The Colorful Story of Aqueous Alteration on Vera Rubin Ridge, Gale Crater, Mars

Credit: Figure 15 of Horgan et al., 2020 showing a) a Mastcam image of Red Cliff on Vera Rubin ridge, b) Mastcam spectra showing the different spectral variabilities associated with red and gray bedrock, and c-e) Enhanced-color stretch close-up images of the areas on Red Cliff outlined in a).

In 2017, the NASA Mars Science Laboratory (MSL) Curiosity rover reached a locality in Gale crater informally named Vera Rubin ridge. The 6.5-kilometer-long and approximately 250-meter-wide Vera Rubin ridge was identified from orbit as a location of interest for the MSL mission because it showed strong hematite signatures in visible and near-infrared observations. Hematite is an iron-oxide mineral of astrobiological interest because it largely occurs on Earth in sedimentary units deposited and/or altered in habitable aqueous environments. When Curiosity reached Vera Rubin ridge, it returned images of laminated mudstones indicating deposition in a lake environment. These lake deposits showed a diversity of colors and alteration features indicative of a complex history of aqueous alteration, which Briony Horgan from Purdue University and collaborators investigated using visible and near-infrared color images and multispectral data from the Mastcam imaging system onboard the rover.

The Vera Rubin ridge can be divided into two geological members, the lower Pettegrove Point member and the upper Jura member, based on textural differences. Overall, the Vera Rubin ridge mudstones are red in color, although deep purple and unusual decameter-scale gray patches are also present, particularly in the uppermost Jura member. The red and gray bedrock colors are shown in the visible and near-infrared images and multispectral data to relate to different proportions of poorly crystalline iron oxides, fine-grained red hematite, and coarse-grained gray hematite. These color variations do not follow the bedding planes, suggesting that the variation in color, and therefore mineralogy, is the result of multiple aqueous alteration events that occurred after deposition of the mudstones. Horgan and collaborators noted that the colors and mineralogy of the Vera Rubin ridge are similar to those of the Moenkopi red beds on Earth. As such, they concluded that alteration by lake waters or runoff from precipitation caused the initial deposition of the red crystalline hematite and poorly crystalline iron oxides. After the mudstones at the Vera Rubin ridge were buried, a later episode of aqueous alteration with a different fluid chemistry dissolved the red crystalline and poorly crystalline hematite to reprecipitate them as coarser grained gray hematite. This study provides further evidence that aqueous fluids existed for a long time in Gale crater, potentially within a subsurface habitable environment, thus increasing our understanding of habitability on Mars. READ MORE