Clay and Sulfate-bearing Rocks in a Stratigraphic Sequence in Gale Crater

Introduction: Gale Crater is a ~150 km diameter impact crater located at 5.3°S, 137.7°E that contains a thick (>5 km) sequence of strata interpreted as sedimentary rocks [1]. Gale is one of four final candidate landing sites for the 2011 Mars Science Laboratory (MSL) rover. Recent data acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument show evidence for clay-bearing rocks overlain by and potentially interbedded with sulfate-bearing units in this stratigraphic sequence. Data and Methods: We examined all existing CRISM ‘targeted’ images (ranging from ~18 – 36 m/pixel) covering Gale’s central mound and surrounding portions of the crater (Fig. 1). CRISM radiance spectra were divided by the solar flux, cosine of the incidence angle, and a scaled atmospheric transmission spectrum to produce I/F spectra. Pixels (spectra) were then averaged for different units of interest and then divided by the spectral average of a dusty or otherwise spectrally ‘neutral’ area. Spectral ratios act to accentuate weak absorption features that might otherwise not be observed in the unratioed I/F spectra and have been used successfully in previous studes [e.g., 2, 3]. High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) images were used to guide the pixel selection process to ensure that averaged pixels corresponded to distinct stratigraphic or geologic units. Band parameters (e.g., band depth of the 1.9 μm H2O feature) described by [4] were also examined to search for mineralogically interesting areas and to help select pixels for averaging. Maps of these band parameters were overlain on CTX and HiRISE images to determine the stratigraphic relationship of different mineral signatures (Fig. 2). Results: CRISM spectra of the bottom ~2 km of stratigraphy in the mound exhibit absorptions consistent with the presence of Fe-rich smectites (i.e., nontronite, Fig. 3), monoand polyhydrated sulfates (likely Mg sulfates, Fig. 4), hematite, pyroxene, and olivine. The uppermost portion of the mound is spectrally similar to dusty regions, consistent with the lower thermal inertia values [5] that also suggest dust mantles the upper rock units. The dark dunes on the crater floor commonly exhibit pyroxene and/or olivine signatures. Similarly, what appears to be the youngest stratigraphic unit also exhibits pyroxene signatures. This unit is found in the northern part of the crater (including areas adjacent to the proposed MSL landing ellipse) and onlaps the lowermost strata of the mound with a dip to the north-northeast. In some locations, small (~100s meters) erosional windows through this unit exhibit hematite signatures, though more data are needed to determine the exact stratigraphic position and extent of these oxides. The majority of beds exposed in the lower ~2 km of the mound exhibit sulfate signatures, primarily polyhdyrated sulfates. These signatures are weak along the northern portion of the mound owing to moderate dust cover, but they become progressively stronger along the western edge of the mound where the dust cover is minimal. In Fig. 2, the reddish-orange units indicative of sulfates can be traced along the mound and are clearly exposed in a large canyon on the west side of the mound. The sulfate spectra are most consistent with polyhydrated suflates, although spectra of the cyan-colored regions in Fig. 2 exhibit features at ~2.1 and ~2.4 μm consistent with a monohydrated sulfate such as kieserite (Fig. 4).