MARTIAN AEROSOLS - NEAR-INFRARED SPECTRAL PROPERTIES AND EFFECTS ON THE OBSERVATION OF THE SURFACE

Imaging spectroscopic measurements of Mars acquired by the ISM instrum ent on Phobos-2 are used to investigate the NIR spectral properties of aerosols and the effects of atmospheric scattering on inferred minera logy of the surface. Estimates of aerosols spectra between 0.77 and 2. 6 mu m are derived above Tharsis and Ophir Planum. The spectral contin ua are consistent with the particle size distribution derived using da ta from the solar occultation experiment on-board the spacecraft (effe ctive radius approximate to 1.2 mu m, with an effective variance appro ximate to 0.2). The aerosols spectra contain water-ice absorption feat ures and possibly absorptions due to clay and/or sulfates. For these p articular observations, aerosol opacity is apparently uncorrelated wit h surface altimetry, suggesting a spatially uniform column density. Th e largest effect of the aerosols on surface spectra is in dark regions , where the continuum spectral slope becomes more negative and the 1-m u m absorption due to Fe in pyroxene is shifted toward longer waveleng ths. The effects of aerosols on spectra of bright regions are insuffic iently large to change mineralogic interpretations based on ISM data, i.e., that bright regions in Tharsis are dominated spectrally by hemat ite, but that additional ferric minerals are probably present in other areas including Arabia. (C) 1994 Academic Press, Inc.