Images from the Mars Science Laboratory (MSL) mission of lacustrine sedimentary rocks of Vera Rubin ridge on Mt. Sharp in Gale crater, Mars, have shown stark color differences ranging from red to purple to gray. Because these color differences cross-cut stratigraphy, they are likely due to diagenetic alteration after deposition of the sediments. However, the chemistry and timing of the fluid interactions is unclear. Determining how diagenetic processes may have modified chemical and mineralogical signatures of ancient environments is critical for understanding the past habitability of Mars and the goals of the MSL mission. Here we use visible/near-infrared spectra from Mastcam multispectral images and ChemCam to determine the mineralogical origins of color changes in the ridge. We show that color variations are consistent with changes in spectral properties related to the crystallinity, grain size, and texture of hematite. Coarse-grained gray hematite dominates in the gray patches and is present in the purple areas, while nanophase and fine-grained red crystalline hematite are present in the red and purple areas. We hypothesize that these differences were caused by grain size coarsening of hematite by diagenetic fluids, as observed in terrestrial analogs with similar color variations. In this model, early primary reddening by oxidizing fluids near the surface was followed during or after burial by bleaching to form the gray patches, with limited secondary reddening after exhumation. Understanding the spectral signatures of these diagenetic processes will help in the future to identify less altered areas with a higher likelihood of organic preservation.

For ~ 500 sols, the Mars Science Laboratory team explored Vera Rubin ridge (VRR), a topographic feature on the northwest slope of Aeolis Mons. Here we review the sedimentary facies and stratigraphy observed during sols 1800-2300, covering more than 100 m of stratigraphic thickness. Curiosity’s traverse includes two transects across the ridge, which enables studies of lateral variability over a distance of ~ 300 m. Three informally named stratigraphic members of the Murray formation are described: Blunts Point, Pettegrove Point, and Jura, with the latter two forming the ridge. The Blunts Point member, exposed just below the ridge, is characterized by a recessive, fine-grained facies that exhibits extensive planar lamination and is crosscut by abundant curviplanar veins. The Pettegrove Point member is more resistant, fine-grained, thinly planar laminated, and contains a higher abundance of diagenetic concretions. Conformable above the Pettegrove Point member is the Jura member, which is also fine-grained and parallel stratified, but is marked by a distinct step in topography which coincides with meter-scale inclined strata, a thinly and thickly laminated facies, and occasional crystal molds. All members record low-energy lacustrine deposition, consistent with prior observations of the Murray formation. Uncommon outcrops of low-angle stratification suggest possible subaqueous currents, and steeply inclined beds may be the result of slumping. Collectively, the rocks exposed at VRR provide additional evidence for a long-lived lacustrine environment (in excess of 10^6 years via comparison to terrestrial records of sedimentation), which extends our understanding of the duration of habitable conditions in Gale crater.