Hydrated Minerals at Yellowknife Bay, Gale Crater, Mars: Observations from Mastcam's Science Filters

The Mastcam imaging investigation on the Mars Science Laboratory Curiosity rover can help constrain the mineralogy of sulfates and phyllosilicates present in the outcrop at Yellowknife Bay within Gale crater. The two Mastcams provide high-resolution morphological information, RGB color, and visible to near-infrared (Vis-NIR) multispectral data using narrowband 'science filters' covering 12 unique wavelengths between 432 and 1013 nm. The longest wavelength filters have some sensitivity to hydrated and/or hydroxylated minerals, as they can detect an absorption due to the 2ν1 + ν3 H2O combination band and/or the 3ν OH overtone when this band minimum occurs between 980 and 1000 nm (in water ice and some carbonates and hydrated sulfates). This narrow hydration band leads to a Mastcam spectral profile that is distinguishable from spectra of iron-bearing minerals with broad absorptions near 1000 nm. Here we use this spectral profile to help interpret the mineralogy hydrated surface materials in Mastcam multispectral images. The light-toned, Ca-sulfate fracture-fills of the Sheepbed Unit at Yellowknife Bay show some evidence for hydration in calibrated Mastcam spectra from sols 133-269. From comparisons to laboratory reflectance spectra of Ca-sulfate minerals convolved to Mastcam bandpasses, the hydration signature near 1013 nm is consistent with the presence of gypsum, but not bassanite or anhydrite. Several narrow fracture-fills observed in the Sheepbed Unit show no evidence for hydration, such as the thin veins at the John Klein and Cumberland drill sites. These observations are in accord with CheMin XRD observations (where bassanite and anhydrite were detected, but not gypsum). Mastcam hydration detections in nearby veins, however, suggest that Ca-sulfate hydration states may vary within the fracture-fill networks on local scales. The phyllosilicate-bearing outcrop of the Sheepbed Unit, when broken by Curiosity's wheels or brushed by the rover's Dust Removal Tool (Mastcam spectra from sols 172-174), has grayish hues and exhibits higher reflectances in the short-wavelength Mastcam filters (447 to 638 nm) than previously observed along the traverse. Some spectra show slightly negative near-IR spectral slopes, consistent with the presence of pyroxene and/or olivine in typical basaltic materials. These fresh surfaces do not exhibit the Mastcam hydration feature near 1013 nm, but the absence of the hydration signature in Mastcam spectra does not necessarily indicate an absence of hydrated minerals, and spectra of most phyllosilicates do not have an absorption detectable to Mastcam's 1013 nm filter. Many smectite spectra exhibit a 937 nm feature that should be detectable to Mastcam, however, and the absence of this feature places constraints on the phyllosilicate phases present at Yellowknife Bay.