Visible to near-infrared reflectance and Raman spectra of evaporites from sulfate-chloride Mars analogue brines

While much attention has been given to the identification and characterization of single-phase salts on Mars, relatively little has been applied to mixed evaporative assemblages. Given the likely existence of these assemblages on Mars (e.g., basin deposits) and the lack of data on their spectral signatures, here we present an experimental study of multicomponent S and Cl-bearing Mars-relevant brines. We modeled, synthesized and evaporated brines in the laboratory under both martian and terrestrial (P, T, pCO2) environmental conditions, and characterized the resulting precipitates using Visible–Near Infrared (VNIR), Raman spectroscopy, XRD and SEM-EDS. We compared these results to mineral assemblages calculated using the FREZCHEM thermodynamic model. For mixed brines primarily containing Na+, K+, Mg2+, Ca2+, SO42− and Cl−, epsomite (MgSO4•7H2O) and bischofite (MgCl2•6H2O) overwhelm VNIR and Raman spectra, while anhydrous crystalline salts and Na- and K-sulfates are unidentified. Mg-sulfates are identifiable in the VNIR and Raman even at low or no modeled mass abundance in an evaporative assemblage and are often the only clearly identifiable salt. These results imply that regions of Mg-sulfate identification on Mars may have only minor amounts of Mg-sulfate present, and significant amounts of halides or other sulfates may be undetectable. This may be due to a combination of late-stage Mg-sulfate precipitation and non-linear spectral mixing. Raman is more sensitive than VNIR to the identification of Ca-sulfate salts in these mixed assemblages. We predict that high abundances of mixed chloride and sulfate salts species will be identified as the Curiosity Rover continues to explore the Sulfate Unit of Gale Crater, and note that the Perseverance rover offers the first opportunity to identify such mixed assemblages in Jezero Crater with this combination of techniques.