Geological significance of hydrated silica on mars as seen by crism data compared to terrestrial analogs

Introduction: Orbital studies of Mars have shown abundant mineralogical evidences of water-rock interactions using the VNIR imaging spectrometers OMEGA and CRISM [1,2]. However, there are only few constraints on the physicochemical conditions of the climate prevailing during early Mars. In particular, relative contributions of hydrothermal activity and continental weathering are still uncertain. Several minerals, such as hydrated silica, can form at low temperature by continental weathering, suggesting surface-atmosphere interactions [3]. But, they can also form at higher temperatures by hydrothermal alteration [3]. In that case, subsurface hydrothermal activity would not imply the occurrence of an open system with surface-atmosphere interactions to form hydrous minerals, and no interpretation on the past climate can be deciphered from the observation of such geological contexts. Previous studies showed that the near-infrared spectral signature of hydrated silica could give information about its crystallinity (hydrated glass, opalA/CT, chalcedony) [3,4,5,6]. Moreover, specific band shape measurements (Concavity-Ratio-Criterion CRC) can provide constraints on the formation processes of terrestrial opals (continental weathering versus hydrothermal alteration) [3]. We applied these spectral criteria to reflectance spectra of terrestrial varieties of hydrated silica and to CRISM data in order to better understand the major geological processes that formed hydrated silica on Mars. Material and methods: We have compared terrestrial data of several types of hydrated silica (hydrated glasses, opal-A/CT, chalcedony) (Fig. 1) with specific spectral criteria: Concavity-Ratio-Criterion [3] and band minimum positions (Fig. 2). NIR spectral acquisitions of terrestrial hydrated silica were carried out at ambient and at Mars-relevant atmospheric pressure (~ 7.5 mbar during ~ 3 to 15 hours) [7]. We have used CRISM data and plot them in the framework of terrestrial data results in order to check for spectral signatures ascribable to weathering or hydrothermal alteration. I/F-converted CRISM images are corrected from atmospheric gas absorptions by the scaled volcanoscan method [8]. Extraction of hydrated silica CRISM spectra have been conducted using the processing tools specified in [9]. These spectral analyses have been correlated with HiRISE and CTX images in order to assess the geomorphological settings of these hydrated silica-bearing deposits.