J. A. McGovern

Hydrated silicate minerals on Mars observed by the Mars Reconnaissance Orbiter CRISM instrument

Phyllosilicates, a class of hydrous mineral first definitively identified on Mars by the OMEGA (Observatoire pour la Mineralogie, L’Eau, les Glaces et l’Activitié) instrument, preserve a record of the interaction of water with rocks on Mars. Global mapping showed that phyllosilicates are widespread but are apparently restricted to ancient terrains and a relatively narrow range of mineralogy (Fe/Mg and Al smectite clays). This was interpreted to indicate that phyllosilicate formation occurred during the Noachian (the earliest geological era of Mars), and that the conditions necessary for phyllosilicate formation (moderate to high pH and high water activity) were specific to surface environments during the earliest era of Mars’s history. Here we report results from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) of phyllosilicate-rich regions. We expand the diversity of phyllosilicate mineralogy with the identification of kaolinite, chlorite and illite or muscovite, and a new class of hydrated silicate (hydrated silica). We observe diverse Fe/Mg-OH phyllosilicates and find that smectites such as nontronite and saponite are the most common, but chlorites are also present in some locations. Stratigraphic relationships in the Nili Fossae region show olivine-rich materials overlying phyllosilicate-bearing units, indicating the cessation of aqueous alteration before emplacement of the olivine-bearing unit. Hundreds of detections of Fe/Mg phyllosilicate in rims, ejecta and central peaks of craters in the southern highland Noachian cratered terrain indicate excavation of altered crust from depth. We also find phyllosilicate in sedimentary deposits clearly laid by water. These results point to a rich diversity of Noachian environments conducive to habitability.

A hematite-bearing layer in Gale Crater, Mars: Mapping and implications for past aqueous conditions

Oversampled Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) visible and near-infrared hyperspectral data over Mount Sharp in Gale Crater, Mars, were used to generate spatially sharpened maps of the location of red crystalline hematite within the uppermost stratum of an ∼6.5-km-long ridge on the mound’s northern flank. Finely layered strata underlie the ridge to the north and have dips consistent with the nearby Mount Sharp sedimentary sequence. Fe-Mg smectites are exposed in a valley to the south of the ridge. Emplacement of the hematite is hypothesized to result either from exposure of anoxic Fe^(2+)-rich groundwater to an oxidizing environment, leading to precipitation of hematite or its precursors, or from in-place weathering of precursor silicate materials under oxidizing conditions. These hypotheses and implications for habitability will be testable with in situ measurements by the Mars rover Curiosity when it reaches Mount Sharp.

New insights into gully formation on Mars: Constraints from composition as seen by MRO/CRISM

Over 100 martian gully sites were analyzed using orbital data collected by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO). Most gullies are spectrally indistinct from their surroundings, due to mantling by dust. Where spectral information on gully sediments was obtained, a variety of mineralogies were identified. Their relationship to the source rock suggests that gully-forming processes transported underlying material downslope. There is no evidence for specific compositions being more likely to be associated with gullies, or with the formation of hydrated minerals in situ as a result of recent liquid water activity. Seasonal CO2 and H2O frosts were observed in gullies at mid- to high latitudes, consistent with seasonal frost-driven processes playing important roles in the evolution of gullies. Our results do not clearly indicate a role for long-lived liquid water in gully formation and evolution.