Near-Infrared Spectral Variations of Martian Surface Materials from ISM Imaging Spectrometer Data

Imaging spectrometer data from the ISM instrument on Phobos 2 were used to characterize spatial variations in near-infrared spectral properties of the martian surface, to determine the correspondence between near-infrared and visible-wavelength spectral variations, and to assess lithologic variations in the surface materials. All data were radiometrically calibrated and corrected for effects of atmospheric gases using previously described methods. The data were also corrected photometrically to a standard geometry, and the estimated contribution of light backscattered by atmospheric aerosols was removed to isolate the reflectance properties of surface materials. At shorter near-infrared wavelengths, the surface varies between three major spectral types which correspond to known visible color units. Dark gray materials have 1- and 2-μm absorptions consistent with a pyroxene-containing lithology, and bright red dust has a shallow 0.9-μm absorption consistent with a poorly crystalline ferric mineralogy. Dark red soils are spectrally similar to dust although lower in albedo. In some cases their 0.9-μm ferric iron absorption is deeper and offset toward longer wavelengths than in dust. These attributes agree well with those determined in situ for comparable materials at the Mars Pathfinder landing site. At longer wavelengths, significant regional heterogeneities are observed in the slope of the spectral continuum and the depth of the 3-μm H₂O absorption. The 3-μm band is stronger in bright red soils than in most dark gray soils, but the strongest absorptions are found in intermediate-albedo dark red soils. Observed spectral variations suggest the presence of at least four surface components, dust, pyroxene-containing rock and sand, one or more crystalline ferric minerals, and a water-bearing phase. These are broadly consistent with four surface components that have been inferred from ground-based, orbital, and landed spectral studies and from in situ compositional measurements. We also conclude from our analysis that most albedo and spectral variations result from the coating of dark mafic rock materials by bright ferric dust. Dark red regions, however, are inferred to have dust-like compositions but lower albedos, due in part to intermixture of a dark, crystalline ferric mineral. Both of these major conclusions are strongly supported by landed investigations by Mars Pathfinder. The layered materials in Valles Marineris are the only geologic formation with distinctive spectral properties, including an enhanced 3-μm H₂O band and pyroxene absorptions which imply a mineralogy distinct from materials in the surrounding highlands. These properties provide important evidence for the layered materials' origins and are most consistent with mechanisms that involve volcanism restricted to the interiors of the chasmata.