MINERALOGY AND STRATIGRAPHY OF GALE CRATER RIM, WALL, AND FLOOR UNITS

Within Gale crater, Mars, lies a large sedimentary mound informally known as Mt. Sharp. Formation hypotheses include deposition of lacustrine sediments, aeolian deposition, or ash fall and subsequent weathering or diagenesis [e.g. 1, 2]. Determining the composition of the bedrock of Gale crater and its surroundings is important for determining the provenance of the Mt. Sharp sediments and the timing and location of aqueous alteration. We have used CRISM infrared spectroscopy combined with HiRISE and CTX images to identify key minerals in the Gale crater wall and rim and in distinct mineralogical/ morphological units on the Gale floor that onlap or interfinger with Mt. Sharp units. Olivine is detected in multiple bedrock locations including the southern wall, Farrah Valles, the NW wall of Gale, and the Northern rim. Fe-Mg smectites have also been detected in the southern and NW walls, floor units, and in Farrah Valles. The Curiosity rover identified ferrian smectites at Yellowknife Bay [3], and nontronite has been identified within a unit of lower Mt. Sharp [4].

Our analyses show Fe,Mg-phyllosilicates are detected over a significant area of Gale and are a key mineralogical component of the bedrock and crater floor units. The Fe-Mg smectites and hydrated phases observed on the floor of Gale and in the Mt. Sharp sediments may be formed in situ or transported. If transported, the sedimentary smectites should match source material spectral properties. However the wavelength of the metal-OH absorption varies slightly in different outcrops. A key difference in all spectra is the presence of a 2.23-micron absorption characteristic of AlFe-OH in Mt. Sharp sediments that is absent in the wallrock and floor materials. Additionally there is regional variability in the strength of water-related absorptions with some regions displaying only absorptions associated with hydroxylation and not water. These imply that the lower Mt. Sharp phyllosilicate unit and the phyllosilicate-bearing materials found elsewhere in Gale differed in starting protolith and/or the chemistry of altering fluids. We are currently mapping the stratigraphic relationships of the wall, valles, and floor units.

[1] Pelkey & Jakosky , 2002, Icarus [2] Anderson & Bell, 2010, Mars J. [3] Vaniman et al, 2014, Science [4] Milliken et al, 2010, GRL