Mars: Definition and Characterization of Global Surface Units With Emphasis on Composition

Units of similar optical properties have been defined for a large part of the surface of Mars using primarily Viking 2 approach maps of red (0.59 •m) and violet (0.45 •m) normal albedo. Of available global data sets, spectral albedo most closely relates to surface mineralogy. The resultant unit map represents Mars at the time the Viking images were acquired (L s = 103 ø-105 ø, morning, northern hemisphere summer in 1976). Computer processing of these data reveals clusters of albedo values which relate to specific, contiguous regions on the planet. Final unit definitions were determined by an interactive, iterative process which included scientific judgment on the part of the researchers; the results therefore are not the product of an unsupervised statistical classification. Some unit boundaries are of a gradational nature, while others are more discrete. Certain of the units are temporal and relate to the presence of condensates at the time of observation. Other units, by virtue of their redness and low albedo, are almost certainly not affected by condensates. A study of the albedo properties of analog materials measured in the laboratory and of telescopic spectrophotometry of Mars indicates that both hydroxylated (low temperature or secondary) and hematitic (high temperature or primary) oxidized materials exist on the surface. The most likely surface expression of global dust and/or high-albedo soils is in the classical bright region Arabia. This material is probably a low-temperature hydrated oxidation product of mafic volcanic glass, such as a palagonite or amorphous iron-silica gel. Most low-albedo features in the region studied have red and violet albedos consistent with a dark substrate (such as basalt) coated by, or mixed with, the fine-grained alteration product mentioned above. There may be no genetic relationship between these two components in this majority of low-albedo surface materials. Certain of th, e other low-albedo regions are also the most red (highest R-V) areas observed on the planet. These areas are consistent with a nonhydroxylated, more hematitic material such as that produced by high-temperature hydrothermal alteration of basalt. Some of these dark, very red regions correlate with exposures of very ancient terrain. At this spatial resolution (10-20 km/pixel) there is no evidence of surface exposures of uncoated or unweathered basaltic rock or glass. Comparisons of the unit map with other data sets available to us revealed no strikin• detailed correlations. Larger-scale correlations certainly do exist, however, among the various albedo data sets and with thermal inertia data.