Constraints on the Formation and Alteration History of Mt. Sharp, Gale Crater, Mars, from a New Crism Mineral Map

Introduction: The primary target for the Mars Science Laboratory (MSL) rover investigation in Gale Crater is the 5 km thick stack of layers that make up the central mound of the crater, known officially as Aeolis Mons and informally as Mt. Sharp [1,2]. Previous near-infrared spectral investigations using the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) have shown that the northwest portion of the mound that MSL will investigate in situ exhibits diverse mineralogy, including smectites, sulfates, and iron oxides, which may indicate changing climates and/or aqueous environments on early Mars [3]. However, to date, no comprehensive map of the mineralogy of the rest of the mound has been published. Here we present new mineral maps of Mt. Sharp derived from CRISM data, and use these new results to constrain hypotheses for the origin of the layers and modes of aqueous alteration in Gale Crater. Methods: Mineral maps were created from CRISM cubes at the TRR3 calibration level and processed to estimated Lambert albedo using the CRISM Analysis Toolkit (CAT) for ENVI, including the standard volcano-scan atmospheric correction [4]. In order to suppress systematic noise, residual atmospheric effects, and dust, spectrally neutral reference spectra were calculated for each column of each cube, and then all spectra were divided by the corresponding reference spectrum for their column. “Neutral” spectra have low or null values for standard CRISM spectral parameters [5] (typically LCPINDEX<0.01, HCPINDEX<0.05, OLINDEX2<0.06, BD1900<0.005, BD2290<0.005, D2300 <0.005, SINDEX<0.02, and BD2500<0.01). Parameters were then recalculated on ratio spectra. To verify mineral detections based on spectral paramters, local band minima locations were found in the regions of 0.8-0.98, 2.0-2.2, and 2.16-2.35 μm relative to linear continua between those endpoints. Spectral units were defined based on parameters and band minima. Spectral maps were projected and co-registered to HiRISE imagery and digital elevation models in ArcGIS. Spectral units: Spectral units from our work are consistent with previous investigations [3,6] (Table 1), including Fe/Mg-smectites, both polyand monohydrated sulfates (PHS/MHS), iron oxides, high-Ca pyroxene (HCP), and a ferrous phase with a strong red spectral slope beween 1.1-1.8 μm. Our unit distributions within the previously mapped MSL future traverse area in the NW mound agree well with previous Table 1: Mineral units and spectral definitions Unit Spectral Definition High-Ca pyroxene Elevated HCPINDEX