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M. D. Smith, J. C. Pearl (NASA/GSFC), B. J. Conrath (Cornell), P. R. Christensen (Arizona St.)
The Mars Global Surveyor (MGS) arrived at Mars in September 1997 near Mars' southern spring equinox. Many infrared spectra returned by the Thermal Emission Spectrometer (TES) contain the clear spectral signature of atmospheric dust. The large majority of spectra are taken in a nadir geometry where Mars is observed through the atmosphere. For these spectra we retrieve column-integrated infrared aerosol (dust) opacities by assuming unit surface emissivity. We report here on 1) the observed horizontal distribution of dust aerosols and their evolution with time, and 2) the spectral shape of the dust feature in the thermal infrared as retrieved from TES data.
During the aerobraking and science phasing portions of the MGS mission, TES has provided monitoring of dust opacities over a large part of the southern hemisphere and limited coverage of the northern hemisphere. At the beginning of the mission (approximately Ls=180), the 9-micron dust opacity at mid-southern latitudes was at a nominal low level (about 0.25). As the season advanced through southern spring and into summer, we observed two large regional dust storms (including the Noachis Terra event of 25 November 1997) where peak 9-micron dust opacities approached or exceeded unity, as well as numerous smaller local storms. Both large and small dust storms exhibited large changes in both spatial coverage and intensity over a time scale of a day. During this entire period, the region at the edge of the retreating southern seasonal polar ice cap was observed to be consistently more active than other latitudes.