In Situ Analysis of Opal in Gale Crater, Mars

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.