Aqueous mineralogy and stratigraphy at and around the proposed Mawrth Vallis MSL Landing Site: New insights into the aqueous history of the region

We have analyzed Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) hyperspectral images from the region within and around the proposed Mawrth Vallis landing ellipse for the Mars Science Laboratory (MSL) rover in order to generate a mineralogical map of the units within the landing ellipse that can be used to guide MSL mission planning. Spectroscopic and morphological studies of the walls of a crater near the ellipse have allowed us to identify a stratigraphic section consisting of at least four primary compositional units including two Fe/Mg-smectite-bearing units at the bottom of the section, an Al-phyllosilicate-bearing unit above that, and in some instances, a Mg- phyllosilicate-bearing unit above that. HiRISE images of the Fe/Mg-smectite-bearing units typically show clear evidence for layering in the unbrecciated portions of the crater walls. The bedding planes are not always parallel and may exhibit truncations, ripple-like bedding and concave forms. The layers under the synclinal forms are not deformed, supporting the hypothesis that these synclinal forms are cross-sections of buried channels. The Al-phyllosilicate-bearing unit often shows evidence for an enhanced concentration of ferrous material at its base, as well as mineralogical evidence for limited exposure to acidic fluids. This unit typically presents fracture patterns near the top, and layering near the base. In the context of an impact crater wall, it is difficult to determine whether this pattern is caused by the impact itself or by some other depositional or diagenetic/pedogenic process. In addition to these minerals, we have identified material with a spectral signature consistent with either the sulfate bassanite or the zeolite analcime just east of the proposed landing ellipse. This material is associated with a unit that has a pitted-and-etched morphology and underlies the layered Fe/Mg-smectite-bearing units (although Fe/Mg-smectites are also prevalent throughout the rest of this unit). We interpret this pitting and etching to be the result of the dissolution of sulfates by groundwater activity. A variety of environmental conditions appear to be preserved within the landing ellipse, with Fe/Mg-smectites preserving evidence for more basic conditions exposed primarily in the eastern portion of the ellipse, and Al-phyllosilicates preserving evidence for more acidic conditions occurring dominantly toward the western portion of the ellipse. The boundary between these two mineralogical zones is sharp, well-exposed, and easily accessible.