Spectrophotometric Characterization of Hawaiian Coated Basalt and Implications for Mars

Visible to near-infrared (VNIR) reflectance spectroscopy is a commonly employed technique used to study the surface of Mars and other bodies, but spectra are sensitive to viewing geometry, non-linear spectral mixing, surface texture, and environmental factors. Furthermore, alteration products like coatings may considerably impact spectra, which is of particular importance for investigating Mars, where orbital spacecraft and rovers have documented extensive surface coatings. Studying how coatings affect the spectra of naturally weathered analog rocks, therefore, is critical to our ability to identify coatings on Mars, determine their compositions, and interpret their formation environments. Previous work using synthetic coatings and limited natural weathered materials has shown that coatings influence spectral slope, can impart unique photometric behavior, mask underlying lithologies, and may alter other absorption features, but further work is needed to characterize the spectral behavior of natural coatings. Coated basalt samples from Hawaii are reasonable natural analogs for coatings on Mars, and previous work has described silica-rich and Fe/Ti-enriched coatings with proposed formation mechanisms. However, no previous studies have quantified their full VNIR spectral and photometric behaviors. Here, we characterize coated Hawaiian basalt samples and constrain the effects of viewing geometry on their VNIR spectra for the first time. We collected samples from the 1920 Kilauea flow along the Kau Desert Trail and the 1970 Kilauea flow along the Puna Coast Trail in Hawaii Volcanoes National Park. Coating composition and morphology were characterized using reflected light microscopy and Scanning Electron Microscope (SEM) with energy dispersive spectroscopy (EDS) elemental maps and backscattered electron (BSE) imaging. Spectral and photometric behavior was characterized via Western Washington University’s TANAGER spectrogoniometer at geometries covering the full scattering hemisphere. VNIR spectra were convolved to bandpasses of the Mars-2020 Perseverance Rover’s Mastcam-Z instrument to be directly compared to Mastcam-Z spectra from Jezero crater, Mars. Mastcam-Z multispectral data from Perseverance’s recent exploration campaigns (sols 415-1198) were assessed for evidence of coatings. We identified three unique coating types with distinct stratigraphic relationships on the Hawaiian samples: (1) “porous” fragment-bearing SiO2 coatings; (2) “Fe/Ti-bearing” coatings; and (3) “layered” SiO2 coating with visible striations and fractures. The coatings significantly mask the VNIR spectral character of the underlying basalt and cause distinct wavelength-dependent photometric behaviors, particularly in the NIR; these may significantly impact interpretations of Mastcam-Z data of coatings of Mars. Our SEM analyses of coating relationships challenge the previously published formation model; further work is needed to determine the coatings’ formation mechanism.