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G. B. Hansen (Planetary Sci. Inst.), T. B. McCord (Univ. Hawaii/Planetary Sci. Inst.), J. B. Adams (Univ. Washington), R. Jaumann, H. Hoffmann (DLR Berlin), G. Neukum (Freie Universität Berlin)
Color images from Mars orbit have been used to study surface composition for three decades, but the ubiquitous dust aerosol in the Martian atmosphere (always with a visible optical depth of at least a tenth) has been recognized as having major effects on the colors obtained from a calibrated sensor. Previous color composites have been assembled from individual images with similar lighting and viewing geometry, allowing for relatively simple atmospheric compensation. The HRSC is a pushbroom imager, with nine separate detector arrays that simultaneously image from ~20o fore and aft of the nadir position. The two end arrays are used to determine topography, the nadir channel is always returned at the highest spatial resolution for context. Four other channels are used for color imaging. These color sensors observe at significantly different viewing angles for the same location on the surface. The different geometries cause the color channels to experience different amounts of atmospheric absorption and scattering. Our goal is to determine an atmospheric correction that allows us to determine a surface reflectance spectrum that can be compared to terrestrial samples measured under standard conditions. The dust colors the direct sunlight and scatters colored light, and the atmosphere also colors the reflected light before reaching the sensor. We are modeling these effects using a multi-stream discrete ordinates radiative transfer code, including dust optical properties from the literature. We are comparing sample surface spectra in a vacuum to the same materials under a Martian atmosphere under variable lighting geometries appropriate to the HRSC.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.