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Twenty multispectral sets of calibrated coregistered Voyager 2 images of the Uranian satellites are analyzed in order to identify spectrally distinct surface units and associate these units with specific endogenic or exogenic surface processes. Several regions on each satellite where significant spectral/compositional diversity could be expected were identified by morphologic unit mapping. All the Uranian satellites were found to exhibit some degree of spectral variability associated with bright impact craters, with the typical trend being that the bright craters and their associated ejecta deposits are less red than the remainder of the satellites by 5 to 30 percent. Oberon shows the greatest variability, followed by Titania and Ariel. Morphologic features on Miranda and Ariel possibly associated with endogenic processes such as tectonism or ice volcanism do not exhibit as high a degree of spectral variability relative to their surroundings as bright impact craters.

The paper presents imaging spectroscopic data and preliminary interpretations acquired during the 1988 perihelic opposition of Mars at the highest spatial resolution achieved from earth for spectrometric measurements (all contiguous spatial sampling for nearly 70 percent of the planet) and at a higher spectral resolution (R = 350) than most previous measurements in the (0.4-1.1)-micron wavelength range. Absorption features appearing near 0.6-0.7 and 0.80-0.95 micron are attributed to Fe(3+) crystal field transitions, confirming previously made measurements at lower spectral resolution. It is shown that attempts to reconcile the overall spectral shape with those of laboratory minerals or Mars analog soils spectra affirm the recent interpretations that hematite may account for nearly all of the 18 wt pct Fe2O3 measured by Viking Landers.

Ground-based telescopic reflectance spectra and analyses of Viking data have led to the conclusion that Martian surface fines are derived from a mafic to ultramafic source material, though the actual composition of these surface fines is still undetermined. A variety of possible compositions were proposed by several authors, however, with salts, particularly sulfates and carbonates, as important components in many of these models. Although salts are predicted components of the Martian surface, they are not identified in telescopic reflectance spectroscopic observations to date. One explanation for the absence of salts in these data is that they are not in quantities large enough to detect. This study was undertaken to determine the detectability of salts in various proposed Martian surface analog mixtures. This study concentrates on the carbonates and sulfate spectral features in the 2.5 to 25 micrometer spectral range because of recent work showing the usefulness of weak bands in the mid infrared for the remote sensing of particulate planetary surfaces. All measurements were made using a Nicolet 5SXC FTIR spectrometer with a bidirectional reflectance attachment, which uses an aluminum mirror as the background against which the sample is compared.

Newly obtained near-infrared reflectance spectra for features on or near the Aristarchus Plateau demonstrate the diversity of compositions in the region. Highland units are of three probable compositions: a feldspar and clinopyroxene assemblage; a probable clinopyroxene, olivine, and possibly feldspar assemblage; and an assemblage composed of olivine or olivine and feldspar. The feldspar-clinopyroxene assemblage is tentatively correlated with the major Th anomaly centered on Aristarchus crater. The regional pyroclastic deposits on the plateau are composed of greater than 90 percent Fe(2+)-bearing glass. The Aristarchus cratering event fully penetrated any mare or pyroclastics in the target. Evidence for mare-related material is found in the ejecta on the mare side of the mare-plateau contact.

In the present analysis of the Voyager global multispectral mosaic of the Galilean satellite Europa, which was conducted to map surface units with similar optical properties, color assignments are indicative of the spectral nature of the given unit. The unit maps are noted to exhibit a marked hemispheric asymmetry which is most clearly seen in the UV/violet albedo ratio image. It appears as if the surface has been most darkened at the center of the trailing hemisphere, with a gradual decrease to a minimum at the center of the leading hemisphere as the cosine of the angle from the antapex of motion; this cosine pattern suggests that the darkening is exogenic in origin, and is interpreted as evidence of surface alteration by in bombardment from the Jovian magnetosphere.

The diversity of reflectance spectra noted among near-earth asteroids that were compared with selected asteroids, planets and satellites to determine possible source regions is indicative of different mineralogical composition and, accordingly, of more than one source region. Spectral signatures that are similar to those of main belt asteroids support models deriving some of these asteroids from the 5:2 Kirkwood gap and the Flora family, by way of gravitational perturbations. The differences in composition found between near-earth asteroids and planetary and satellite surfaces are in keeping with theoretical arguments that such bodies should not be sources. While some near-earth asteroids furnish portions of the earth's meteorite flux, other sources must also contribute.

Comparisons of Europa and Ganymede multispectral data show that Ganymede is less spectrally variable than Europa. Four major spectral units dominate Ganymede, corresponding to the ancient cratered terrain and the grooved terrain in the leading and trailing hemispheres. A hemispheric asymmetry in UV absorption definitely exists on Ganymede, although it is not so strong as that on Europa. Comparison of normalized spectra for the four major units shows that the sense of the asymmetry (more absoption toward shorter wavelengths on the trailing hemisphere) is also the same on the two bodies. This hemispheric asymmetry is interpreted as evidence of alteration of the surface by magnetospheric bombardment or micrometorite bombardment. It is concluded that the pattern observed represents a steady state involving both of these exogenic modifying agents. The spectral changes which could be produced by these two processes are grain size alteration and changes in composition. The spectral effects of variation in water ice grain size are fairly well known. Laboratory experiments are being conducted to study the spectral effects of sulfur irradiation on water ice.

Studies of the trajectory of the Galileo mission to Jupiter recently revealed that the spacecraft can pass close to one of the largest asteroids (#29 Amphitrite). NASA has therefore altered the mission plan of the Galileo spacecraft to include a possible close flyby of Amphitrite in early December 1986, if the condition of the spacecraft allows. If this option is actually implemented, Amphitrite will become the only asteroid for which any high-spatial resolution images and reflection spectra will be available. To evaluate the value of this data and place Amphitrite in the context of the more than 600 asteroids for which some compositional information exists. Existing data was reexamined, new telescopic spectra of Amphitrite were obtained, and simulated Galileo data sets were constructed.

The mineralogy and petrology of 17 near-earth asteroids are characterized using reflectance spectroscopy with ground-based telescopes. Assuming the surfaces are composed of cosmically abundant materials, the presence of certain mineralogical species can be determined from diagnostic absorption features and spectral characteristics which have been studied under known laboratory conditions and understood in terms of crystal-field theory. With one possible exception, the surface composition of near-earth asteroids consists of common rock-forming minerals such as olivine, pyroxene, and phyllosilicates. Opaque components are present but cannot be mineralogically identified with existing experimental data. The spectrum of 2201 Oljato cannot be interpreted in terms of common rock-forming minerals and is examined for cometary features because its high orbital eccentricity suggests a possible relation to comets, but no common cometary features are identified. The predominance of mafic silicate absorption features in spectra of near-earth asteroids (compared to the majority of main-belt asteroids) may be a primary compositional feature or may be the signature of relatively fresher asteroid material.

Near infrared reflectance spectra for the Aristachus region, obtained using the 2.2m UH telescope at the Mauna Kea Observatory, were reduced and analyzed. The spectra obtained for the central peak, southern floor, southwestern wall, eastern wall, and northwestern wall of Aristachus crater exhibit shallow continuum slopes, relatively strong feldspar bands, pyroxene bands stronger than those typically seen in the spectra of fresh higland features, and pyroxene band centers near l micrometer suggesting the dominance of Ca rich clinopyroxene. The spectrum of the south rim of Aristachus is quite distinct from those of other crater units. The position of Aristrchus on the plateau/mare boundary raises questions concerning compositional variations in crater ejects deposits.

Viking 2 approach maps of red and violet normal albedo are used to define units of similar optical properties for a large area of the Martian surface. Computer processing of these data reveals clusters of albedo values relating to specific, contiguous regions of the planet, with some unit boundaries being of a gradational nature while others are more discrete. A study of the albedo properties of analog materials measured in the laboratory, and of telescopic spectrophotometry of Mars, indicates that both hydroxylated and hematitic oxidized materials exist on the surface. The most probable surface expression of global dust and/or high albedo soils is in the classical bright region, Arabia. Most low albedo features in the region studied have red and violet albedos that are consistent with a dark substrate, such as basalt, coated by, or mixed with, a fine grained alteration product.

Both morphological and compositional information are needed to define and characterize surface geologic units on Mars. A description is presented of new, near-infrared spectra (0.65 to 2.50 micrometers) for 11 regions on the Martian surface observed in 1978. The high photometric quality of these data combined with increased near-infrared spectral coverage provide new information about the spectral behavior and, therefore, the composition and physical nature of Martian surface materials. The spectral reflectances were obtained with the aid of a 2.2-m telescope located on Mauna Kea, Hawaii. A cooled (to 77 K) circular variable filter spectrometer with an InSb detector was used to measure alternatively Mars and the standard star Beta Geminorum. Attention is given to general spectral characteristics, the dark region composition, spectral evidence for water, and the 2.3 micrometer absorption.

A description is presented of new earth-based reflectance spectra of the Martian north residual polar cap. The spectra indicate that the composition is at least mostly water ice plus another component with a 'gray' reflectance. The other minerals in the ice cap appear to be hydrated. The data were obtained with a cooled circular variable filter spectrometer on February 20, 1978, using the 2.2-m telescope on Mauna Kea, Hawaii. It is pointed out that the identification of water ice in the north polar cap alone does not indicate that water makes up all or even most of the bulk of the cap. Kieffer (1970) has shown that a small amount of water will mask the spectral features of CO2.

Spectral reflectance contains direct information on the mineralogy of the surface soils and rocks. A problem concerning the employment of remote spectral-reflectance sensing techniques for mineralogical studies is related to the lack of photometrically precise spectra of planetary surfaces in the region from 1.0 to 2.5 micrometers, where additional important mineral absorptions occur. Recently, a new instrument was developed, and an observational program is underway to provide the required near-infrared spectra. The first, comprehensive near-infrared spectroscopic results for the moon are presented. It is shown that it is equally possible to derive quantitative mineralogical information about the lunar surface by using remotely sensed reflectance spectra as it is by using laboratory reflectance measurements of lunar samples.

High-spectral-resolution images of Saturn over the wavelengths of the 6196.8-A feature of CH4 were obtained and used to extract line profiles for the observed feature over the disk of Saturn and to determine the spatial variation of the equivalent width at the time of the observations. The result of the observations is consistent only with a single class of models for the vertical structure of the atmosphere of Saturn, namely, the homogeneous scattering models. Reflecting-layer or two-layer models are at a striking variance with the observations. With these data, the requirements for new observations to determine the finer details of the vertical structure of Saturn's atmosphere can be specified.

The most recent comprehensive results on spectrophotometric remote sensing of planets and satellites are reviewed. The moon and terrestrial planets are considered in terms of individual surface elements, reflectance spectra being analyzed to show the composition of the soils of these bodies. For more distant, unresolved objects, including the asteroids, the Galilean satellites, the small satellites of Jupiter, the rings and satellites of Saturn and Uranus, as well as Triton and Pluto, the global or hemispheric averages of surface composition are the objects of study. The absorptions due to methane gas and frost are indicated.

One hundred eighty-seven reflectance spectra (0.33-1.10 microns) of the Galilean satellites have been obtained. Solar phase angle color correction coefficients were derived and the spectra corrected to a solar phase of 6 deg. Solar phase angle coefficients beyond 0.55 micron are presented for the first time. The spectra as a function of orbital phase angle are presented in the form of images to display hemispheric spectral variations. Io and Europa are redder on their trailing hemispheres while Callisto is redder on its leading hemisphere. Ganymede shows small longitudinal color variations despite the complex albedo structure visible in Voyager images. Comparisons of these data with previous measurements reveal that most differences can be attributed to the solar calibration. Reflectance measurements of Io at 0.73 micron observed 8.5 years apart show a 6% global reflectance decrease. However, it is difficult to unambiguously attribute this particular decrease in reflectance to a change in Io's surface composition.

A new high-photometric-precision reflectance spectrum of Saturn's rings covering the spectral region 0.65 to 2.5 microns is presented and three previously unreported absorption features at 1.25, 0.85, and probably 1.04 microns are identified. The 1.25- and 1.04 micron absorptions are due to water ice. The 0.85 microns feature may be due to a combination of 0.81- and 0.90 micron ice absorptions but this feature appears too strong relative to the 1.04 micron band to be completely explained by water ice. Another possibility is that the 0.85 micron band is due to Fe(3+)-bearing minerals in an ice-mineral mixture. This explanation could also account for the drop in the visible and ultraviolet reflectance and the rise in reflectance around 3.6 microns. Finally, a composite spectrum from 0.325 to 4.08 is presented which will be useful for future analysis and laboratory studies.

Images of the lunar surface were obtained at several wavelengths using a silicon vidicon imaging system and groundbased telescopes. These images were recorded and processed in digital form so that quantitative information is preserved. The photometric precision of the images is shown to be better than 1 percent. Ratio images calculated by dividing images obtained at two wavelengths (0.40/0.56 micrometer) and 0.95/0.56 micrometer are presented for about 50 percent of the lunar frontside. Spatial resolution is about 2 km at the sub-earth point. A complex of distinct units is evident in the images. Earlier work with the reflectance spectrum of lunar materials indicates that for the most part these units are compositionally distinct. Digital images of this precision are extremely useful to lunar geologists in disentangling the history of the lunar surface.

The depth of the absorption feature near 1 micron in the reflectance spectra of immature lunar soils is determined primarily by the amount of pyroxene present in the soil. As a soil matures, the agglutinitic glass content increases and the Fe(2+) glass band dominates the absorption feature. An empirical relationship between the 1 micron band depth and the percentage of magnetic agglutinates, and hence maturity, is demonstrated for mature highland soils of a given composition. A similar variation of 1 micron band depth is observed in the telescopic spectra of lunar highland craters and is positively correlated with the surface maturity as determined by the small-impact erosion model of Soderblom (1970). Knowledge of both maturity and the 1 micron band depth of a lunar surface feature allows compositional information to be derived.

Telescopic reflection spectra of mature mare surfaces are used to identify and characterize major basalt types on the frontside of the moon. The spectra are classified according to (1) continuum slope and (2) near-infrared features. This study indicates that there are major lunar basalt types that are unlikely to have been sampled during the landing missions. Regions of basalt exist in the western maria with a TiO2 content comparable to that of Apollo 11 but with infrared characteristics that indicate a distinctly different composition. Samples from two landing sites, Apollo 12 and Luna 16, may contain fragments of a nearby basalt unit compositionally different from the dominant basalt type of the landing area.

New spectrophotometry for 31 asteroids, and improved data for nine previously observed, is presented, raising the total sample to 98. Several important new spectral types have been found. Asteroid 349 Dembowska is the first large main-belt asteroid found to resemble ordinary chondritic meteorites in spectral properties (it is similar to LL6 chondrites in pyroxene/olivine content). The first two measured Trojan asteroids show unusual spectra not compatible with carbonaceous chondrites or other known meteorites. The spectrum of Mars-crosser 887 Alinda is compatible with unequilibrated chondrites. Most fainter asteroids (especially those in the outer half of the belt) have flat spectra indicating probable carbonaceous composition. Compositional heterogeneity of Hirayama families is common among the 16 families studied to date. But individual asteroids seem to have remarkably uniform surface compositions, indicated by the usual lack of spectral changes with rotation. Spectra of a preliminary sample of proposed meteorite source-bodies are consistent with derivation of meteorites by proposed mechanisms, but further observations are needed.

Mare Humorum has been found to contain at least nine distinct units. Mare basalt units typical of those at the Apollo 12 and 15 sites and central Mare Serenitatis exist also in Mare Humorum, but the early, high-titanium basalts typical of Mare Tranquillitatis and southeast Mare Serenitatis are absent. Basalts containing about 5% TiO2, not sampled at any Apollo site, also are found in Mare Humorum. Emplacement of extensive low-titanium basalts occurred in Mare Humorum in roughly the same period as emplacement of similar basalts at Apollo 12 and central Mare Serenitatis. These were followed by emplacement of the higher titanium basalts during a later period. These and other units are characterized and their extent mapped using a combination of remote sensing and Apollo sample studies. The results are interpreted in the context of the evolution of the Humorum region.

We present visible and near-infrared spectral reflectance curves for 35 asteroids, making a total sample of 67 including our previous work. Several previously unknown spectral types have been discovered. Absorption bands and other features, from which information about the surface composition of the asteroids can be derived, continue to appear. Sampling statistics suggest we have observed about half of the significantly different mineralogical assemblages present in the asteroid belt. There is a general, but imperfect, correlation between spectral type and semimajor axis, which is consistent with the hypothesis that meteorites are derived from the asteroid belt. The color-frequency histogram for the 67 objects is bimodal, probably associated with 'stony' objects and 'carbonaceous' objects. Hirayama family pairs usually show disparate spectral types. 19 Fortuna may show strikingly different photometric properties on opposite sides.

Minerals partly composing the surfaces of 14 asteroids are determined by using asteroid reflectance spectra and optical properties of meteorites and other materials. Individual electronic absorption features are identified in the asteroids' spectra. The energies, relative strengths, and shapes of these features are interpreted by using laboratory and theoretical studies. Analysis of the initial 14 asteroid reflectance spectra indicates the presence of the following types of surface materials: six carbonaceous chondrite-like; two stony-iron-like (metal/silicate ratio approximately equal to 1); one iron meteorite-like; one basaltic achondrite-like; and four silicate-metal assemblages (metal/silicate ratio approximately equal to 0.25). These results support the conclusion that the asteroid belt is a source of at least some meteoritic material, and they show a relation between certain asteroids and certain classes of meteorites.

Crystallized spheres of orange glass from Shorty Crater at the Apollo 17 site are shown by spectral reflectance data to be the characteristic ingredient of the dark mantling deposit of the Taurus-Littrow region. This deposit, mapped on spectral vidicon images taken using earth-based telescopes, apparently is thickest 50 km northwest of the Apollo 17 landing area, and has a gradational contact with the surrounding materials. The irregular areal distribution of the dark-mantle deposit and the gradational borders support conclusions based on laboratory studies of the orange soils that these are volcanic pyroclastic materials. Similar deposits are identified by spectral reflectance properties near Rima Bode, Schroeter W, Fauth H, and Higinus W. The same material is likely to occur at Sulpucius Gallus based on visual and photographic observations of orange soil. The apparently restricted occurrence of the orange-glass deposits in a belt along the edges of major mare basins implies structural control of volcanic vents and a possible deep-seated origin.

The evolution, geology, geochemistry and topology of the Flamsteed region of Oceanus Procellarum are examined, considering remote sensing data including multispectral images and spectral reflectance measurements along with crater degradation studies and radar backscatter. Seven spectrally distinct basaltic units ranging in age from 2.5 plus or minus 0.5 to 3.5 plus or minus 0.5 billion years have been identified. The earliest units of the mapped area are composed of highlands material and include partially flooded impact craters, and the oldest surface exposed mare basalts are very low Ti basalts of the Telemann formation.

New near-infrared (0.65-2.5 microns) reflectance spectra of the Galilean satellites are presented. These spectra more precisely define the water ice absorption features previously identified on Europa, Ganymede, and Callisto at 1.55 and 2.0 microns. In addition, previously unreported spectral features due to water ice are seen at 1.25, 1.06, 0.90, and 0.81 microns on Europa, and at 1.25, 1.04, and possibly 0.71 microns on Ganymede. Unreported absorption features in Callisto's spectrum occur at 1.2 microns, probably due to H2O, and a weak, broad band extending from 0.75 to 0.95 microns due possibly to other minerals. The spectrum of Io has only weak absorption features at 1.15 microns and between 0.8 and 1.0 microns. No water absorptions are positively identified in the Io spectra, indicating an upper limit of areal water frost coverage of 2% (leading and trailing sides). It is found for Callisto, Ganymede, and Europa that the water ice absorption features are due to free water and not to water bound or absorbed onto minerals. The areal coverage of water frost is approximately 100% on Europa (trailing side), approximately 65% on Ganymede (leading side), and 20-30% on Callisto (leading side). An upper limit of approximately 5% bound water (in addition to the 20-30% ice) may be present on Callisto, based on the strong 3-micron band seen by other investigators. A summary of spectra of the satellites from 0.325 to about 5 microns to aid in laboratory and interpretation studies is also presented.

Visible and near-infrared reflectance spectra and multispectral maps of the Martian surface are discussed, and implications of the data for the composition of the Martian surface are considered. Spacecraft and earth-based telescopic observations have confirmed the generally bimodal albedo distribution of the planet, dividing the surface into bright and dark regions. Mars spectra are characterized by the presence of strong Fe(+3) absorption, which is attributed to various ferric oxide minerals. Interpretations of the spectra from the dark regions indicate a basaltic or ultramafic source rock. Evidence for water ice or a highly desiccated metal hydrate has been obtained, along with evidence for CO2-ice only in the south polar cap. Mariner 9 observations of Martian dust suggest the presence of rather acidic rock or mineral particles, or a montmorillonite-type clay. Prospects for the future study of Martian surface composition include continuing earth-based spectrophotometric studies, and high-spectral-resolution mapping of a significant portion of the surface by the Galileo spacecraft and the next Mars mission.

The near-infrared (0.65-2.5 micron) spectral albedo of Jupiter and Saturn with 1.5% spectral resolution is presented for the center of disk and for the limb. There is a distinct difference in the continuum slope between Jupiter and Saturn which may be attributed to a difference in the dust content or composition of the two atmospheres. There is an indication of limb brightening in the deepest CH4 bands on Saturn. No limb brightening is found for Jupiter.

Multispectral mapping of the Apollo 15-Apennine region has allowed the identification of numerous dark mantle deposits of probable pyroclastic origin. The deposits display a low albedo, appear to mantle and slightly subdue subjacent terrain, are spectrally distinct on the multispectral maps (high in the infrared but low in the ultraviolet) and generally exhibit a weak depolarized 3.8 cm radar echo. These characteristics are consistent with an origin by pyroclastic eruption. The regional dark mantle deposits are commonly associated with vents along marginal fractures and faults near the base of the Apennines, thereby emphasizing the role of basin-controlled weaknesses in providing channels for the upward migration of magma generated at depth. The spectral properties of the pyroclastic deposits are incompatible with those of Apollo 15 green glass but the deposits may be composed of material similar to the Apollo 15 brown or yellow glass. If so, the widespread distribution of the deposits suggest that mare basalts genetically related to the brown or yellow glass may occur in the Apollo 15 region. Moreover, pyroclastic volcanic activity has apparently been a more important and widespread process in the Apollo 15-Apennine region than has previously been thought.

Additional spectral reflectance data are presented for the Mare Crisium region. This new remote sensing information supports and supplements the subdivision of Mare Crisium into three major basalt units and shows further that: (1) the Luna 24 landing site is on a small region apparently surrounded by distinctly different units, but is clearly correlated with basalt Group IIA of Head et al. (1978); (2) exposures of Group IIB basalt in the northern and southern parts of the basin are spectrally similar; and (3) Group I basalts along the eastern part of the basin, which have been proposed to be relatively young (Boyce and Johnson, 1977), are spectrally similar to older Group I basalts.

Images of the lunar surface are being obtained at several wavelengths using a silicon vidicon imaging system and ground-based telescopes. These images are recorded and processed in digital form so that quantitative information is preserved; their photometric precision has been shown to be 1% or better. Ratio images, calculated by dividing those obtained at two wavelengths (0.40/0.56 and 0.95/0.56 micron), were presented in an earlier article, and more are presented here for a total coverage of about 60% of the lunar frontside. Spatial resolution is about 2 km at the subearth point. A complex of distinct units is evident in the images. Earlier work with the reflectance spectrum of lunar materials indicates that for the most part these units are compositionally distinct. These digital images are being used by lunar geologists to interpret the history of the lunar surface.

Several distinct absorption features, some recognized for the first time, are evident in a newly obtained reflectance spectrum (wavelengths = 0.62-2.6 microns, reciprocal dispersion = 83) of the integral disk of Mars. The effects of Martian atmospheric CO2 have been removed from the spectrum to arrive at a reflectance spectrum that is believed to be due mostly to surface material. Absorptions, at 1.22, 1.55, and 2.05 microns are interpreted to indicate the presence of H2O ice plus high desiccated mineral hydrate, although an H2O ice phase with strongly shifted fundamental frequencies cannot be ruled out. Ferrosilicate and ferric oxide bands near and shortward of 1.0 micron are confirmed. The new observations were made on April 21-23, 1976, universal time, at the 2.24-m telescope on Mauna Kea, Hawaii, using a newly developed infrared spectrometer.

A reflectance spectrum (0.62-2.6 microns) of Mars (integral disk) is obtained using a newly developed IR spectrometer at the 2.25 m telescope on Mauna Kea, Hawaii. Details of the instrument, observations, and data reduction are presented. Several distinct absorption features are evident that were apparently caused by H2O, but the positions and intensities of the features are quite unusual. In summary, the regolith is probably not as desiccated and dehydrated as the full disk reflectance spectrum and Viking soil analyses would suggest. The surface materials become desiccated and dehydrated as a result of solar UV effects and because of the relative adsorption and desorption rates during the strong diurnal cycling. There may be significant amounts of H2O at depth, and in the Solis Lacus region the H2O reservoir may extend to within a few centimeters of the surface. The Solis Lacus region may therefore be an important target for future Mars landing or sample return mission.

Spectral reflectance measurements of five Luna 24 samples and new telescopic reflectance spectra of 10-20 km areas of seven sites in Mare Crisium have been used to calibrate multispectral images of mare units. Based on these data, three major mare units are defined in the Crisium basin and their stratigraphy is interpreted. The oldest mare unit is exposed in the ejecta of the craters Picard and Peirce and along the outer edge of the southeastern part of the basin. The next younger unit includes the Luna 24 site and generally follows a topographic annulus along the basin margin. The youngest mare unit occupies the central part of the basin. It is concluded that subsidence occurred throughout the emplacement of mare units, including extensive warping and downfaulting of the inner part of the Crisium basin.

The Gruithuisen and Mairan domes on the moon represent morphologically and spectrally distinct nonmare extrusive volcanic features of Imbrian age. The composition, morphology, and age relationships of the domes indicate that nonmare extrusive volcanism in the northern Procellarum region of the moon continued until about 3.3 x 10 to the 9th to 3.6 x 10 to the 9th years ago and was partially contemporaneous with the emplacement of the main sequence of mare deposits.

The use of general and specific diagnostic spectral features and parameters to interpret most of the published high-quality reflectance spectra of asteroids is discussed. Such diagnostics are shown to provide the most complete and sophisticated mineralogically and petrologically based interpretation of the asteroid surface materials. Previous investigations of asteroid surface materials are exhaustively reviewed, emphasizing the general approaches employed, sources of information, previous characterizations of asteroid surface materials, and asteroid spectral groups. The interpretive methodology is then applied to spectral reflectance data for many individual members of the various spectral groups. A range of mineral assemblages similar to certain meteorite classes is identified, and evidence is presented for a selective high-temperature event that melted certain asteroid parent bodies and permitted their differentiation as some function of distance from the sun or the protosun and of the size of the protoasteroid. Possible candidates are proposed for the source bodies of different meteorite groups.

Remote sensing and Luna 24 sample data are used to develop a summary of the regional stratigraphy and geologic history of Mare Crisium. Laboratory spectra of Luna 24 samples, telescopic reflectance spectra in the 0.3 to 1.1 micron range and orbital X-ray data have identified three major basalt groups in the region. Group I soil is derived from iron- and magnesium-rich titaniferous basalts and was apparently emplaced over the majority of the basin, however is presently exposed as a shelf in the southwest part. Group II soils, derived from very low titanium ferrobasalts, were emplaced in two stages subsequent to Group I emplacement and now appear as part of the outer shelf and topographic annulus. Subsidence of the basin interior preceded and continued after the emplacement of the third basalt group, a soil derived from a low titanium ferrobasalt. The Luna 24 site is found to be within a patch of Group II material.

Reflectance spectroscopy and multispectral mapping are the techniques likely to be most useful for determining asteroid surfaces. Several other techniques should be considered for providing complementary information.

The interpretation of asteroid reflectance spectrophotometry in terms of mineralogical types gives inferred mineral assemblages for about 60 asteroids. Asteroid surface materials are compared with similar materials that make up many meteorites. The absence of asteroids with spectra that match identically the ordinary chondrites is noted.

Topics discussed include the need for: the conception and development of a wide spectrum of experiments, instruments, and vehicles in order to derive the proper return from an exploration program; the effective use of alternative methods of data acquisition involving ground-based, airborne and near Earth orbital techniques to supplement spacraft mission; and continued reduction and analysis of existing data including laboratory and theoretical studies in order to benefit fully from experiments and to build on the past programs toward a logical and efficient exploration of the solar system.

Earth-based astronomical observations and laboratory analysis of meteorites provide the only scientific data available on asteroids. These data are summarized and subjects for future investigations are explored. The measurements required for potential missions are discussed and concepts for a multi-asteroid rendezvous mission in the mid-1980's are outlined.

The paper discusses imaging of Mars at 20 wavelengths in the spectral region from 0.35 to 1.10 microns performed during the 1973 opposition with a photometric imaging system that uses a silicon vidicon tube as the detector. The imaging system and data-reduction procedures are described in detail, and relative reflectance spectra for several small areas in the images are presented along with vidicon images obtained during early phases in the development of the 1973 dust storms. Several anomalous blue brightenings are examined and attributed to ground fog and H2O frost. It is suggested that anomalously large amounts of H2O were pulsed into the atmosphere from the regolith by the storm system. An apparent fading and recession of the north polar hood is noted, surface units revealed in contrast-enhanced ratio images are identified, and the need for higher-spatial-resolution orbital spectroscopy of dark areas is emphasized.