Voluminous silica precipitated from martian waters during late-stage aqueous alteration

Mars' transition from an early "warm and wet" to the "cold and dry" environment left fingerprints on the geological record of fluvial activity on Mars. The morphological and mineralogical observations of aqueous activity provided varying constraints on the condition and duration of liquid water on martian surface. In this study, we surveyed the mineralogy of martian alluvial fans and deltas and investigated the hydrated silica-bearing deposits associated with these landforms. Using CRISM data, we identified 35 locations across Mars with hydrated silica in proximity to fan/deltas, where the spectral characteristics are consistent with immature or dehydrated opal-A. In a few stepped fan/deltas, we find hydrated silica occurs within the bulk fan deposits and form sedimentary layers correlated with elevation, corroborating the formation of hydrated silica through precipitation. Meanwhile in the older fan/deltas silica mostly occur at distal locations and the relation to primary sedimentary deposits is more complex. We propose that the hydrated silica-bearing deposits in stepped fan/deltas likely formed authigenically from martian surface waters, mainly during the Late Hesperian and Early Amazonian [Hauber et al., 2013]. These silica-bearing deposits could be a tracer for the temperature of water involved in the formation of these deposits, given more precise and detailed observations of the sedimentary context, accessory minerals, the concentration of hydrated silica and sediment-to-water ratio. Therefore, we consider that silica-bearing deposits should be among the most critical samples to investigate for future Mars missions, which are accessible in the landing sites of Mars 2020 and ExoMars missions.