Wavelength dependence of limb-darkening of Mars from visible and near-IR telescopic spectral imaging

We are investigating the photometric properties of Mars using Earthbased telescopic spectral imaging obtained during 1988. Each spatial pixel consists of 300 spectral channels from 0.44 to 1.02 microns, calibrated to radiance factor (r(sub F)) through a careful procedure involving standard star observations in 1988 and 1990. Calibrated data include 3 spectral images with solar phase angle (g) of 12 degrees and 4 images with g of 4 degrees. Pixels near the sub-Earth point have a spatial footprint of about 280 x 150 km. Preliminary work using Hapke photometric function reveals a dependence of both single-scattering albedo and roughness parameter on wavelength. The former is in agreement with observed spectra of the martian surface, but the latter is unexpected.