical features of slump blocks on Earth. In addition, some craters show flow features that suggest downslope movement of regolith. On steep slopes, features similar to rock falls and avalanches are apparent. Also, tracks and scars resulting from boulders rolling and sliding downslope have been identified in many areas.
On the Moon as on other bodies that lack atmospheres and active tectonic systems, impacts are the major disturbing forces producing mass movement. Some of the unanswered questions concerning mass movement on the Moon include the following: (1) What is the role of mass movement in effacing small craters on slopes in the lunar highlands ? (2) Does the cumulative effect of numerous small impacts exceed that of the rarer large events in inducing mass movement? (3) Have slopes on the lunar surface reached a state of equilibrium? (4) To what extent does slope retreat occur on the Moon?
Although crustal materials on Mercury and the Moon are approximately similar, gravity on Mercury is about twice that on the lunar surface. Thus, it would be informative to compare analogous expressions of mass movement on the two bodies. Unfortunately the best images of Mercury have resolutions no better than 0.5 km, making such comparisons impossible at present. Nevertheless, the coverage does contain evidence of mass movement: slump terraces on the inner walls of craters and on the faces of escarpments. Significantly perhaps, no mass movement features comparable in scale to the largest such features on Mars have been detected on Mercury. Although both planets have similar surface gravities, the martian regolith may well be saturated locally with ice, whereas that of Mercury is almost certainly dry.
2.6.3. Mass Movement on Mars
The surface of Mars shows abundant examples of mass movement, many of which are enormous compared to those on Earth. Huge landslides occur within large impact craters, in Valles Marineris, and on the flanks of the large volcanoes (Sharp, 1973; Lucchitta, 1978; Carr, 1981). Spectacular longitudinal troughs and ridges on the lower parts of many landslides, as well as the backward rotation of slump blocks, are clearly seen. In addition, debris....
....flows along the regional escarpment, separating the southern highlands from the northern plains, show long flow lines. Important questions concerning mass movement on Mars include the following: (1) Why are some of the features produced by mass movement so large? (2) Is the melting of subsurface ice involved in some of these flows? (3) What is the relation of the chaotic terrain and the catastrophic floods believed to have produced the major channels?
2.6.4. Mass Movement on Other Bodies
The Galilean satellites present some interesting new problems in understanding mass movement as a planetary process. Callisto, Ganymede, and Europa are icy bodies that have comparatively low relief, probably due to plastic flow of the ice that makes up their  crusts. On Callisto, mass movement may be represented by deposits along scarps in the Valhalla ring system. On Ganymede mass movement may be involved in the evolution of the grooved terrain by filling of extension fractures and graben. Prominent scarps exist on lo, and mass movement must occur on the surface of this sulfur-covered satellite. Many of the icy satellites of Saturn are now known to have rugged surfaces that exhibit high relief. What types of mass movement occur on their cold, icy surfaces? Unfortunately, at present we lack the high-resolution images needed to address these questions, but in the case of the Galilean satellites at Ieast, such data should be provided by the Galileo mission to Jupiter.
Mass movement is one of the few universal geologic processes operating on planetary surfaces. We know from studies on Earth that perhaps the single most important factor influencing mass movement is the presence of water. Water infiltrates the pore space, acts as a lubricant, and adds weight to the slops material. Another factor is alternating freezing and thawing, which rapidly breaks down the solid rock into angular fragments. Yet in the surfaces of other bodies of the solar system (with the possible exception of Mars), water does not exist in a fluid state. One of the more significant problems is to determine how features produced by mass movement on other planetary bodies (especially the Moon and Mercury) can resemble those on Earth without the presence of air and water. In the absence of water, what are the most significant factors affecting mass movement? What role does temperature play in mass movement on the icy planetary bodies? Is mass movement an important surface process on small bodies such as asteroids, comets, and small satellites? Evidence of mass movement on the two tiny satellites of Mars has already been detected (Thomas and Veverka, 1980) even though surface gravity is only 1O-3g.
There is also the important question of slope retreat. On Earth, slope retreat is intimately associated with a drainage system. The slope can be thought of as an open system with a major input of material from the steeper slopes, movement of material downslope, and removal of material by the drainage system. How does slope retreat operate on Mars or lo, where gigantic scarps are known to exist? What role does sapping play in slope retreat on Mars? Do martian winds accelerate this process?
 2.7. Glacial and Periglacial Processes
This section deals with planetary surface features and processes associated with cold regions and ice, and unless noted otherwise, the word ice refers to water ice throughout. Because the terminology involved is often misunderstood, this section begins with a review of definitions (from Gary et al., 1972):
A review of the various environments in the solar system shows that all planets and satellites except Venus experience temperatures below freezing. Glacial and periglacial processes definitely occur on Earth and on the icy satellites of the outer planets. On Mars, ground ice almost certainly exists, and periglacial processes probably play a major role in the evolution of the planet's surface.
2.7.1. Physical Properties of Water Ice of Interest to Geology
Water ice is a major constituent of many solid bodies found beyond the asteroid belt. Yet all of our knowledge about the natural properties of ice comes from observations under terrestrial conditions (e.g., Mellor, 1964). Data on the physical properties of natural ice at higher pressures and lower temperatures than encountered on Earth's surface are urgently needed (Parmentier and Head, 1979a). Some information on density, strength, rheology, and thermal behavior is available from laboratory experiments; other data must be estimated by extrapolation.
Pure water ice occurs in a wide range of polymorphs (Glen, 1974, 1975), designated Ice-I through Ice-lX. Ice-I occurs naturally on Earth and presumably Mars and many of the outer planet satellites. Ice-II, -III, -V, -VI, and -VII have been formed in the laboratory  at pressures greater than 2 x 103 bars. Ice-IV, -VIII, and -IX are considered metastable phases. High-pressure forms of ice are presumed to exist in the interiors of large icy satellites, and a knowledge of the physical properties of these polymorphs is essential to a more complete understanding of the evolution and surface history of these objects.
The density relation of pure water and Ice-I is anomalous. Pure water expands about 9 percent upon freezing, and this expansion can set up large stresses at the margins of a constrained body of water such as an aquifer or a filled crack. The expansion also results in a significant decrease in density, from about 1.0 g cm-3 at 4°C to 0.92 at 0°C. Thus, the freezing of water can lead to a gravitationally stable configuration consisting of low-density ice floating on top of liquid water. If the surface of such an ice layer were to be covered by silicate material derived from meteoritic impact, for example, the density could increase sufficiently for foundering and sinking of the ice to occur (Parmentier and Head, 1979b). In contrast to Ice-I, the high-pressure polymorphs all have specific gravities greater than that of pure water. Formation of Ice-I at depth in a planet's interior could lead to the rise of diapirs to the surface.
The thermal properties of Ice-I and water are also unusual. The specific heats of water (1 cal g-1) and Ice-I (0.5 cal g-1) as well as the latent heat of fusion (79.7 cal g-1 at 0°C) are abnormally high (Fletcher, 1970). Hence water or ice in a system may act as a heat sink, modulating wide fluctuations in temperature within large bodies of ice or water. In addition, the melting point of Ice-I decreases as pressure increases. This mechanism is considered responsible for regelation, a process by which solid particles can pass through ice. Sufficient pressure on one edge of a solid particle, usually due to its own weight, lowers the ice melting point and causes melt water to flow from the leading to the trailing edge of the particle, where it refreezes in the lower-pressure region.
Mechanical properties of polycrystalline ice depend on the orientations of individual crystals. On Earth, polycrystalline ice has three main structural forms: (1) randomly oriented, (2) columnar with c-axes perpendicular to column length, and (3) columnar with c-axes parallel to column length. In addition, many complex shapes, orientations, and arrangements of crystals exist in highly stressed regions of ice. Some mechanical anisotropies are inherently associated with the hexagonal structure of individual Ice-I crystals. Bulk properties such as elastic moduli may be calculated fairly accurately  by averaging the moduli of individual crystals. Other rheologic properties of polycrystalline ice are strongly anisotropic, and averaging of individual crystal properties may not be valid.
Surface features on an icy or ice-saturated body result largely from processes of flow and fracture. Although ice is often modeled as a Newtonian viscous fluid, experiments indicate that it is more properly considered a power-law or pseudoplastic fluid that deforms by creep (e.g., Glen, 1974, 1975). In a Newtonian fluid, the rate of strain is linearly proportional to the applied stress, and the viscosity is the ratio of strain proportional to the stress raised to some power, so that as the stress level is increased, the material deforms more and more rapidly. The result is that a power-law fluid like ice appears to become less viscous at higher rates of strain. Ice-I has a power-law exponent of about 3.1 measured for stresses in the range of 10 to 100 Pa (Glen, 1974, 1975); similar data for high-pressure polymorphs are not yet available. The viscosity of ice at a given stress level decreases with increasing temperature. Thus, in the presence of a thermal gradient, the viscosity will decrease with depth. Ice also has a yield strength, a level of stress that must be exceeded before any permanent deformation can occur. Generally, this strength is so low that it can be ignored.
Under very rapid strain rates, such as during an impact event, ice behaves more like a brittle elastic material than a fluid and may be assigned a tensile strength. The tensile strength of Ice-I increases slightly from about 12 bars at 0°C to about 16 bars at 40° C (Croft et al., 1979). These values compare with about 75 bars for granite to a few hundred bars for basalt. The tensile strength of ice-saturated sand is less than that of pure ice near the freezing point, but becomes much greater at lower temperatures ranging from around 3 bars at 60°C to an extrapolated value of 50 bars at -40°C. Thus, ice will tend to fracture easily under surface and near-surface conditions of Earth and Mars, but should become quite strong at the very low temperatures that prevail on the surfaces of bodies in the outer solar system.
The above discussion has summarized some of the physical properties of pure water ice that are of major interest to the planetary geologist. A fundamental complication is that the ice encountered on any real surface will seldom, if ever, be pure. As already indicated even small amounts of impurities affect some of the physical properties of ice (e.g., its strength) quite drastically.
 2.7.2. Ice Masses in the Solar System
In this section we consider ice masses on planetary surfaces and for simplicity subdivide these into glaciers (originating at least partly on land from precipitation) and floating ice (derived primarily from bodies of liquid water).
On Earth, glaciers are classified as either valley glaciers (or Alpine glaciers) if they occupy valleys within mountains or as ice sheets (also called continental glaciers or ice caps) if they are large masses not contained by valleys. Most glaciers of both types are composed of two parts, an accumulation area where snowfall exceeds melting each year and a wastage area where melting exceeds snowfall; the volume gained and lost in these two areas constitutes the "glacial budget" as described by Sharp (1960) and determines whether the glacier will advance or "retreat." All glaciers move downslope or outward, and a "retreating" glacier simply means that the melting and ablation in the wastage area exceeds the rate of forward movement by the glacier.
On Earth, precipitation of snow in the area of accumulation forms a deposit that is about 20 percent ice and 80 percent air. Melting and refreezing plus compaction converts the snow to spherical ice particles called firn. As the firn accumulates, further compaction causes a recrystallization to form the main ice mass of the glacier that typically has less than 10 percent air.
Movement of terrestrial glaciers occurs through three mechanisms: (1) slipping of the mass over the surface; slipping is usually lubricated by a melt water layer between the ice and the ground, (2) plastic flow (ice is a viscous medium), and (3) fracturing and sliding, in which blocks of ice in the brittle parts of the glacier break apart and move forward; this occurs principally on the surface and along some edges of the glacier. Movement of the ice mass is seldom uniform in either space or time, and differential deformation leads to surface features, some of which are large enough to be observed on aerial photography. The principal surface features are crevasses, which are elongated cracks. Crevasses are subdivided into various types as functions of their geometry and mode of origin. Some of these patterns can provide clues to the deformational history of the glacier.
 Most glaciers incorporate rocky materials within the ice mass. This material can include dust and other airborne particles, as well as chunks of rock gouged by the ice as it moves across a surface or from the accumulation of debris that falls, onto the glacier from valley walls. Surface material often coalesces into medial moraines that show as dark parallel stripes reflecting flow by the glacier. On Earth, material carried by the ice eventually reaches the melting front of the glacier, where it is released. It may be deposited in situ, or carried away by melt water. The finer material is often picked up by the strong winds generated along ice margins. At least some loess deposits (windblown silts) are considered to have glacier origins. Coarse glacial deposits, termed drift, may assume a wide range of geometries that can be used to interpret the form and position of glaciers after the ice mass has "retreated."
On Earth, glaciers have effected extensive changes in the landscape. U-shaped valleys, grooves, and striations parallel to the flow of ice, and amphitheater-shaped cirques in the headward parts of valleys are indicative of glacial erosion and can be seen on aerial and orbital images. In our solar system only Earth and Mars (and possibly Titan and Triton?) can have glaciers originating at least in part from precipitation. Features similar to ice sheets definitely occur in the polar regions of Mars: the permanent or residual ice caps (Murray et al., 1972; Cutts et al., 1976; 1979). Although valley glaciers have not been found on the planet, some of the high-latitude surface features may in part owe their existence to past glacial processes. At lower latitudes there are numerous channels (section 2.5) that have been interpreted by some investigators as having been cut by ancient ice streams associated with ice sheets (Lucchitta et al., 1981).
Most satellites of the major planets have surfaces dominated by ice, making glacier-like activity a possibility insofar as the relaxation of crater topography is concerned. Theoretical considerations of the rheology of ice indicate that static loading, such as in crater walls, could cause movement when the strength of the ice is exceeded; observed surface temperatures for the larger Jovian satellites (Ganymede and Callisto) imply that topographic relief should be modified noticeably by relaxation within about 106 years (Johnson and McGetchin, 1973). Voyager images reveal both fresh, normal-appearing craters and circular features ("palimpsests"), suggesting that some craters "relaxed" and flowed, whereas others have.....
.....remained preserved (Smith et al., 1979b). In ice, the relaxation time of topographic features is very temperature dependent due to the strong dependence of viscosity on temperature (Parmentier and Head, 1979b); thus, the gradual cooling of crustal temperatures could explain the nonrelaxation of more recent craters (Reynolds and Cassen, 1979). Statistical studies of craters and palimpsests can be used to infer the thermal history of the crust of an icy satellite (Phillips and Malin, 1980; Passey and Shoemaker, 1982; Passey, 1983).
B. Floating Ice
On Earth, ice that forms from freezing of liquid water includes both sea ice and ice on fresh water bodies; both are generally referred to as floating ice (Weeks and Assur, 1967; Weeks, 1967). A  wide range of surface features can form on floating ice, depending on the freezing sequence and the influence of exogenic processes such as deformation by winds.
Many of the deformational features observed in floating ice may be analogous to features observed on some of the icy satellites, most notably Europa. The most common deformational surface features on sea ice are linear arrays of broken ice called pressure ridges. Ridges may form either from shear when two adjacent ice flows move parallel to one another along fractures or cracks, or from the collision of two ice floes moving toward one another. Ice ridges are composed of blocks of fractured ice. Voids between the blocks can cause the densities of the ridge to be less than that of solid ice. The porosity of the ridge in the initial stages of formation varies from 10 to 40 percent.
2.7.3. Periglacial Processes on Planetary Surfaces
On Earth, the term periglacial refers to a specific climatic zone (Davies, 1969) in which the processes of solifluction (the slow, viscous, downslope flow of water-saturated, unconsolidated materials), gelifluction (the flow of ice-saturated materials; Washburn, 1980), and nivation (the erosion of rock or soil by snow and ice, by frost action, and by chemical weathering) are characteristic, and within which such geomorphic features as permanently frozen ground (permafrost), patterned ground, and thermokarst topography are readily developed (Stearns, 1965, 1966). The occurrence of a periglacial region is not genetically related to the proximity of glaciers or continental ice sheets, contrary to what is implied by its etymology (e.g., Washburn, 1980). This broader definition is useful in that it allows us to consider the possible operation of periglacial-type processes on the surfaces of other objects in the solar system.
A. Inner Planets
The presence and action of water are essential for most periglacial processes to occur. Destruction of lithified and unconsolidated material by nivation, the transport of this material by solifluction or gelifluction (mass wasting), and the subsequent sorting and redistribution by freeze-thaw mechanisms to form ice wedges, pingos, ground ice, and patterned ground are all characteristics of a relatively wet environment. On Earth, the resulting morphologies are generally well studied (e.g., King, 1976), but the extension of these.....
.....terrestrial processes and landforms to other planetary surfaces is a broad topic of current interest.
The possible existence of water ice in nearly permanently shaded regions at high lunar latitudes has been suggested by Arnold (1979). Extensive permafrost-bearing plains units have been inferred for Mars, particularly in the northern latitudes (Carr and Schaber, 1979). This deduction is based on observation and interpretations of mass wasting, some types of polygonally patterned ground, and the radically striated, apparently fluidized ejecta blankets surrounding many craters. Some of these landforms appear to  be more highly developed in the higher latitudes and lower elevations, where the amount of water is inferred to be greater.
Photogeologic study of possible martian periglacial landforms is an integral part of the study of Mars and provides insight into basic physical processes that act on the planet's surface. Detailed study of terrestrial periglacial features provides the "ground truth" necessary for more efficient analysis of both the landforms themselves and the related planetwide processes on Mars. Some of these processes include water redistribution among regolith, polar deposits, and the atmosphere; of subglacial and subaqueous volcanics; phreatomagmatic eruptions and the production, deposition, modification and redistribution of pyroclastic deposits; and mass wasting and rheology of crater ejecta deposits formed from volatile-rich materials. On the other hand, knowledge gained from the Mars Viking Orbiter data may prove useful in interpreting Landsat images of the periglacial, high-latitude regions of Earth.
B. Outer Planet Satellites
The Voyager 1 and 2 encounters with Jupiter in 1979 provided a wealth of information about the four Galilean satellites, Io, Europa, Ganymede, and Callisto. With the exception of Io, all are considered to have complex icy crusts that may contain sizable fractions of silicates. Thus, the intriguing possibility exists for mass wasting and surface modification processes similar to those in terrestrial glacial and periglacial terrains. A major problem is the low resolution of available images, in which features smaller than about 500 m cannot be recognized.
Many of the Saturnian satellites imaged by Voyager are topographically rugged (Smith et al., 1981). However, periglacial and associated mass wasting processes could be inhibited by the cold surface temperatures (<100 K) and the low gravitational accelerations on these satellites.
Processes similar to glacial and periglacial processes on Earth probably occur on other solid bodies in the solar system. In all cases studied to date, water ice is the important ice. Therefore those physical properties of water ice of fundamental interest to planetologists must be determined, especially under conditions that pertain to studies of outer planet satellites (at low temperatures for  surface studies; at high pressure for interior models). The effects of impurities (other ices or silicates) on these properties are of crucial importance.
Studies of the behavior and surface morphology of glaciers may have some significance to investigations of the polar caps of Mars and perhaps to some features on the surfaces of icy satellites. Investigations of the characteristics and surface morphology of floating ice may be of relevance to the study of those icy satellites (e.g., Europa) which, at some point in their histories, may have had liquid mantles covered by thin ice crusts.
Periglacial studies are of great relevance to Mars, where periglacial processes were dominant in sculpturing the surface in many areas, and perhaps to investigations of the surfaces of icy satellites in the outer solar system.
Although to date the emphasis has been on water ice, it must be realized that the properties of other ices (methane, ammonia, and their clathrates) will become increasingly important to planetary geology as our exploration of the outer solar system continues. By 1986 Voyager 2 will have studied the satellites of Uranus, bodies whose surfaces may not consist of water ice only. Even today we have enough precise data about Titan to suspect that its surface consists, at least in part, of frozen methane. Finally, it should be recalled that on Mars deposits of carbon dioxide frost occur at least in the annual polar caps; further investigations of the physical and mechanical properties of frozen carbon dioxide and of its clathrate form are highly desirable.
* United Nations Educational, Scientific, and Cultural Organization.