We present Alpha‐Particle X‐ray Spectrometer (APXS) data for the active Bagnold dune field within the Gale impact crater (Mars Science Laboratory (MSL) mission). We derive an APXS‐based average basaltic soil (ABS) composition for Mars based on past and recent data from the MSL and Mars Exploration Rover (MER) missions. This represents an update to the Taylor and McLennan (2009) average Martian soil and facilitates comparison across Martian data sets. The active Bagnold dune field is compositionally distinct from the ABS, with elevated Mg, Ni, and Fe, suggesting mafic mineral enrichment and uniformly low levels of S, Cl, and Zn, indicating only a minimal dust component. A relationship between decreasing grain size and increasing felsic content is revealed. The Bagnold sands possess the lowest S/Cl of all Martian unconsolidated materials. Gale soils exhibit relatively uniform major element compositions, similar to Meridiani Planum and Gusev Crater basaltic soils (MER missions). However, they show minor enrichments in K, Cr, Mn, and Fe, which may signify a local contribution. The lithified eolian Stimson Formation within the Gale impact crater is compositionally similar to the ABS and Bagnold sands, which provide a modern analogue for these ancient eolian deposits. Compilation of APXS‐derived soil data reveals a generally homogenous global composition for Martian soils but one that can be locally modified due to past or extant geologic processes that are limited in both space and time.
Modern Martian dust is similar in composition to the global soil unit and bulk basaltic Mars crust, but it is enriched in S and Cl. The Alpha Particle X‐ray Spectrometer (APXS) on the Mars Science Laboratory Curiosity rover analyzed air fall dust on the science observation tray (o‐tray) in Gale Crater to determine dust oxide compositions. The o‐tray dust has the highest concentrations of SO3 and Cl measured in Mars dust (SO3 8.3%; Cl 1.1 wt %). The molar S/Cl in the dust (3.35 ± 0.34) is consistent with previous studies of Martian dust and soils (S/Cl = 3.7 ± 0.7). Fe is also elevated ~25% over average Mars soils and the bulk crust. These enrichments link air fall dust with the S‐, Cl‐, and Fe‐rich X‐ray amorphous component of Gale Crater soil. Dust and soil have the same S/Cl, constraining the surface concentrations of S and Cl on a global scale.
Zinc and germanium enrichments have been discovered in sedimentary rocks in Gale Crater, Mars, by the Alpha Particle X‐ray Spectrometer on the rover Curiosity. Concentrations of Zn (910 ± 840 ppm) and Ge (65 ± 58 ppm) are tens to hundreds of times greater than in Martian meteorites and estimates for average silicate Mars. Enrichments occur in diverse rocks including minimally to extensively altered basaltic and alkalic sedimentary rocks. The magnitude of the enrichments indicates hydrothermal fluids, but Curiosity has not discovered unambiguous hydrothermal mineral assemblages. We propose that Zn‐ and Ge‐rich hydrothermal deposits in the source region were dispersed in siliciclastic sediments during transport into the crater. Subsequent diagenetic mobilization and fractionation of Zn and Ge is evident in a Zn‐rich sandstone (Windjana; Zn ~4000 ppm, Ge ~85 ppm) and associated Cl‐rich vein (Stephen; Zn ~8000 ppm, Ge ~60 ppm), in Ge‐rich veins (Garden City; Zn ~2200 ppm, Ge ~650 ppm), and in silica‐rich alteration haloes leached of Zn (30–200 ppm). In moderately to highly altered silica‐rich rocks, Ge remained immobile relative to leached elements (Fe, Mn, Mg, and Ca), consistent with fluid interaction at pH ≪ 7. In contrast, crosscutting Ge‐rich veins at Garden City suggest aqueous mobilization as Ge‐F complexes at pH < 2.5. Multiple jarosite detections by the CheMin X‐ray diffractometer and variable Zn concentrations indicate diagenesis of lower Mount Sharp bedrock under acidic conditions. The enrichment and fractionation of Zn and Ge constrains fluid events affecting Gale sediments and can aid in unraveling fluid histories as Curiosity's traverse continues.
Alpha Particle X-ray Spectrometer (APXS) results for Phase 2 of the Bagnold Dunes campaign, focusing on the linear dunes, complement those from Phase 1 (barchan dunes) and add to our understanding of active Martian dune systems. This work highlights both compositional similarities and differences across the dune field. The concentration of elements associated with mafic minerals in coarser grains and along active ripple crests, previously identified in the barchan dunes, highlights differences in Martian and terrestrial weathering, transport, and sorting processes. Concentration of a Cr-Ti mineral phase within the linear sands is identified. The inactive ripple fields are geochemically similar to soil, while active ripple fields are similar to sands. Inferred dust content (S + Cl + Zn concentrations), derived from APXS analyses, provide geochemical confirmation of current and seasonal activity variations, within the barchan and linear dunes, as well as revealing a continuum between inactive soils and active sands. During 2015During -2017, the Mars Science Laboratory rover, Curiosity, in Gale Crater, Mars, crossed the active Bagnold Dunes, which comprise both barchan (crescent shaped) and linear dunes. This has enabled the first in situ appraisal of an active dune system on another planet, including the geochemical analysis (major elements, plus Ni, Zn, and Br) of sand samples via the Alpha Particle X-ray Spectrometer. We find a concentration of elements (MgO and Ni) associated with mafic minerals (particularly olivine) along the crests of active ripples and in coarse-grained samples. Elements such as SiO 2 , Al 2 O 3 , and K 2 O, associated with more felsic minerals such as plagioclase, are concentrated in off-crest sand and finer-grained portions. We find evidence for a Cr 2 O 3 -TiO 2 mineral phase, concentrated in the linear dunes. Dust content (indicated by SO 3 , Cl, and Zn levels) indicates that activity levels are higher in the linear dunes than in the barchan dunes. Plain Language Summary
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