Photometric investigations of weathering rinds and coatings with implications for Mars

Reflectance spectroscopy is a major technique for characterizing the composition of planetary surfaces, and has led to many key findings in planetary geology. In laboratory measurements, reflectance spectrometers typically acquire data using a standard, fixed viewing geometry. Measurements from spacecraft, however, may be acquired at a wide range of viewing geometries, depending on the orientation of the instrument relative to the target surface and the Sun. For many materials, the impact of viewing geometry on reflectance is minor; however, some materials' spectral signatures can be influenced by these photometric effects. In particular, spectra of weathering rinds and rock coatings are likely to show photometric variability. In this work, our goal is to better constrain how photometric properties of Mars-analog surfaces vary as a function of the composition and microtexture of weathering rinds and coatings. We developed a new automated goniometer to enable the collection of reflectance spectra across a range of viewing geometries similar to those of spacecraft observations. We acquired spectrogoniometric measurements for synthetically-coated and naturally-weathered Mars analog samples, which we further characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), and x-ray diffraction (XRD). Our results show that thin silica coatings on basalt substrates are not detectable at standard viewing geometries but exhibit diagnostic photometric effects in the visible and near-infrared. Additionally, we find that weathering processes may influence the shapes of scattering lobes, with some weathered samples found to change from forward scattering to backscattering depending on the degree and nature of sample weathering.