SPECTRAL VARIABILITY OF WEATHERED BASALTS AND SANDSTONES

Rock weathering on Mars is an important indicator of past environmental conditions. In order to understand the reflectance spectra taken from the rovers and orbiters on Mars, and how rock weathering rinds and coatings influence spectral observations, analog rocks can be studied in the lab. The Martian surface is covered in basaltic sands, and clastic sedimentary rocks are present in many key locations. Previous laboratory studies have characterized weathering rinds on basalts, which are mineralogically analogous to the rocks on the Martian surface, but few studies have sought to characterize the weathering surfaces of sandstones, which are texturally analogous to many rocks on the Martian surface. In this study, we use a spectrogoniometer to measure visible/near-infrared (VNIR) reflectance spectra of naturally weathered surfaces of basalts and arkosic sandstones under different viewing geometries. We compare the photometric behavior of naturally weathered sandstones and basalts to investigate how those behaviors relate to textural similarities or differences of their weathered surfaces. We used scanning electron microscopy (SEM) and X-ray diffraction (XRD) to characterize the texture and mineralogy of their weathered surfaces, and compared them to their rock interiors. Arkosic sandstone and basalt weathering rinds have different absorption features and different near-infrared slope parameters due to their different mineral compositions; however, we focus on understanding the textural effects on the photometric behavior of different weathering rinds. Initial results show that weathered sandstones are backscattering, which is consistent with the SEM images showing rough topography on the 10-100s of micrometer scale. We will further investigate if sandstones and basalts with texturally similar weathering rinds show photometric similarities. Photometric differences that have been noticed are also hypothesized to be related to the textural properties of the weathering rinds and will be further investigated. This investigation will enable us to better understand how spectra taken by the Mars rovers and orbiters are affected by the texture of the weathering rinds of sandstones and basalts, and how those can be linked to past environmental conditions and the geologic history of weathering on Mars.