Morphology, chemistry, and spectral properties of Hawaiian rock coatings and implications for Mars

We studied coatings on five glass‐rich basalts from the Big Island of Hawaii. The coatings are characterized by complex morphologies and their thicknesses range from 3 to 80 μ m. Coating chemistries are predominantly hydrated silica with minor amounts of Fe‐, Ti‐, and S‐bearing materials. Visible, near infrared, and thermal infrared spectra of the coatings demonstrate that coatings as thin as 3 μ m mask the spectral character of the substrate basalt. The Fe‐, Ti‐, and S‐bearing components control the VNIR coating spectra. The hydrated silica component of the coatings dominates the thermal infrared coating spectra. The coating chemistries are consistent with leaching and/or dissolution of basalt glass or tephra in aqueous, acidic, and oxidizing conditions and subsequent precipitation of insoluble phases. Similar alteration conditions are thought to have occurred on Mars, making formation of such coatings on Martian lithologies feasible. Given the capabilities of various Mars missions, if coatings like those of this study were present on Martian lithologies, their chemical and/or spectral signatures would be detectable. Chemical and spectral data from thin (≤10 μ m), Fe‐ and Ti‐bearing coatings are consistent with phases capable of explaining the high‐SiO 2 component of Type 2 surfaces previously identified in the Thermal Emission Spectrometer data set and are potentially consistent with spectral data from the Mars Exploration Rovers.