Spectral, Compositional, and Physical Properties of the Upper Murray Formation and Vera Rubin Ridge, Gale Crater, Mars

During 2018 and 2019, the Mars Science Laboratory Curiosity rover investigated the chemistry, morphology, and stratigraphy of Vera Rubin ridge (VRR). Using orbital data from the Compact Reconnaissance Imaging Spectrometer for Mars, scientists attributed the strong 860 nm signal associated with VRR to the presence of red crystalline hematite. However, Mastcam multispectral data and CheMin X-ray diffraction (XRD) measurements show that the depth of the 860 nm absorption is negatively correlated with the abundance of red crystalline hematite, suggesting that other mineralogical or physical parameters are also controlling the 860 nm absorption. Here, we examine Mastcam and ChemCam passive reflectance spectra from VRR and other locations to link the depth, position, and presence or absence of iron-related mineralogic absorption features to the XRD-derived rock mineralogy. Correlating CheMin mineralogy to spectral parameters showed that the similar to 860 nm absorption has a strong positive correlation with the abundance of ferric phyllosilicates. New laboratory reflectance measurements of powdered mineral mixtures can reproduce trends found in Gale crater. We hypothesize that variations in the 860 nm absorption feature in Mastcam and ChemCam observations of VRR materials are a result of three factors: (1) variations in ferric phyllosilicate abundance due to its similar to 800-1,000 nm absorption; (2) variations in clinopyroxene abundance because of its band maximum at similar to 860 nm; and (3) the presence of red crystalline hematite because of its absorption centered at 860 nm. We also show that relatively small changes in Ca-sulfate abundance is one potential cause of the erosional resistance and geomorphic expression of VRR. Plain Language Summary Results from near-infrared spectral measurements showed that Vera Rubin ridge (VRR) in Gale crater, Mars, has a unique spectral signature compared to the surrounding Mt. Sharp units, which was confirmed using instruments onboard the Curiosity rover. This paper describes the spectral, compositional, and physical properties of eight rocks that were drilled and analyzed by the Curiosity rover, including three rocks that are on VRR. Quantitative mineralogy, determined using the CheMin instrument, found that all eight rocks had plagioclase, clinopyroxene, ferric phyllosilicates, and red crystalline hematite in significant abundances. These minerals all have unique spectral signatures that contributed to spectra obtained by the Mastcam multispectral imager of the eight drilled rocks. By comparing spectral parameters of drill tailings from the eight rocks to their quantitative mineralogy, this paper found that the spectral differences of VRR and surrounding units are the result of variations in ferric phyllosilicates, clinopyroxene, and red crystalline hematite. New lab spectral measurements successfully reproduced the same trends as seen in data from the Curiosity rover. The results of this work help us better understand how different minerals affect the spectral signals we measure on Mars, which is extremely important to understanding its geologic history.