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L. A. Soderblom (United States Geological Survey)
During November 1999 new global reflectance spectra of Mars for two full rotations were acquired with the short-wavelength infrared (SWIR) channel of the MICAS (Miniature Integrated Camera and Spectrometer) aboard DS1 (New Millennium Deep Space 1 S/C). Earlier spectra, acquired in May 1999, had shown very tentative new absorption features between 1.5-1.9 microns, possibly related to surface mineralogy (Soderblom, 2000, LPSC abstract). During the November 1999 sequence Mars was at a much closer range (55 versus 115 million km). The conditions for the November observations were as follows: phase angle~53 deg., heliocentric longitude (Ls) ~240 deg. (northern Autumn), sub-S/C latitude ~18.1 S, and sub-solar latitude ~21.8 S. This is an opportune geometry because most of the surface contrast among surface materials likely to show mineralogical variance is in the belt from the equator to about 40 S (c.f. Viking global color mosaics). The south annual CO2 cap was near its full extent (~60 S). MICAS acquired 48 spectra in with the SWIR keyhole (16 groups of three on 3-hour centers) covering roughly two full rotations of Mars. Because Mars' sidereal day is about 24h and 40m, the two rotations are shifted about 9 degrees relative to each other; all spectra that were within +/- 5 degrees of each other were averaged to provide longitude coverage every ~45 degrees. The data were calibrated using a new MICAS SWIR spectral response file derived Arcturus observations. The new calibration allows extension of the useful spectral range out to ~2.7 microns. The highest quality (highest SNR) spectral region is from 1.6 to 2.0 microns bounded on both sides by atmospheric CO2 bands. A series of spectral features are recognized in this band appear to be related to surface mineralogy because their strength is modulated by rotation of the planet. The features were tentatively identified at 1.74 and 1.87 microns in the May data (Soderblom, 2000) are far more clearly visible in the November data. Search for compositional candidates for these spectral features is ongoing including isotopic CO2, CO, and H2O gas and ice bands. So far it appears that these features are most likely related to simple mineralogies (owing to their spectral narrowness) such as salts, clays, sulfates, etc. In any case the results are important and relevant to consideration of future Mars orbital mapping experiments.