Spectrophotometric properties of materials observed by Pancam on the Mars Exploration Rovers: 4. Final mission observations

The last sets of Panoramic Camera (Pancam) visible/near-infrared (432–1009 nm) multispectral observations made under varying viewing and illumination geometries by the Mars Exploration Rovers Spirit and Opportunity were examined using radiative transfer models to study the surface scattering and microphysical nature of rock and soil units at both sites. Nearly 12,000 individual measurements were collected for this study of soil, dust, and rock units over phase angles of ~0° to ~150°. Images were acquired on sols 1944–1946 (June 2009) at Troy, the final resting place of Spirit on the western side of Home Plate in Gusev crater, and by Opportunity at three locations on the western rim of Endeavour crater in Meridiani Planum between sols 2785 (November 2011) and 3867 (December 2014). Sky models were developed from observations of atmospheric opacity, which enabled corrections for diffuse skylight when combined with surface facet orientations determined from stereo images. Model results were improved by removing data affected by scattered light evident in some high phase angle images (resulting from minor dust contamination on the camera windows). At Troy, relatively dust-free “gray” rock units exhibited narrow, forward scattering behaviors akin to previous analyses of similar gray rock units at Gusev crater. Soils and “red” rocks coated with greater amounts of dust were more backscattering. Red rocks exhibited higher single scattering albedo (w), macroscopic roughness (θ), and opposition effect width (h) parameters, indicative of rough, low-porosity surfaces perhaps with more uniform grain size distributions. At Meridiani Planum, rubbly soils near São Gabriel crater and Cape Tribulation exhibited w values typical of previous soil analyses. However, the large drift “dust” deposits found in depressions on the northern tip of Cape York near Turkey Haven demonstrated elevated w values with a downturn toward 1009 nm, consistent with minor hydration of these materials. The dust deposits were modeled with the lowest θ values and highest h values of all soil units analyzed during the Opportunity mission, indicative of a smooth surface with homogeneous grain size distribution and/or lower porosity than other units. The dust unit scattering function was dissimilar to those for atmospheric and airfall-deposited dusts, however, suggesting that the originally deposited materials had been modified, perhaps by hydration and ongoing aeolian effects. Analyses of phase curve ratios among the units studied here and from laboratory data of analog soils suggested that surface scattering is a major control on the peak phase angle position of the “arch” in phase curve ratios, alongside the effects of particle-scale roughness.