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Abstract 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.
Sun et al. (2018) studied this question.