Classification of surface units in the equatorial region of Mars based on Viking Orbiter color, albedo, and thermal data

Viking Orbiter color, albedo, and thermal inertia data for the equatorial region of Mars were utilized as a five‐dimensional data set to search for clusters that correspond to mappable surface units. A principal components analysis shows that 84% of the variance within the data for the equatorial region (30° of equator) can be carried along two vector directions. These vector directions typify the dominant trend of Martian surface materials. The first direction indicates that as the surface materials become brighter at red wavelengths they also brighten at green and violet wavelengths. The second direction implies that as the albedo increases, the thermal inertia decreases. These dominant trends were deemphasized by stretching the data from a five‐dimensional elliptical swarm into a hypersphere through the use of principal component techniques. The decorrelated data were then plotted in a triangle diagram with red/violet, albedo, and thermal inertia apices to facilitate discrimination of inherent clusters. Unit maps for the region within 15° latitude of the equator and bounded by 0° to 50° and 280° to 360°W longitude were generated on the basis of the clusters. Up to eight clusters can be identified and mixing between clusters appears to be important. The three major clusters consist of bright, red materials with low thermal inertia; dark, relatively gray materials with high inertia; and materials with intermediate values of color, albedo, and thermal inertia. Mantling by aeolian deposits or volcanic materials is prevalent in the brightest, reddest unit with the lowest thermal inertia, while the darker, grayer units with higher thermal inertia are dominated by marelike plains. The mantling is apparent in Viking Orbiter images with resolutions from 8 to 30 m per pixel.