Preliminary results on photometric properties of materials at the Sagan Memorial Station, Mars

Reflectance measurements of selected rocks and soils over a wide range of i llumination geometries obtained by the Imager for Mars Pathfinder (IMP) cam era provide constraints on interpretations of the physical and mineralogica l nature of geologic materials at the landing site. The data sets consist o f (1) three small "photometric spot" subframed scenes, covering phase angle s from 20 degrees to 150 degrees; (2) two image strips composed of three su bframed images each, located along the antisunrise and antisunset lines (ph otometric equator), covering phase angles from similar to 0 degrees to 155 degrees; and (3) full-image scenes of the rock "Yogi," covering phase angle s from 48 degrees to 100 degrees. Phase functions extracted from calibrated data exhibit a dominantly backscattering photometric function, consistent with the results from the Viking lander cameras. However, forward scatterin g behavior does appear at phase angles >140 degrees, particularly for the d arker gray rock surfaces, Preliminary efforts using a Hapke scattering mode l are useful in comparing surface properties of different rock and soil typ es but are not well constrained, possibly due to the incomplete phase angle availability, uncertainties related to the photometric function of the cal ibration targets, and/or the competing effects of diffuse and direct lighti ng. Preliminary interpretations of the derived Hapke parameters suggest tha t (1) red rocks can be modeled as a mixture of gray rocks with a coating of bright and dark soil or dust, and (2) gray rocks have macroscopically smoo ther surfaces composed of microscopically homogeneous, clear materials with little internal scattering, which may imply a glass-like or varnished surf ace.