Compositional investigation of the proposed chloride-bearing materials on Mars using near-infrared orbital data from OMEGA/MEx

Several hundred occurrences of chloride-bearing salt deposits have been proposed in terrains within the southern highlands of Mars on the basis of Thermal Emission Imaging System and Thermal Emission Spectrometer infrared observations. The spectral identification of chloride salts by remote sensing is challenging because they are transparent over much of the thermal infrared portion of the spectrum. Further ambiguity arises from the diverse geologic settings in which the putative chloride-bearing materials are found. In order to better constrain the composition of these unique compositional units, we perform a global survey of these materials in the Near-Infrared (NIR) domain with the Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activite (OMEGA) imaging spectrometer. The spectral signatures of the deposits are consistent with - although not specific of - chlorides. We do not observe olivine to be associated with the deposits, which confirms that sulfides are an unlikely alternative candidate. Our systematic search reveals the global lack of association with hydrated minerals (phyllosilicates, sulfates, hydrated silica) except for a few deposits (noteworthy in northwestern Terra Sirenum) where a small fraction of chloride material overlaps Fe/Mg-rich clay-bearing terrains. Even in these locations, the morphology and crosscutting relationships of the deposits suggest two separate episodes of mineralization, first phyllosilicates then chlorides, followed by subsequent formation of sulfates. Our study shows that local groundwater upwelling seems to be the most frequent source for the water involved in the formation of chloride, rather than surface runoff.