Elemental Composition and Chemical Evolution of Geologic Materials in Gale Crater, Mars: APXS Results From Bradbury Landing to the Vera Rubin Ridge

The Alpha Particle X‐ray Spectrometer (APXS) on the rover Curiosity has analyzed the composition of geologic materials along a >20‐km traverse in Gale crater on Mars. The APXS dataset after 6.5 Earth years (2,301 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 measurement of each of the 22 drilled and scooped samples that were delivered to the instruments Chemistry and Mineralogy (CheMin; X‐ray diffractometer) and Sample Analysis at Mars (SAM; mass spectrometer and gas chromatograph) during this period. The APXS results demonstrate that the basaltic and alkali‐rich 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 ∼750–2,301) has compositions indicating pervasive alteration. Diagenetic features are common and show fluid interaction with the sediment after (and possibly during) lithification. A 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 Curiosity, indicates that Gale crater was habitable, and that liquid water was stable for extended periods. Plain Language Summary The Mars rover Curiosity uses the Alpha Particle X‐ray Spectrometer (APXS) located on Curiosity's robotic arm to determine the composition of surface materials at the Mars Science Laboratory landing site in Gale crater. The APXS has measured more than 700 2‐cm‐wide spots over a >20 km traverse during 2,301 martian days, or 6.5 Earth years. This study presents those data acquired to date, and an overview of the results from this period of Curiosity's mission. The APXS results demonstrate that the geologic materials in Gale crater have diverse elemental compositions and include several broad groups. Most of the bedrock is made of small grains of rock transported by surface processes that may include fluvial, aeolian, impact, volcanic, and mass wasting into a lake system where they were deposited and cemented. Many of the bedrock layers show evidence for aqueous processes that can be traced using soluble elements in the