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Opaline silica deposits have been identified on Mars from orbital near-infrared (NIR) reflectance spectra, but opal types (e.g., -A,/CT) have not yet been conclusively determined due to insufficient laboratory spectra acquired at Mars-relevant conditions. NIR reflectance spectra of siliceous deposits acquired by the Compact Reconnaissance Imaging Spectrometer for Mars instrument are compared with new laboratory spectra of terrestrial opal samples that we measured under Mars-relevant conditions. Opal-A occurrences on Mars are commonly associated with bedrock exposures, whereas more crystalline hydrated silica (opal-CT and quartz/chalcedony) is primarily observed in unconsolidated sediments. These differences suggest some opal-bearing deposits may have experienced prolonged interaction with near-surface water or diagenetic fluids, and bedrock exposures dominated by opal-A may have experienced less post-formation diagenesis compared with those containing more crystalline hydrated silica. NIR data thus provide a useful tool to assess opal maturity and relative potential for biosignature preservation for different siliceous deposits on Mars.

Plain Language Summary Opals form in habitable aqueous environments and are known to preserve biosignatures on Earth. Opal-bearing deposits have also been identified on Mars, but the distribution of opal type is not well-understood and carries astrobiological implications. Opal-CT and quartz phases are more crystalline than opal-A and often result from post-formation interaction of opal-A with water; such progressive diagenesis could lead to less pristine biosignature preservation. In this work, we distinguish between opal-A and more crystalline hydrated silica (opal-CT and quartz/chalcedony) on Mars for the first time using orbital data. These orbital observations are compared to analogous near-infrared reflectance spectra of terrestrial opal-A, opal-CT, quartz, and chalcedony. On Mars, we find that opal-A is associated with bedrock, whereas more crystalline hydrated silica is associated with aeolian sediment. Because opal-A transforms into more crystalline forms in the presence of water, these findings suggest that silica in aeolian deposits has experienced longer interaction with water than opal-bearing bedrock throughout Martian history. These methods can help better identify and prioritize hydrated silica outcrops on Mars according to their likely degree of post-formation interaction with water and thus their potential to preserve biosignatures. These results have applications towards future astrobiological investigations such as the National Aeronautics and Space Administration's Mars 2020 rover.