Hydrated silica in Oxia Planum, Mars

Oxia Planum, Mars, is the future landing site of ESA’s ExoMars Rosalind Franklin rover (EMRF, launching 2028), which will search for physical and chemical biosignatures at the surface and subsurface using its analytical suite of instruments, the ‘Pasteur’ payload [1]. Oxia Planum contains detections of deposits containing hydrated silica (SiO2·nH2O; opal [2]). Hydrated silica is important in understanding aqueous processes and habitability on Mars owing to its numerous formation pathways which invariably require liquid water, and its excellent preservation potential for physical and chemical biosignatures that may be present. CRISM data indicate the presence of opal-bearing material (hydrated silica unit; HSU) in two main physiogeographic locations within Oxia Planum. Firstly, HSU is present in a thin (~5 m), bright-toned, blueish-white unit positioned stratigraphically below the sedimentary fan, and above the phyllosilicate-bearing plains. The fan body also contains exposures of meter-scale, laterally-discontinuous outcrops of bright-toned, similarly-colored strata. CaSSIS CBRCs indicate that these outcrops are identical in color to the CRISM detections of HSU in the larger, exposed outcrops. This, as well as their similar relationship to the fan, indicates that they are likely to also be HSU. Outcrops of HSU are also present infilling topographic lows south of the sedimentary fan, at the margins of Pelso Chasma. The position of the 1.4 µm and 2.2 µm BDR values from targeted CRISM cubes indicate that the HSU in Oxia Planum is predominantly amorphous (Opal-A); ten out of fourteen spectra plot within the Opal-A field, three plot within the crystalline opal (Opal-CT) field, and one plots in the overlap region. The mean crystallinity of opal in Oxia Planum is similar to the mean crystallinity of opal in fans elsewhere on Mars. CRC values for the 1.4- and 1.9- micron features both indicate that opal in the HSU is predominantly weathering-derived, instead of hydrothermally-derived. Aqueous alteration of hydrated silica under martian conditions can alter its crystallinity through dissolution and reprecipitation by circulating fluid, over time converting relatively more amorphous opal (Opal-A) into relatively more crystalline opal (Opal-CT; [3]). The observation of Opal-A at Oxia Planum, situated directly above clay-bearing plains that underwent aqueous alteration [4], implies an unconformity exists between the alteration of the clay-rich plains and the deposition of the overlying hydrated silica-bearing unit, and therefore also between the plains and the sedimentary fan. References: [1] Vago et al. (2017) Astrobio. 17, 471-510; [2] Quantin-Nataf et al. (2021) Astrobio. 21, 345-366; [3] Sun & Milliken (2018) GRL. 45, 10,221-10,228; [4] Mandon et al. (2021) Astrobio. 21, 464-48