Global and regional/seasonal color mosaics of Mars

Four regional mosaics of Mars acquired during different seasons, along with their composite as a single global mosaic, have been completed in two colors (red and violet) at scales of 1/16 and 1/64 degrees/pixel. These mosaics were put together from a set of 51 separate mosaics, each acquired from a single Viking orbiter spacecraft orbital revolution. Special techniques were developed and applied to suppress large variations between mosaics introcued by highly variable, optically thin, condensate hazes. The techniques utilize a combination of the spatial characteristics of the hazes (generally broad, low-frequency) along with their modulation of the reginal color ratios (strongly enhancing the violet/red ratios). Photometric-function normalization was applied following the haze removal. Most of the single-orbit mosaics consist of red and violet or red, green, and violet filters, but a few mosaics with only red-filter data were included to fill gaps in global coverage at high northern latitudes. Global coverage is approximately 99 percent complete in red-filter mosaics and approximately 95 percent and approximately 60 percent complete in corresponding violet- and green-filter mosaics, respectively. All of the mosaics are geometrically tied to the 1/256 deg per pixel Mars Digital Image Map (MDIM), which is available on Compact Disk (CD), and which will be used as the base map for Mars Observer data sets. Early in 1993, the single-orbit color mosaics will be distributed to the science community in a six-volume set of CDs. Perhaps the most scientifically interesting parts of this dataset are the overlap regions, which show significant temporal variations in surface and atmospheric features. Surface changes can be categorized as (1) changes that probably occurred during the great dust storms of 1977; (2) changes that occurred soon after 1977 storms due to removal of redistribution of recently deposited dust; (3) changes in the northern lowlands that probably occurred during the dusty southern summer of 1979 (when no great dust storm occurred); and (4) changes associated with strong slope winds in the Tharsis and Elysium regions

The Distribution and Ages of Regional Lithologies in the Lunar Maria

A research program was designed (1) to map regional lithological units of the lunar surface based on measurements of spatial variations in spectral reflectance, and, (2) to establish the sequence of the formation of such lithological units from measurements of the accumulated affects of impacting bodies. Spectral reflectance data were obtained by scanning luminance variations over the lunar surface at three wavelengths (0.4µ, 0.52µ, and 0.7µ). These luminance measurements were reduced to normalized spectral reflectance values relative to a standard area in More Serenitotis. The spectral type of each lunar area was identified from the shape of its reflectance spectrum. From these data lithological units or regions of constant color were identified. The maria fall into two major spectral classes: circular moria like More Serenitotis contain S-type or red material and thin, irregular, expansive maria like Mare Tranquillitatis contain T-type or blue material. Four distinct subtypes of S-type reflectances and two of T-type reflectances exist. As these six subtypes occur in a number of lunar regions, it is concluded that they represent specific types of material rather than some homologous set of a few end members. The relative ages or sequence of formation of these more units were established from measurements of the accumulated impacts which have occurred since more formation. A model was developed which relates the integrated flux of particles which hove impacted a surface to the distribution of craters as functions of size and shape. Erosion of craters is caused chiefly by small bodies which produce negligible individual changes in crater shape. Hence the shape of a crater can be used to estimate the total number of small impacts that have occurred since the crater was formed. Relative ages of a surface can then be obtained from measurements of the slopes of the walls of the oldest craters formed on the surface. The results show that different maria and regions within them were emplaced at different times. An approximate absolute time scale was derived from Apollo 11 crystallization ages under an assumption of a constant rote of impacting for the last 4 x 10^9 yrs. Assuming, constant flux, the period of mare formation lasted from over 4 x 10^9 yrs to about 1.5 x 10^9 yrs ago. A synthesis of the results of relative age measurements and of spectral reflectance mapping shows that (1) the formation of the lunar maria occurred in three stages; material of only one spectral type was deposited in each stage, (2) two distinct kinds of maria exist, each type distinguished by morphology, structure, gravity anomalies, time of formation, and spectral reflectance type, and (3) individual maria have complicated histories; they contain a variety of lithic units emplaced at different times. </p

Radiometric performance of AVIRIS: Assessment for an arid region geologic target

Data from several AVIRIS flight lines were examined to assess instrument stability and response. Both scene and in-flight calibration data were analyzed statistically. The data clearly indicates that, although the instrument output was noisy and unstable at the time of the data acquisition, valuable spectral signatures can still be extracted and analyzed. Some first order calibration corrections can be performed by forcing internal consistency within the data. AVIRIS data are delivered in band-interleaved-by-line format, but high efficiency routines were developed which access the data as either image or spectral planes and enable effective statistical and visual examination of both AVIRIS scenes and ancillary files. Two methods were used to extract spectral information from segment 4 of the Kelso Dunes flight. Both successfully identified at least three distinct spectral signatures, but neither has positively identified a specific material

The surface of Mars 1. Cratered terrains

Mariner 6 and 7 pictures show that craters are the dominant landform on Mars and that their occurrence is not correlated uniquely with latitude, elevation, or albedo markings. Two distinct morphological classes are recognized: small bowl-shaped and large flat-bottomed. The former show little evidence of modifications, whereas the latter appear generally more modified than lunar upland craters of comparable size. A regional maria/uplands dichotomy like the moon has not yet been recognized on Mars. Crater modification on Mars has involved much greater horizontal redistribution of material than in the lunar uplands. It is possible that there are erosional processes only infrequently active. Analysis of the natures and fluxes of bodies that have probably impacted the moon and Mars leads to the likelihood that most of the large flat-bottomed craters on Mars have survived from the final phases of planetary accretion. Significant crater modification, however, has taken place more recently on Mars. Inasmuch as the present small bowl-shaped craters evidence little modification, the postaccretion crater-modification process on Mars may have been primarily episodic rather than continuous. The size-frequency distribution of impacting bodies that produced the present small Martian bowl-shaped craters differs from that responsible for post-mare primary impacts on the moon by a marked deficiency of large bodies. Survival of crater topography from the end of planetary accretion would make any hypothetical earthlike phase with primitive oceans there unlikely. The traditional view of Mars as an earthlike planetary neighbor in terms of its surface history is not supported by the picture data

The surface of Mars 2. Uncratered terrains

Mariner 6 and 7 photographs reveal two types of uncratered terrain on Mars. These are descriptively termed chaotic and featureless. Chaotic terrain is younger than cratered terrain and displays features strongly suggestive of slump and collapse. The speculation is offered that it may be an expression of geothermal developments within Mars that only recently have begun to affect the surface. Featureless terrain, identified only within the large circular area Hellas, is devoid of any discernible topographic forms larger than the limit of resolution, about 500 meters. Manner 7 data indicate that Hellas is a topographically low and structurally old basin. Smoothness of its floor could be the product of a recent event or of continuous processes that obliterate craters. Local processes of high efficacy, unusual surface materials, or both, are probably involved. Through its chaotic terrain the martian surface displays a development that does not seem to be recorded, at least in the form of preserved recognizable evidence, on the moon or earth

Spectral decomposition of AVIRIS data

A set of techniques is presented that uses only information contained within a raw Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) scene to estimate and to remove additive components such as multiple scattering and instrument dark current. Multiplicative components (instrument gain, topographic modulation of brightness, atmospheric transmission) can then be normalized, permitting enhancement, extraction, and identification of relative reflectance information related to surface composition and mineralogy. The technique for derivation of additive-component spectra from a raw AVIRIS scene is an adaption of the 'regression intersection method' of Crippen. This method uses two surface units that are spatially extensive, and located in rugged terrain. For a given wavelength pair, subtraction of the derived additive component from individual band values will remove topography in both regions in a band/band ratio image. Normalization of all spectra in the scene to the average scene spectrum then results in cancellation of multiplicative components and production of a relative-reflectance scene. The resulting AVIRIS product contains relative-reflectance features due to mineral absorption that depart from the average spectrum. These features commonly are extremely weak and difficult to recognize, but they can be enhanced by using two simple 3-D image-processing tools. The validity of these techniques will be demonstrated by comparisons between relative-reflectance AVIRIS spectra and those derived by using JPL standard calibrations. The AVIRIS data used in this analysis were acquired over the Kelso Dunes area (34 deg 55' N, 115 deg 43' W) of the eastern Mojave Desert, CA (in 1987) and the Upheaval Dome area (38 deg 27' N, 109 deg 55' W) of the Canyonlands National Park, UT (in 1991)

The surface of Mars 4. South polar cap

The south polar cap of Mars occupies a region of cratered terrain. Immediately outside the shrinking cap craters appear no more modified than those in areas farther north that are not annually frost covered. Craters showing through the frost mantle are locally as abundant as elsewhere on Mars. Only in a central region close to the pole are craters sparse. Both far- and near-encounter views reveal a highly irregular pole-cap edge. Photos of the same sector taken six days apart are near duplicates, suggesting that the irregularity is primarily ground controlled. No evidence of the classical polar collar is seen. Within the marginal zone, frost is preserved largely in crater bottoms and on slopes inclined away from the sun. Preferential retention in low spots supports the earlier suggestion that the Mountains of Mitchel may actually be depressions. An argument based on insolation as the prime factor in frost wastage and the narrow width of the marginal zone suggests that slopes of topographic features therein are mostly gentle, on the order of a few degrees. The frost cover of the pole-cap interior may range widely in thickness, obscuring parts of some craters and seemingly enhancing topographic visibility elsewhere, possibly through variations in thickness and reflectivity. Unusually bright areas on the cap surface, and differences in luminance between bright rims and the more somber floors of craters and other depressions, may be due in large part to differences in related frost textures and to the local history of evaporation and sublimation. Irregularly angular depressions within the polecap frost termed ‘etch pits’ may be the product of differential ablation or the undermining by wind of a slabby surficial crust. Encircling the south pole is a region of subdued relief with a paucity of craters, which displays enigmatic quasi-linear markings believed to be ground features. Although no satisfactory explanation of these markings has been formulated, it seems likely that this region has been occupied repeatedly by perennial masses of CO_2 ice, formed and maintained during those phases of the martian precessional cycle that resulted in short cool summers in the southern hemisphere. Such ice masses may play a role in producing the unusual features of the central polar region. Physical relationships suggest a local maximum frost thickness as great as tens of meters. The possibility should be kept in mind that remnants of perennial CO_2 ice of still greater thickness may exist locally, for example, in the ‘etch pit’ area

The surface of Mars 3. Light and dark markings

The Mariner 6 and 7 pictures have provided significant clues to the nature of the light and dark markings on Mars, but do not yet provide an adequate foundation for any complete explanation of the phenomena. They display detail never before seen or photographed and demonstrate that there is no network of dark lines (i.e. canals) on the planet. A variety of shapes and of boundaries between major markings are recorded in the pictures. No substantial correlation of albedo markings with cratered or chaotic terrain has been recognized; featureless terrain conceivably may be genetically related to light areas. Within and surrounding the dark area Meridiani Sinus there is evidence of local topographic control of albedo markings; light material is found in locally low areas. Also, characteristic patterns of local albedo markings are exhibited by craters there. Aeolian transportation of light material with deposition locally in low areas is suggested as an explanation of these markings and may be useful as a working hypothesis for subsequent exploration. Across some light/dark boundaries crater morphologies are unchanged; across others craters in the light area appear smoother. If there is a relationship between cratered terrain modification and surface albedo it is an indirect one