[143] Traditionally, progress in planetary geology has resulted from an active interplay between the study of the surfaces of distant objects and detailed field work, laboratory investigations, and supporting remote sensing measurements here on Earth. We expect this close interrelationship to continue in the future.
The purpose of this chapter is to outline some directions that we see such studies taking during the next decade in support of the planetary geology goals outlined in the preceding chapters. The three major areas involved: (1) terrestrial analog studies in the field; (2) laboratory measurements and simulations; and (3) remote sensing observations, are discussed in turn in the following sections.
8.1. Analog Studies
In this report, we have laid great stress on understanding planetary processes by analogy with those observed on Earth. This procedure has obvious advantages and has proven its value during the past twenty years. First, Earth's surface is far more accessible for detailed investigation of common processes than that of any other planet. Second, due to this ready accessibility, the range of applicable techniques and instrumentation is far wider. Third, in view of the different surface environments involved, detailed comparisons of similar processes on Earth and other planets often make it possible to assess the relative importance of the many variables that might affect a particular process. Specific examples of important analog studies that should be carried out are described in chapter 2 and chapter 4. The Planetary Geology Working Croup stresses the fundamental importance of such analog studies to the continued [144] vigorous development of our field. As our understanding of the planets increases, our ability to ask incisive questions grows: the process is an evolving one.
8.2. Laboratory Studies
In many scientific disciplines, laboratory simulations and experiments serve as an essential bridge between observation and theory. This remark is especially true of many branches of planetary geology, in which time scales ranging from microseconds to billions of years, processes varying across scales of micrometers to the dimensions of the solar system, and environments as disparate as the surfaces of asteroids and of Venus must be treated. The exploration of the solar system continues to challenge present theories and models with observations of processes that are beyond our immediate experience and intuitive grasp. In such cases, laboratory work is of utmost importance. Some examples, chosen from different areas of planetary geology, are discussed below.
8.2.1. Cratering
Insofar as craters are the most ubiquitous landform in the solar system, it is not surprising that impact cratering was one of the first planetary geology processes to be simulated in the laboratory. In particular, the light gas gun at the NASA Ames Research Center (e.g., Gault et al., 1968) was designed and built specifically for this purpose in the early 1960s. It remains the principal facility for impact cratering experiments under simulated planetary conditions and serves as an excellent example of a laboratory facility designed to investigate a fundamental planetary process. Not only can projectile size, shape, composition, velocity, and impact angle be controlled, but a wide variety of targets can be used in a vacuum chamber equipped with high-speed motion picture equipment. Additionally, targets can be dropped at various accelerations during impact experiments to simulate different gravitational fields, ejecta kinematics and characteristics can be documented through various techniques, and the resulting craters can be preserved and dissected to investigate subsurface phenomena.
Work at this facility and the results of impact and explosion experiments elsewhere have yielded extremely important data on how various parameters influence the cratering process (Roddy et al., 1977). These results, in turn, have been applied to the study of [145] much larger craters. However, to date, the bulk of the experiments involved rocky materials. Following Voyager's investigations of the satellites of Jupiter and Saturn, there is an immediate need to carry out similar experiments to understand cratering in ice and ice-rich targets. Comprehensive programs to determine the equations of state of planetary materials under high shock stress conditions will also contribute toward our overall understanding of this ubiquitous planetary process.
8.2.2. Volcanic Processes
It is clearly difficult to simulate volcanic processes in the laboratory. Much attention must be paid to detailed scaling to guarantee effective modeling. Before reliable volcanic models can be constructed, the properties of magmas, lavas, and hot solids must become better known over the wide range of complex conditions of interest in planetary processes. Volatile content, eruption temperature, density, specific heat, yield strength, thermal conductivity, and temperature dependence of viscosity are but a few of the quantities necessary to describe the physical state of a volcanic liquid or a hot particle suspension. As might be expected, only a few of these variables can be controlled at any one time during a modeling experiment.
In spite of these difficulties, laboratory models such as those being developed by Greeley and co-workers (Womer et al., 1980) provide the potential to understand in much greater detail the phenomena associated with planetary-scale silicate volcanism. Such work should be extended to other eruptive processes (sulfur volcanism on Io, ice volcanism on the icy satellites, etc.).
8.2.3. Tectonics
Because of their large scales, tectonic features are difficult to simulate in the laboratory. Nevertheless, laboratory-derived data on the behavior of various geologic materials are important in constraining hypotheses that attempt to explain tectonic forces, mechanisms, and the resulting structures. Information on the mechanical properties of various materials of planetary interest under diverse conditions are needed. For example, in the context of the outer planet satellites we need information on low-density materials such as ice, ice-clathrates, salts, and ice-rock mixtures at very low temperatures. In the context of Venus we need data on the properties of hot silicates over a wide range of confining pressures.
[146] 8.2.4. Fluvial Processes
As a result of various engineering, geomorphologic, and hydrologic studies, a wealth of fluid mechanical data exists for direct application to planetary problems of fluid flow. Yet there remain many planetary geology questions concerning fluvial processes the can most readily be answered by laboratory studies. For instance what would be the characteristics of a liquid methane flow on the surface of Titan? What might have been the behavior of a suspension of ice particles and silicate grains in turbulent flow on the surface of Mars? How would the erosional efficiency of such flows vary as a function of substrate structure, slope, and other variables?
8.2.5. Aeolian Phenomena
The details of wind-related processes are often so complex that they require laboratory simulations to describe them adequately. Although the flow patterns around geometrically uniform objects have been studied thoroughly, this is certainly not the case for Irregularities associated with craters, hills, canyons, and other geologic features. Thus, the resolution of conflicting interpretations of specific erosional and depositional features on Earth and Mars will rely on scale-model or other simulations.
Environments that will be equally challenging in terms of eolian processes are those presented by Venus and Titan. Although the surface environment of' Titan remains poorly known, the eolian regime on Venus is reasonably well defined. Specialized wind tunnel facilities currently under development should contribute substantially to our understanding of wind-related erosional and depositional phenomena under Venus' extreme pressure and temperature conditions (e.g., Greeley et al., 1980b).
8.2.6. Cometary Processes
Cometary nuclei are bodies with which we have no geologic experience. Their inferred weak, volatile-rich structure and exceedingly small gravitational fields undoubtedly give rise to unusual processes that have no immediate analogs on the larger, more familiar bodies of the solar system. Due to the current lack of hard data on the properties of comet nuclei, it is difficult to suggest specific topics for laboratory studies at the present time.
8.2.7. Regolith Processes
Laboratory simulations provide unique means of studying regolith and weathering processes (e.g., Huguenin, 1973a, 1973b, 1978).
[147] Three types of' questions can be addressed by such experiments:
For many planetary bodies the environmental parameters are not known in detail. More importantly, the interactions of the sample and environment are complex. Consequently, more than one set of experimental parameters will often produce reasonable agreement with the ground-truth data within the constraints of our current knowledge of environmental conditions.
The applicability of the results of simulation studies will depend directly on how accurately the environmental conditions can be reproduced in the laboratory. These conditions include temperature, pressure, composition of the gas atmosphere, the spectral distribution and flux of the electromagnetic radiation impinging on the surface, and the normal planetary variations in these parameters. It is not necessary or even desirable, however, to exactly reproduce some of the environmental parameters if they can be scaled.
8.3. Remote Sensing
Remote sensing measurements have played a key role in the development of our ideas about the geologic nature of the planets and satellites in our solar system. Although such observations, either from Earth or from a spacecraft, do not fall within the traditional techniques of geology, they nevertheless continue to provide essential data about the surface environments of the bodies we study.
Conventionally, remote sensing techniques are classified to first order by the spectral range in which they operate and then according to the spectral resolution they employ. On such bases, one finds terms such as photometry, colorimetry, spectral reflectance, spectroscopy, infrared radiometry, radar, etc.
Radar and radio remote sensing have provided important data of geologic interest and are discussed in a separate section below. Remote sensing of X-rays and y[gamma]-rays from lunar orbit have provided essential information about the distribution of rock types on the Moon (Adler et al., 1972; Arnold et al., 1972). Infrared radiometry [148] has provided unique data about the texture of planetary and satellite surfaces from observations such as the eclipse cooling an posteclipse heating rates of the Galilean satellites (Morrison, 1977 It has provided one of the most reliable means of determining the sizes and albedos of asteroids and other small, remote object (Morrison and Lebofsky, 1979). It has been used to monitor the extent of volcanic activity on Io, and. in one of its most sophisticated developments to date, the Viking IRTM* experiment (Kieffer et al., 1976), it provided numerous invaluable pieces of information about Mars: surface temperatures, textures, atmospheric opacity, etc.
Since gases display much sharper spectral features than .solids the highest spectral resolution measurements (spectroscopy) are often devoted to investigations of atmospheric composition and structure, topics that are of interest to planetary geology only insofar as they relate to the surface environment of the planet, rates of weathering, and possibly to rates of outgassing. Nevertheless. spectral measurements at high wavelength resolution have produced and will continue to produce, some data of crucial importance to the geologic studies of the planets. Examples include the Mariner 9 IRIS** spectral constraints on the composition of the dust in Mars' atmosphere (Hanel et al., 1972), the detection and monitoring of sodium, sulfur, and other clouds in the vicinity of Io's orbit (Pilcher and Strobel, 1982), as well as the very high resolution infrared spectra of satellites and asteroids obtained by Larson and Fink, Pincher, Ridgway, and others (e.g., Pilcher et al., 1972, Fink et al 1973; Larson and Veeder, 1979). Our basic knowledge that many of the satellites of the outer planets have surfaces made of water ice comes from such observations (Sill and Clark, 1979; Cruikshank, 1979, 1980).
Generally speaking, the spectral region between 1 and 5 micrometers (µm) is the most diagnostic in terms of composition of solid surfaces. Important Earth-based measurements in this region include the detection of S02 frost as a major constituent of Io's surface (Fanale et al., 1979; Nash and Nelson, 1979) and the discovery of methane frost on the surface of Pluto (Cruikshank et al. 1976). Such measurements have also been used to demonstrate that the surface of Ceres, the largest asteroid, is similar in composition to some type 2 carbonaceous chondrites and that it certainly [149] contains water of hydration and perhaps even particles of surface frost (Lebofsky, 1978, 1981). The; have also been used in attempts to match the spectral colors of the dark side of lapetus with those of Phoebe (Cruikshank et al., 1982) and in attempts to understand the composition of the residual caps on Mars (Clark and McCord, 1982). Fundamentally new information will come when instruments such as the Galileo Orbiter NIMS*** will be able to make spectrally resolved maps of objects within the 0.7 to 5 µm window.
Most of the remote sensing observations shortward of 1 µm can be conveniently divided into photometry, colorimetry, and spectral reflectance measurements. The actual wavelength range covered by most of these observations is about 0.3 to 1.2 µm, due to a combination of three main factors: transparency of Earth's atmosphere, spectral range of common detectors, and spectral distribution of the Sun's energy. Generally speaking, measurements below 3000 Å, although of crucial importance to the remote sensing of atmospheres, are not of prime importance to the study of solid surface. However, important exceptions exist; for example, the detection of S02 frost on the trailing hemisphere of Europa by Lane et al. (1981) using observations made by the IUE**** spacecraft.
Photometry of planetary and satellite surfaces may be defined as the study that aims to determine how a particular surface scatters incoming sunlight (e.g., Veverka, 1977). The result is usually expressed in the form of a photometric function, which may or may not depend significantly on the wavelength. Understanding how the surface of an object scatters incident sunlight is one of the fundamental parameters about it that we must know. Such knowledge is needed to determine accurately the albedos and colors of the various surface units and is a prerequisite to the construction of meaningful albedo and color maps. Precise albedos and photometric functions are also needed to calculate accurate values of surface temperatures, which are essential to discussions of the stability of frosts on various surfaces (e.g., SO2 frost on Io, H20 frost on the surface of Callisto, etc.). Accurate albedos may also be useful in answering important specific questions, such as, is Phoebe dark enough to be the source of some of the dark material on the leading hemisphere of lapetus (Thomas et al., 1983)?
[150] Photometric measurements also provide information on the texture of surfaces. Although there is some debate on how uniquely such information can be extracted from photometric data, there i. no question that such determinations are useful in a comparative sense. Thorpe (1978) has attempted to derive the relative textures of certain eolian features on Mars from their photometric behavior. whereas Bowell and Lumme (1979), as well as others, have used disk-integrated measurements to compare the surface textures of various minor planets. Information on the texture of surfaces has also been inferred in the past from measurements of the degree of linear polarization of the scattered sunlight, but in recent years such polarization measurements have been used mostly to estimate the albedos, and hence sizes, of numerous minor planets (Zellner,1979)
Colorimetry is the term used to describe broadband measurements of the colors of planets or of individual features on their surfaces. As the spectral resolution of such measurements increases colonmetry merges into spectral reflectance measurements. Typical spectral resolution in spectral reflectance measurements is between 0.02 and 0.1 µm, the common spectral interval covered being approximately 0.3 to 1.2 µm. Among the very important spectral reflectance measurements made during the past decade, one must single out the work of McCord and his many co-workers dealing with the Moon, Mars, asteroids, and satellites (e.g., McCord et al 1982a, b). Such data have been used to correlate spectral properties with the geologic character of lunar surface units and features (Adams and McCord, 1970; Head et al., 1978; Pieters et al., 1980, 1981), and have served as the basis for inferring the compositions of many asteroids (Chapman and Gaffey, 1979; Gaffey and McCord 1979). Perhaps the most famous single result obtained by this technique was the identification of the surface of Vesta as corresponding to a basaltic achondrite (McCord et al., 1970). The important information that there is a gradation in asteroid composition with increasing distance from the Sun in the asteroid belt also comes in part from such measurements (Gradie and Tedesco, 1982).
Even though our progress in the direct exploration of the planets and satellites has been phenomenal during the past fifteen years, the role of remote sensing in studying these objects cannot be expected to diminish in the future. We cannot expect to send spacecraft to all the bodies in the solar system about which we [151] require information. This comment applies especially to the multitude of small bodies, asteroids, and comets discussed in chapter 7. We also must anticipate that the power of remote sensing techniques will continue to grow in the future, as will the ability of such techniques to address important new questions in the context of planets and satellites that our spacecraft have visited in the past. Inevitably, most spacecraft in the past have carried remote sensing instruments of interest to planetary geology. Certainly, this trend will continue in the future.
8.3.1. Remote Sensing Using Earth-Based Telescopes
In spite of the continuing direct exploration of planets, there remain key remote sensing observations to be made by Earth-based and airborne telescopes working in the ultraviolet, visible, and infrared parts of the spectrum. The following brief list is meant to be illustrative, rather than exclusive. The PGWG ***** has made no attempt to attach relative priorities; the discussion is given in order of increasing distance from the Sun.
Moon. We expect the Moon to continue to be a focus for many remote sensing efforts, both in support of increasingly detailed geologic investigations and as the diagnostic sensitivity of our techniques continues to improve. These efforts will continue to provide important new information (e.g., Pieters, 1981).
Mars. Efforts to monitor the overall appearance of the surface of Mars (extent, color, and albedo of major markings; maximum extent and recession of the polar caps), as well as the state of the atmosphere (dust storms, polar hoods, general opacity) should continue. Additional spectral mapping of individual regions is also needed.
Mars Satellites. The major outstanding data that can be obtained using state-of-the-art instrumentation are accurate spectra of Phobos and Deimos between 0.4 and 1.2 µm (at least). The optimum time to carry out such measurements will be during the favorable oppositions of the mid-1980s.
Asteroids. Statistics on Earth- and Mars-crossing asteroids should be augmented. The gathering of high-quality spectral reflectance curves of minor planets should continue, with special attention to faint but important objects such as the Trojans, Apollos, and Atens. [152] Improved determinations of asteroid sizes, shapes, and spin rates are of interest in understanding the collisional evolution of the asteroid belt.
Io. A major effort is needed to monitor lo's volcanic activity by means of infrared radiometry (Simon, 1980). Complementary high resolution spectroscopy as a function of orbital longitude could yield information on possible changes in the areal extent of SO2 frost deposits m response to variations in volcanic activity
Outer Planet Satellites. Fine single most important task in this are; is to obtain high-resolution spectra in the visible and infrared to define surface compositions as well as possible. Whenever practicable, such data should be obtained as a function of orbital longitude to look for surface heterogeneity. Special attention should be devoted to the little-studied faint satellites. For example, some of the important questions concerning the small satellites in the Jupiter system that can be addressed are: (1) How similar are the surfaces of the outer satellites, and how do they compare with those of' the Trojans? (2) How similar are the surfaces of the newly discovered small inner satellites, and how do they compare with that of Amalthea?
Pluto System. So little is known about the Pluto/Charon system that any sort of quantitative information is highly desirable. Since Pluto's distance from the Sun changes appreciably, it is important to monitor this planet over periods of decades to see if any noticeable changes in its lightcurve or in the amount of CH4 in its atmosphere occur in response to changing insolation.
Comets. From the point of view of planetary geology, it is comet nuclei and their possible evolutionary connection with certain types of asteroids that are of major interest. Good statistics on the sizes and rotation rates of cometary nuclei are needed for comparison with asteroid data, as are high-quality spectral reflectance measurements.
8.3.2. Observations from Earth Orbit
Many of the objectives outlined in section 8.3.1 can also be addressed by instruments in Earth orbit. However, due to potentially higher spatial resolutions and spectral coverage, such instruments can carry out additional investigations that are not possible from the Earth's surface. As an example, we discuss some observations of high interest to planetary geology that could be carried out by Space Telescope in the second half of the 1980s.
Space Telescope is a 2.4 m Ritchey-Chretien reflecting telescope that, together with a complement of five optical science instruments, is scheduled to be placed in orbit (approximately 400 km above Earth's surface) in 1986. The first-generation set of instruments includes two imaging systems, the wide field/planetary camera and the faint-object camera; two spectrographs, the faint-object spectrograph and the high-resolution spectrograph; and a high-speed photometer. In addition, the fine guidance subsystem can be used for astrometric measurements. A complete description of the Space Telescope science instruments can be found m Bahcall and O'Dell (1979).
The imaging capabilities of the Space Telescope will exceed those of ground-based telescopes both in spatial resolution and spectral coverage (except in the infrared region of the spectrum). For example, the planetary camera has a spectral range extending from 1150 to 11 000 Å and a picture element (pixel) size that [154] equals 40 km/AU when projected on the target. Because the modulation transfer function (MTF) of the planetary camera CCD****** is so superior to the vidicons flown on Mariner, Viking, and Voyager spacecraft, the actual resolution of a single frame is likely to be equivalent to that of a vidicon operating at 60 rather than 80 km/ line pair/AU. Due to Space Telescope's subpixel pointing accuracy, it will be possible to reach resolutions of 40 km/line pair/AU by adding several images of a target.
The planetary camera can carry out many observations of high interest to planetary geology. Examples include the following.
Mercury. Less than half of Mercury was imaged by Mariner 10. Space Telescope could observe Mercury near greatest elongation (typical distance, ~ 1 AU) with a resolution of about 40 km/line pair. At this resolution it should be possible to map features larger than about 100 km, including large craters, basins, and plains units.
Io. Space Telescope provides a unique opportunity to monitor volcanic activity on lo and associated changes in surface albedo patterns. Io can be observed from a range of 4 to 5 AU, corresponding to resolutions of 160 to 200 km/line pair.
Asteroids. Main belt asteroids can be observed at a range of about 1.5 AU, yielding resolutions of 60 km/line pair. The image of a 300 km asteroid would subtend 10 pixels, whereas Ceres, the largest of the asteroids (diameter = 1000 km), would span 33 pixels. Such images can provide unique information on the shapes of the larger asteroids as well as on the albedo heterogeneity of their surfaces. Space Telescope will also provide an unequivocal answer to the question of whether multiple asteroids really exist.
Mars. Mars can be observed at ranges of 0.5 to 1.5 AU, corresponding to resolutions of 20 to 60 km/line pair, adequate to monitor global atmospheric activity as well as seasonal variations in the polar caps and albedo markings.
8.3.3. Radar and Radio Observations
Although early radio measurements provided some important information about the planets (e.g., the temperature of the surface of Venus), recently the more exciting results have come from radar observations. The list of impressive firsts includes measurement of spin rates for Mercury, Venus, and several asteroids; altimetry of Mercury, Venus, Moon, and Mars; imaging of Venus; and determi [155] nation of the surface scattering characteristics of Mercury, Venus, Moon, Mars, Galilean satellites, Saturn's rings, and several minor planets (Pettengill, 1978; Pettengill and Jurgens, 1979; Campbell and Burns, 1980; Ostro, 1982, etc.). It should also be noted that radar ranging provides the basis of many highly accurate ephemerides.
At present, there are two facilities in the United States that have the capability to conduct radar astronomy observations: the National Astronomy and ionosphere Center in Arecibo, Puerto Rico, and the NASA Deep Space Instrumentation Facility in Goldstone, California. The two observatories differ in several respects. Arecibo is equipped with L- and S-band radars and Goldstone with S- and X-band. The Arecibo S-band system is more sensitive by about 7 db than the Goldstone S-band system. Whereas the Goldstone system can track targets from horizon to horizon, the Arecibo coverage is limited to 20° about the zenith.
From Arecibo, Venus and Mercury will be visible periodically during the 1980s, but Jupiter and Saturn will be largely inaccessible; Mars will be a difficult target until late in the decade, but the Moon and numerous minor planets will be readily observable. Mapping of Venus will continue to complement the Pioneer Venus observations, and in support of future missions. Backscatter maps, topography, and surface properties may also be obtained for Mercury, especially in those areas for which Mariner 10 imaging does not exist. Acquisition of the Mars topographic data will continue. New experiments (initiated at Arecibo in 1980) on depolarizing and diffuse scattering properties of Mars' surface should provide new data on the distribution of small-scale (meter-sized) surface structures.
Radar observations of comets and asteroids will also be pursued vigorously at Arecibo throughout the 1980s. (By 1982 over a dozen asteroids and two comet nuclei had been investigated from Arecibo.) These observations will yield basic parameters such as size, shape, and scattering characteristics of these objects. They may also distinguish iron-rich from iron-poor regoliths and set stringent constraints on the existence of satellites of minor planets.
Since the Goldstone antenna is steerable, every solar system object visible in the northern sky is, in principle, available. Goldstone observations of Mercury in the 1980s should yield topographic profiles in the planet's equatorial region and refined determinations of the planet's figure and spin vector.
[156] Three major experiments involving Venus are under consideration at Goldstone: (1) continued S-band tristatic observations, which would yield high-resolution (down to I km) topographic and back-scatter maps of the equatorial regions; (2) dual S- and X-band altimetry along the sub-Earth track to improve planetary ephemerides and verify both the Pioneer Venus altimetry and the Goldstone tristatic altimetry; and (3) use of the X-band to detect possible precipitation in the Venus atmosphere.
Goldstone ranging of Mars started in 1971, and by the mid 1980s one complete cycle of latitude coverage will have been completed. One full cycle of martian oppositions, spanning a period of fifteen years, is required to complete coverage of' the ± 23° latitude band visible to radar from Earth. The X-band observations will have a higher signal to noise ratio, whereas the dual S- and X-band measurements may provide insights into the fraction of surface covered by wavelength-sized scatterers.
The Goldstone radar, if modified to become operational at Xband, would provide the only facility to observe Galilean satellites during the entire decade of the 1980s. These X-band observations would complement the existing Arecibo S-band data. In addition, the Goldstone radar could provide further data on Saturn's rings.
To complement the radar observations that are anticipated, a vigorous program of supporting theoretical and laboratory work is needed. For example, theoretical work on the polarization and high-angle scattering behavior of rough surfaces should be combined with high-quality laboratory experiments on the properties of analog materials to improve our ability to infer geologic characteristics from radar data. In terms of laboratory measurements, investigations of the radar reflectance properties of silicate/metal regolith analogs are needed urgently to fully interpret the accumulating set of asteroid observations.
**Infrared interferometer spectrometer.
*** Near-infrared mapping spectrometer.
**** International Ultraviolet Explorer.
***** Planetary Geology Working Group.
