Diagenesis of Vera Rubin ridge, Gale crater, Mars from Mastcam
multispectral images
Abstract
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 variations from red to purple to gray.
These color differences cross-cut stratigraphy and are likely due to
diagenetic alteration of the sediments after deposition. However, the
chemistry and timing of these fluid interactions is unclear. Determining
how diagenetic processes may have modified chemical and mineralogical
signatures of ancient martian environments is critical for understanding
the past habitability of Mars and achieving the goals of the MSL
mission. Here we use visible/near-infrared spectra from Mastcam and
ChemCam to determine the mineralogical origins of color variations in
the ridge. Color variations are consistent with changes in spectral
properties related to the crystallinity, grain size, and texture of
hematite. Coarse-grained gray hematite spectrally dominates in the gray
patches and is present in the purple areas, while nanophase and
fine-grained red crystalline hematite are present and spectrally
dominate 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. 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,
possibly with limited secondary reddening after exhumation. Diagenetic
alteration may have diminished the preservation of biosignatures and
changed the composition of the sediments, making it more difficult to
interpret how conditions evolved in the paleolake over time.