Chemical data acquired by Curiosity’s Alpha Particle X-ray Spectrometer (APXS) during examination of the contact between the upper Mount Sharp group and overlying Stimson formation sandstones at the Greenheugh pediment reveal compositional similarities to rocks encountered earlier in the mission. Mount Sharp group strata encountered below the Basal Siccar Point group unconformity at the base and top of the section, separated by >300 m in elevation, have distinct and related compositions. This indicates enhanced post-depositional fluid flow and alteration focused along this contact. Sandstone targets exposed immediately above the unconformity have basaltic compositions consistent with previously encountered eolian Stimson formation sandstones, except at the contact, where they show the addition of S. Resistant sandstone outcrops above the contact have higher K, Mn and Na and lower Ni concentrations that primarily reflect changes in provenance. They are compositionally related to cap rock float blocks encountered as Curiosity climbed through the Mount Sharp group, and Bradbury group sandstone outcrops. The higher K, pediment sandstones are interpreted to have a similar provenance to some Bradbury group sandstones, further evidence for widespread, alkaline source rock within and/or in the vicinity of Gale crater. The Bradbury and Siccar Point groups may both be younger than the Mount Sharp group. Alternatively, an alkaline source area in and around Gale crater has been eroded by both water and wind at different times (both before and after deposition of the Mount Sharp group), during the evolution of the crater and its infill.

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.

The Glen Torridon stratigraphic sequence marks the transition from the low energy lacustrine-dominated Murray formation (Mf) (Jura member: Jm) to the more diverse Carolyn Shoemaker formation (CSf) (Knockfarril Hill member: KHm; Glasgow member: Gm), indicating a change in overall depositional setting. Alpha Particle X-ray Spectrometer (APXS) results and statistical analysis reveals that the bulk primary geochemistry of Mf targets are broadly in family with CSf targets, but with subtle compositional and diagenetic trends with increasing elevation. APXS results reveal significant compositional differences between Jm_GT and the stratigraphically equivalent Jura on Vera Rubin ridge (Jm_VRR). APXS data defines two geochemical facies (high-K or high-Mg) with a strong bimodal grain distribution in Jm_GT and KHm. The contact between KHm to Gm is marked by abrupt sedimentological changes but a similar composition for both. Away from the contact, the KHm and Gm plot discretely, suggesting a zone of common alteration at the transition and/or a gradual transition in provenance with increasing elevation in the Gm. APXS results point to a complex history of diagenesis within Glen Torridon, with increasing diagenesis close to the Basal Siccar Point unconformity on the Greenheugh pediment, and with proximity to the beginning of the clay sulfate transition. Elemental mobility is evident in localized enrichments or depletions of Ca, S, Mn, P, Zn, Ni. The highly altered Hutton interval, in contact with the unconformity on Tower butte, is also identified on Western Butte, indicating that the “interval” was once laterally extensive.

In Gale crater on Mars, the rover Curiosity has discovered evidence of fluid mobilization of the redox-sensitive element manganese. We present results for Mn from Curiosity’s Alpha Particle X-ray Spectrometer (APXS), which show that the average MnO concentration in mudstone-dominated sedimentary units (0.22 wt%) is about one-half of the concentration in the average Mars crust (0.44 wt%). Geochemical trends indicate that Mn in the sedimentary bedrock, most of which has a basaltic provenance, was leached by chemical alteration and dissolution. In >350 vertical meters of mudstone-dominated strata, the apparent leaching of Mn and retention of Fe in Fe-O-H phase(s) resulted in the fractionation of Fe and Mn, indicating relatively moderate Eh-pH fluid conditions that were not highly alkaline, reducing, or oxidizing. Exceptions are fracture-associated, silica-rich haloes where both Mn and Fe were leached by low pH fluids. The rover also discovered Mn-rich veins, nodules, and patchy, dark coatings on rock surfaces, which are variably associated with enrichments in Fe, P, Cl, and/or Zn. These Mn-rich features represent ~1% of the 1,029 APXS measurements acquired over ~25 km of rover traverse. A thermochemical model shows that dissolved Mn2+ could have been concentrated via evaporation, sublimation, and/or freezing. Manganese was then likely precipitated in localized features when > 99.99% of the Mn2+-bearing water was removed from the system. These findings indicate that Mn was mobile in Gale crater and therefore bioavailable as a potential energy source for life.

The Alpha Particle X-ray Spectrometer (APXS) on the rover, , has analyzed the composition of geologic materials along a >20-kilometer traverse in Gale crater on Mars. The APXS dataset after 6.5 Earth years (2301 sols) includes 712 analyses of soil, sand, float, bedrock, and drilled/scooped fines. We present the APXS results over this duration and provide stratigraphic context for each target. We identify the best APXS analysis of each of the 22 drilled and scooped samples that were delivered to the instruments CheMin (X-ray diffractometer) and SAM (mass spectrometer and gas chromatograph) during this period. The APXS results demonstrate that the basaltic and alkalic units in the Bradbury group (sols 0-750) show minimal alteration indicating an arid climate. In contrast, the Murray formation of the Mount Sharp group (sols ~750-2301) has compositions indicating pervasive alteration. Diagenetic features are common and show fluid interaction with the sediment after (and possibly during) lithification, which is consistent with burial and diagenesis. A lithified sandstone unit, the Stimson formation, overlies part of the Murray formation. This has a composition similar to the basaltic sand and soil, suggesting a shared source. Cross-cutting, fracture-associated haloes are evidence of late-stage fluid alteration after lithification of the sediment. The APXS dataset, evaluated in concert with the full science payload of , indicates that Gale crater was habitable, and that liquid water was stable for extended periods.