Smectites in the Rim of Endeavour Crater, Mars, Detected Using Along-Track Oversampled CRISM Observations

Smectite minerals in the rim of the Noachian aged Endeavour Crater demonstrate that aqueous alteration under relatively mild acidity and oxidation potential was a pervasive process, probably in early Martian history. Reflectance spectra retrieved from along-track oversampled Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) observations have been key in directing the Opportunity rover to relatively small outcrops with smectite signatures. CRISM I/F data were converted to single scattering albedo (SSA) using the radiative code package DISORT to best identify subtle smectite absorption features without dependence on column ratios or assumed atmospheric transmission spectra. Scattering and absorption by atmospheric dust and ice aerosols are modeled from first principles, and CO2, CO and water vapor absorptions are modeled as a function of atmospheric column mass. Surface scattering assumes a Hapke surface model with a modestly backscattering single particle phase function. Whereas SSA values are retrieved from 0.4 to 2.65 μm, we primarily consider the 2.1 and 2.5 μm region. Metal-OH combination absorptions are clearly evident on Mars at these wavelengths and are characteristic of smectites. Spectral noise in this region is minimized by both instrumental and physical factors. Shallow absorptions at 2.3 and 2.4 μm, indicating Fe and Mg bearing smectites, were detected in the floor of Marathon Valley, on the western edge of Endeavour Crater, and were evident in several overlapping CRISM observations. Exploration of these outcrops by Opportunity establishes the geologic context and paleoenvironmental conditions associated with the formation of the smectites. Al-bearing smectites are also detected in several scenes over the western rim of Endeavour, particularly in sun-facing and soil-free outcrops.