ChemCam First Discovery of High Silica Sediments in Gale Crater
Abstract:On sol 991, The Curiosity rover ascended a steep slope to Marias Pass in Gale Crater. Close to the top, ChemCam analyzed the rock target Elk from an apparent bright unit. Utilizing the new elemental calibration implemented for ChemCam in the summer of 2015, four of five points on Elk were measured to contain 76-82 wt% SiO2 and >3 wt% TiO2, whereas the last point showed elevated CaSO4. The Elk target is identified to be part of the Murray formation, and hence related to the Pahrump area mudstones that were subjected to intensive studies by the Curiosity rover team over the sols 758-948. While the Murray formation west of Elk did show elevated SiO2 (~65 wt%) compared to the Pahrump area, no targets with similarly high SiO2 wt% as Elk were observed, thus prompting - together with detection of anomalously high DAN H signals in the same area - the Curiosity rover to return to the Elk target area for additional analyses. This return led to numerous additional high Si observations (targets Pistol, Mary, Shepard, Dublin Gulch and Frog) that all corroborated the initial high Si observation at Elk. Additionally, the Alpha Particle X-ray Spectrometer (APXS) analyzed the target Lamoose and found SiO2 content in excess of 72 wt% and moderately elevated TiO2. Considering the difference in footprint (1.7 cm for APXS vs ~400μm for ChemCam) and the fact that the target couldn’t be brushed, this is considered a good corroboration of the very high Si observed with ChemCam.
These targets suggest that the Elk-area targets represent an end-member of the Murray formation, but there are multiple working hypotheses for the origin of the high SiO2 and TiO2 in these: 1. primary precipitates from the water column of a lake, 2. a post-depositional leaching/weathering front and 3. a hydrothermal silica precipitate.