INFLUENCE OF SILICA COATINGS ON THE REFLECTANCE SPECTRA OF MARS ANALOG BASALTS

Reflectance spectroscopy is a key technique for understanding the mineral composition and other surface characteristics on Mars. However, interpretations of rock composition from spectra are complicated by the weathering rinds and coatings. In this study, we investigate the visible to near-infrared (VNIR) spectral and photometric variability in Mars analog basalts that have been coated with amorphous silica. If silica is detected in spectra from the Martian surface, it can imply aqueous alteration and provide insight into past environments. Specifically, our goal is to determine the thickness required for silica absorptions to become detectable in coated rock spectra, and to better understand how varying the coating thickness and the surface roughness of the rock affects the scattering behavior of light from a basalt surface. The sample used in this study was collected in the Columbia River Flood Basalts at Frenchman Coulee in Eastern Washington State. Multiple slabs were prepared for different rock textures and varying silica-coating thicknesses. Multiple silicon carbide grits were used to create different scales of roughness on the rock surfaces and amorphous silica was precipitated onto basalt slabs to form coatings following the methodology of Kraft et al (2003, AGU, doi:10.1029/2003GL018848). VNIR spectra were acquired at a range of incidence and emission angles using a spectrogoniometer, as described by Hoza et al. (2019, LPSC, Abstract #2958). We can use these measurements to quantify how these effects of coatings on the basalt spectra vary as a function of basalt surface roughness. Initial results show that the silica increases reflectance values in standard and forward scattering geometries but has minimal impacts on absorption features. In addition, using a Scanning Electron Microscope (SEM), we have explored the micron-scale morphology and chemistry of the rock surfaces and coatings, which in turn can affect the spectra. The SEM images show that coatings tend to pool in topographic lows. With rougher textures, the coatings on the topographic highs were minimal or absent. Future steps include continuing to vary the coating thicknesses and textures, varying the coating composition, and use of an Atomic Force Microscope to characterize topography of the surfaces before and after coatings are prepared.