Silica enrichments resulting in up to ~90 wt% SiO₂ have been observed by the Curiosity rover's instruments in Gale crater, Mars, within the Murray and Stimson formations. Samples acquired by the rover drill revealed a significant abundance of an X‐ray amorphous silica phase. Laser‐induced breakdown spectroscopy (LIBS) highlights an overall correlation of the hydrogen signal with silica content for these Si‐enriched targets. The increased hydration of the high‐silica rocks compared to the surrounding bedrock is also confirmed by active neutron spectroscopy. Laboratory LIBS experiments have been performed to calibrate the hydrogen signal and show that the correlation observed on Mars is consistent with a silica phase containing on average 6.3 ± 1.4 wt% water. X‐ray diffraction and LIBS measurements indicate that opal‐A, amorphous hydrated silica, is the most likely phase containing this water in the rocks. Pyrolysis experiments were also performed on drilled samples by the Sample Analysis at Mars (SAM) instrument to measure volatile content, but the data suggests that most of the water was released during handling prior to pyrolysis. The inferred low‐temperature release of water helps constrain the nature of the opal. Given the geological context and the spatial association with other phases such as calcium sulfates, the opal was likely formed from multiple diagenetic fluid events and possibly represents the latest significant water‐rock interaction in these sedimentary rocks.

Mars’ transition from an early “warm and wet” to a “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 the Martian surface. In this study, we surveyed the mineralogy of Martian alluvial fans and deltas and investigated the hydrated silica-bearing deposits associated with some of these landforms. Using CRISM data, we identified 35 locations across Mars with hydrated silica in proximity to fans/deltas, where the spectral characteristics are consistent with immature or dehydrated opal-A. In a few stepped fans/deltas, we find hydrated silica occurs within the bulk fan deposits and form sedimentary layers correlated with elevation. Meanwhile, in the older fans/deltas, silica mostly occurs at distal locations, and the relation to primary sedimentary deposits is more complex. We propose that the hydrated silica-bearing deposits in stepped fans/deltas likely formed authigenically from Martian surface waters, mainly during the Late Hesperian and Early Amazonian. These silica-bearing deposits could be a tracer for the temperature or duration 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 2022 missions.