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N. Thomas, W.J. Markiewicz, H. U. Keller (Max-Planck-Institut fuer Aeronomie), P. H. Smith (Lunar and Planetary Lab., University of Arizona)
Scattering by dust in the atmosphere produces a bright red sky on Mars which also shows evidence of an absorption. The sky brightness contributes significantly to the illumination of the surface. The ratio of the diffuse flux to the direct flux varies strongly with the geometry of the observed surface feature as well as with time of day. A model of the sky brightness has been produced and tested against observations by the Imager for Mars Pathfinder. The model has been used to quantify both the total diffuse flux and the diffuse flux onto inclined surfaces. The diffuse illumination in shadowed areas is strongly reddened while areas illuminated directly by the Sun at near normal incidence see a more solar-type spectrum. Quantitative corrections for the reddening in shadowed areas are demonstrated. It is also shown quantitatively, that the unusual appearance of the rock, Yogi (the east face of which appeared relatively blue in images taken during the morning but relatively red during the afternoon) can be explained purely by the changing illumination geometry. We conclude that the reflectances of surfaces viewed under different illumination geometries cannot be investigated for spectral diversity unless an accurate correction has been applied which removes the influence of the reddened diffuse flux.