Optical scattering properties of terrestrial varnished basalts compared with rocks and soils at the Viking Lander sites

Basalt flows at the Pisgah Volcanic Field, California, are mantled by aeolian silt-sized deposits. The bright mantle is overlain by a dense pavement of basalt cobbles smoothed by aeolian abrasion and coated with glossy dark gray and red desert varnishes. In contrast, the two Viking landing sites on Mars lack desert pavements, and rocks have glossy surfaces that in some cases are brighter than soils. To pursue further the scattering characteristics of varnished basalt cobbles for comparison to Mars rocks, laboratory measurements of bidirectional reflectance (0.69 mu m center wavelength) were made as a function of incidence and emission angle along the principal plane for varnished and fresh basalt surfaces. The reflectance data, fit with a scattering model that includes specular reflections from rough surfaces and volume penetration and reflection, show that the red varnished surfaces have higher single scattering albedoes and higher surface roughness values than the dark gray varnished surfaces (0.56 and 0.42; 21 degrees and 17 degrees, respectively). Fresh cut surfaces have lowest values (0.30 and 6 degrees). Results are consistent with observations made from thin sections for these surfaces and with field observations that show dark gray varnished surfaces are smoothed by exposure to wind abrasion. Dark red varnished surfaces tend to be rougher because they are embedded in the aeolian mantle and develop in a protected environment. Application of the model to atmospherically corrected, radiometrically calibrated Viking Lander images (0.60 to 0.74 mu m) shows that many rocks are smoother and have single scattering albedoes higher than soils (9 degrees versus 27 degrees; 0.71 versus 0.63, respectively). Results are consistent with a model in which rocks at the surface of Mars become smoothed by aeolian abrasion and coated with atmospheric dust. Lack of desert pavement on Mars is probably associated with a scarcity of cobbles and a lack of soil forming processes. Similar bright, smooth coatings are expected at other locations on Mars, including the Pathfinder Ares Vallis site.