ELEMENTAL COMPOSITION AND CHEMICAL EVOLUTION OF SEDIMENTARY ROCKS IN GALE CRATER, MARS: APXS RESULTS FROM THE FIRST 25 KM OF THE ROVER CURIOSITY’S TRAVERSE

The Alpha Particle X-ray Spectrometer (APXS) on the rover Curiosity has investigated the elemental composition of geologic materials along a >25-km traverse in Gale crater on Mars. After 8.5 Earth years (3,100 martian days), the APXS dataset includes ~1,060 analyses of soil, regolith, and bedrock. Curiosity's traverse is effectively a cross-section of the layered sedimentary strata in Gale crater, which were likely deposited in fluvio-lacustrine and aeolian systems. This traverse enables the APXS investigation to infer the geologic and climatic history, establish whether liquid water was stable for extended periods, constrain the geochemical conditions of ancient liquids, and deduce the provenance of sedimentary materials in Gale crater.

Three stratigraphic groups have been explored, and each has distinct elemental characteristics: (1) The Bradbury group has a diverse igneous provenance with a mixture of basaltic, alkali-rich, and potassic endmembers within mudstone, sandstone, and conglomerate. Open-system chemical alteration is minimal, indicating an arid climate. (2) The Mt. Sharp group, in contrast, has compositions indicating pervasive chemical alteration of a basaltic precursor. The alteration has resulted in a remarkably uniform bedrock composition in ~400 m of laminated strata. The bedrock is enriched in Si, and depleted in Mg, Ca, and Mn. Diagenetic nodules, coatings, and veins are common. Ca-sulfate veins are prominent, crosscutting every member of the group. The Mt. Sharp and Bradbury groups have Zn and Ge enrichments (10-100X) that may be fingerprints of hydrothermal activity in the source region. (3) The Siccar Point group overlies the Mt. Sharp group and contains a cross-bedded sandstone unit, the Stimson formation. This has a basaltic composition, indicating limited open-system chemical alteration consistent with aeolian deposition in an arid climate. At least one unit in the Siccar Point group has elevated Na and K that is like the Bradbury group and may indicate a genetic link. Cross-cutting, fracture-associated, high Si 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.