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N.M. Hoekzema, W.J. Markiewicz, H.U. Keller (Max-Planck-Institute for Aeronomy)
It is now well known that the solar photons scattered in the Martian atmosphere produce a significant diffuse illumination of the surface. The spectral characteristics of this illumination reflect the optical properties of the aerosols and are different from that of the direct solar illumination. Interpretation of the orbiter images requires careful separation of this atmospheric effect. The required analysis yields information on the compositions and properties of the aerosols. Probably the most significant parameter of the problem is the total optical depth of the atmosphere. Here, we present a method to estimate the optical depth directly from orbiter images. The method compares shadowed and illuminated areas of the surface. Since only the diffuse light illuminates shadowed regions an estimate of the optical depth with minimal number of assumptions can be made. Effects of shadows on scales beyond the resolution (micro-shadowing) can also be included. We tested our method by comparing the results from Viking orbiter images of the Viking Lander sites with direst data from the landers. The accuracy of the method is shown to be a few percent. In one set of the images of the Viking I lander we are able to identify ground fogs and/or hazes by direct inspection of the images and the variations of the optical depth with wavelength in different parts of the image (different local solar time). These variations correlate with variations measured by the lander.