NEW COMPOSITE SPECTRA OF MARS, 0.4-5.7 MU-M

About 15 areas were observed in the equatorial regions of Mars by the infrared spectrometers IRS (Mariner 6 and 7) and ISM (Phobos-2). The c omparison between the spectra shows a remarkable consistency between t wo data sets acquired 20 years apart and calibrated independently. Thi s similarity demonstrates the accuracy of ISM calibration above 2 mu m , except for a possible stray light contribution above 2.6 mu m, on th e order of similar to 1-2% of the solar flux at 2.7 mu m. Most differe nces in spectral shapes are related to differences in spectral/spatial resolution and viewing geometries. No important variation in surface properties is detected, except for a spot in southern Arabia Terra whi ch has a much deeper hydration feature in IRS spectra; differences in viewing geometries and spatial resolutions do not seem to account for this difference that could result from shifting or dehydration of surf ace materials. Composite spectra of several types of bright and dark m aterials are computed by modeling the thermal emission and are complet ed with telescopic spectra in the visible range. Modeled reflectance i n the 3.0-5.7 mu m range is consistent with basalts and palagonites. T he bright regions and analog palagonite spectra are different from hem atite in this range, but resemble several phyllosilicates. We infer th at(1) although hematite dominates the spectra in the 0.4- to 2.5-mu m range, the silicate-clay host is spectrally active beyond 3 mu m and c an be identified from this domain; (2) phyllosilicates such as montmor illonite or smectite may be abundant components of the martian soils, although the domain below 3 mu m lacks the characteristic features of the most usual terrestrial clay minerals. (C) 1997 Academic Press.