Potential Martian Evaporites and their Spectral Signatures

<p>Evaporite deposits, including chlorides, sulfates, and carbonates, have been detected remotely on the surface of Mars. These salts are not only indicative of interaction with water, but can provide clues to the environmental conditions of their formation. Briny solutions have been proposed as one way to explain Martian salt deposits, and several studies have previously examined those briny solutions most likely to form under past Martian conditions. Brines have also been invoked to explain Martian surface features including dunes, duricrust, and recent surface flow features, and previous work has suggested brines that would be metastable within the present-day Martian surface or subsurface. The majority of the brines previously analyzed have only been modeled; this study aims to understand experimentally the salts likely to precipitate under varied conditions, and to analyze and catalog the visible and near-infrared spectral signature produced by likely multi-component salt suites. Solutions were chosen from previously proposed, theoretical Martian brines to represent diverse potential compositions, and when necessary, modeled to appropriate concentrations. These brines were evaporated within a fume hood under monitored "Earth" conditions and within a simulated Martian environment. Precipitates were analyzed using X-ray diffraction (XRD), scanning electron microscope (SEM), and VNIR (350 - 2500nm) spectroscopy.</p> <p>Anhydrous halides, as expected, were undetectable in the VNIR. Those brines with significant carbonate in their starting chemistry produced hydrated carbonates, detectable by their water, rather than the C-O absorptions currently used to detect Martian carbonates. Spectral signatures were typically dominated by hydrated sulfates, whether or not they were the most volumetrically significant mineral class. Mg and Fe brines produced two distinct Mg and Fe sulfates, neither of which perfectly matched the spectra of the hydrated end-member sulfates. Ferric sulfates, when present, dominated the visible portion of the spectra, even when present in minute volumes. Amorphous material was common within the most hydrated brines, and created broader, less pronounced absorptions than their respective crystalline varieties.</p>