Effect of halite and calcite coatings on thermal infrared spectra wih implications for Mars exploration

Knowing the occurrence and distribution of soluble ionic salts allows us to model the aqueous history and geochemical conditions of surface solutions on Mars. Salts deposited as coatings are particularly important for attaining this knowledge because the thermal infrared (TIR) remote sensing method for determining mineral abundance is limited to the top few hundred micrometers of the surface. To better understand the TIR properties of salt coatings, we investigated the effects of coating thickness and texture on the TIR reflectance spectra of haliteand calcite-coated glasses. We evaluated salt coating textures that could occur on Mars: 1) continuous, uniformly thick layers of salt without exposed substrate; 2) discontinuous particulate salt coatings with variable thickness and exposed substrate. Salt-coated glasses have spectral features attributable to the substrate and the coating. Partially transmissive coatings decrease the intensity of substrate absorption bands as thickness increases. This decrease in intensity is exponential for continuous and coarse particulate (< 50 μm) halite coatings and linear for fine particulate halite coatings (< 2 μm). All halite coating textures and continuous calcite