On The spectral reflectance properties of materials exposed at the Viking landing sites

Bidirectional reflectances, calibrated to within 10% uncertainties, for blue (0.40–0.52 μm), green (0.50–0.59 μm), and red (0.60–0.74 μm) channels were determined for 31 block and soil exposures imaged by the Viking Lander Cameras. Reflectances were corrected for atmospheric attenuation by using optical depth measurements obtained by the Lander Cameras and for skylight illumination by subtracting the brightness values of adjacent shadowed areas. Reflectances for two facets on each of two large blocks were computed for a common geometry by determining facet orientations from high resolution Lander stereo images and by fitting the Hapke‐Irvine photometric function to reflectances estimated for several geometries. Results indicate that individual facets on the same block have different optical properties. The darkest, grayest block facets have properties similar to laboratory reflectance spectra of mafic rocks that are thinly coated with ferric iron‐rich palagonite. Most block surfaces have reflectances consistent with thin to optically thick covers of palagonitic material. Limited exposures of soil have spectra that are also similar to palagonitic material. The majority of the soils sampled and a few block surfaces have spectra with steeper slopes and greater spectral curvatures than the palagonite analog, consistent with greater degree of ferric iron crystallinity. The soils could be an intimate mixture of palagonitic material and other materials not seen in end member form, but, if so, these other materials would need an even greater slope and curvature than observed soils. The differences in reflectance properties for common block surfaces compared to common soils imply that local weathering of blocks has not contributed significantly to the soil exposed at the landing sites.