Chloride salt-bearing deposits are widely distributed across the southern highlands of Mars. Because chloride salts are highly water-soluble, these deposits may be representative of the last significant period of stable liquid water at the Martian surface. Therefore, these deposits are key to understanding the fate and evolution of surface waters on Mars. Yet, little consensus exists about the formation conditions of these deposits, and their origins remain enigmatic. This is due in part because remote spectroscopic detection and quantification of many chlorides is hampered by a lack of easily discernible diagnostic absorption features. To address this issue, we present a novel Hapke radiative transfer model (RTM)-based method to estimate hydration states and salt abundances of Martian chloride salt-bearing deposits using visible/near-infrared (VNIR) reflectance spectra. VNIR laboratory spectra are used to derive water abundances of analog chloride-bearing materials, establishing an experimental basis for application of these methods to Mars. These methods are then applied to orbital Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data to create maps of hydration state and modeled salt abundance of chloride-bearing deposits. When overlain onto high resolution 3D digital terrain models (DTMs), these methods produce the highest resolution, site-specific salt abundance maps currently available, enabling new discoveries and understanding of geologic context. As an example, deposits in the Terra Sirenum region are observed to have higher estimated salt abundances than previously recognized, exhibiting spatial variations in both abundance and surface morphology.

Martian soils and rocks contain a significant fraction of amorphous materials, based on previous lander-based x-ray diffraction and orbital infrared measurements. However, the exact nature and chemistry of the phases that make up this component are not well constrained. The upcoming Mars2020 and ExoMars rovers will carry Raman and visible/near-infrared (VNIR) reflectance spectrometers, offering new methods for characterizing Martian surface materials in-situ. Raman spectroscopy in particular has the potential to discriminate between amorphous phases; however, many of the candidate amorphous phases are absent from Raman spectral databases. We synthesized and spectrally characterized candidate x-ray amorphous phases for Martian soils (amorphous ferric sulfate, allophane, ferrihydrite, allophane with adsorbed sulfate and phosphate, and ferrihydrite with adsorbed sulfate and phosphate) with Raman and VNIR spectroscopy and document the Raman peak locations for these materials. We found that sulfate and phosphate anions were Raman-detectable when adsorbed to allophane, but were not observed when adsorbed to ferrihydrite; a possible cause for this includes decomposition of the adsorbed species during the Raman acquisition. We show that candidate sulfur-bearing species – amorphous ferric sulfate and allophane with adsorbed sulfate – are distinguishable in Raman data. Allophane, ferrihydrite and amorphous ferric sulfate exhibit distinctive VNIR spectra, but are not likely to be distinguishable in the VNIR if mixed with other materials. The potential for detecting adsorbed species is a unique strength of Raman spectroscopy compared to other spectral methods, however further studies are needed to understand the acquisition conditions, abundances and matrix compositions under which adsorbed species can be detected.