Mars surface composition from reflectance spectroscopy: A summary

Visible and near-infrared (0.3--2.6 μm) reflectance spectra of the martian surface have been obtained primarily from earth-based telescopic observations, and multispectral images have been obtained both from spacecraft and earth-based observations. Observations in this wavelength region have confirmed the bimodal albedo distribution of surface materials, first observed visually. All spectra of Mars are characterized by strong Fe3+ absorptions from the near-UV to about 0.75 μm. Darker regions show this effect to a lesser degree, and are interpreted to be less oxidized materials. In addition, dark areas have Fe2+ absorptions near 1.0 μm, attributed primarily to olivines and pyroxenes. There is evidence at infrared wavelengths for highly dessicated mineral hydrates and for H2O-ice and/or adsorbed H2O. Observations of the north polar cap show a strong H2O-ice spectral signature but no spectral evidence for CO2-ice, while only CO2-ice has been identified in spectra of the south polar cap. While the brightest materials on Mars are widespread and correlate with aeolian dust, darker materials show greater mineralogic variability and are thought to be closer in petrology and physical location to their parent rock. At present the best model for the dark materials is somewhat oxidized basaltic or ultramafic rock, regionally variable in composition and details of oxidation. The bright materials appear to be finer-grained assemblages of primarily highly oxygen-sharing dessicated mineral hydrate, some ferric oxides, and other less major constituents, including a small amount of relatively unaltered mafic material. The bright materials seem likely to be primary and/or secondary alteration products of the basaltic or ultramafic dark materials