Spectral variability in naturally weathered rock surfaces and implications for Mars

Rock weathering products are important clues for understanding past environmental conditions on Mars. They can be identified using reflectance spectroscopy because the formation of new minerals and textures on a rock surface will change its spectral signature. Previous studies demonstrate that the spectral signature of coated rock surfaces can vary with viewing geometry (the angle between incident and emitted light); however, these photometric effects have not been extensively characterized for naturally weathered rocks. Our goal is to quantify how both weathering and viewing geometry affect visible to near-infrared (VNIR) reflectance spectra of dunites, basalts, and andesites so that we can better interpret VNIR data from orbital and in-situ Mars missions. The Western Mars Lab's hemispheric goniometer and spectrometer were used to collect reflectance spectroscopy data from natural rock surfaces at several geometries covering the full scattering hemisphere. Within this sample suite, weathered samples tend to have more backscattering behavior than relatively unaltered samples of the same rock type. Improving our understanding of how weathering changes the spectral signature of terrestrial samples can then be applied to Martian rocks to make inferences about their original compositions and environmental conditions that formed any weathering products present.