SPECTRAL VARIABILITY IN WEATHERED ULTRAMAFIC TERRESTRIAL ANALOGS FOR MARTIAN ROCKS

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. Here we study a suite of weathered dunite samples, collected at Twin Sisters Mountain in western Washington State, as analogs for ultramafic rocks on Mars. Previous studies demonstrate that the spectral signature of weathered rock surfaces can vary with viewing geometry; however, these photometric effects have not been characterized for weathered ultramafic rocks. Our goal in this study is to quantify how both weathering and viewing geometry affect visible to near-infrared (VNIR) reflectance spectra of dunites, as analogs to Martian rocks, so that we can better interpret VNIR data from orbital and in-situ Mars missions. Weathered surface compositions and textures were compared to their unweathered rock interiors using powder X-ray diffraction and scanning electron microscopy. The dunites in this sample suite have a mix of tan, orange, and dark brown weathered surfaces with less altered green interiors.

Western Washington University’s hemispheric goniometer and spectrometer were used to collect reflectance spectroscopy data from natural surfaces on the samples at several geometries covering the full scattering hemisphere. Preliminary spectral results show hydration in most samples, regardless of the amount of weathered material present on exterior surfaces. Samples that have formed weathering rinds have spectral signatures consistent with mixtures of clay minerals, ferric oxides, and serpentine. Within this sample suite, the most weathered samples tend to have higher reflectance values in backscattering viewing geometries. An improved understanding of how weathering changes the spectral signature of terrestrial analogs can then be applied to spectral data from Martian rocks in order to infer their original compositions, the amount of weathering they have experienced, and the past environmental conditions and processes that formed any alteration products present.