The Mars Science Laboratory Curiosity rover performed coordinated measurements to examine the textures and compositions of aeolian sands in the active Bagnold dune field. The Bagnold sands are rounded to subrounded, very fine to medium sized (~45–500 μm) with ≥6 distinct grain colors. In contrast to sands examined by Curiosity in a dust‐covered, inactive bedform called Rocknest and soils at other landing sites, Bagnold sands are darker, less red, better sorted, have fewer silt‐sized or smaller grains, and show no evidence for cohesion. Nevertheless, Bagnold mineralogy and Rocknest mineralogy are similar with plagioclase, olivine, and pyroxenes in similar proportions comprising >90% of crystalline phases, along with a substantial amorphous component (35% ± 15%). Yet Bagnold and Rocknest bulk chemistry differ. Bagnold sands are Si enriched relative to other soils at Gale crater, and H 2 O, S, and Cl are lower relative to all previously measured Martian soils and most Gale crater rocks. Mg, Ni, Fe, and Mn are enriched in the coarse‐sieved fraction of Bagnold sands, corroborated by visible/near‐infrared spectra that suggest enrichment of olivine. Collectively, patterns in major element chemistry and volatile release data indicate two distinctive volatile reservoirs in Martian soils: (1) amorphous components in the sand‐sized fraction (represented by Bagnold) that are Si‐enriched, hydroxylated alteration products and/or H 2 O‐ or OH‐bearing impact or volcanic glasses and (2) amorphous components in the fine fraction (<40 μm; represented by Rocknest and other bright soils) that are Fe, S, and Cl enriched with low Si and adsorbed and structural H 2 O.

The resistant~50 m thick Vera Rubin ridge (VRR) situated near the base of Mount Sharp, Gale crater, Mars, has been deemed a high priority science target for the Mars Science Laboratory mission. This is because of (1) its position at the base of the 5 km layered strata of Mount Sharp and (2) the detection of hematite from orbit, indicating that it could be the site of enhanced oxidation. The compositional data acquired by the Alpha Particle X-ray Spectrometer (APXS) during Curiosity's exploration of VRR help to elucidate questions pertaining to the formation of the ridge. APXS analyses indicate that VRR falls within the compositional range of underlying lacustrine mudstones, consistent with a continuation of that depositional environment and derivation from a similar provenance. Lower Fe concentrations for VRR compared to the underlying strata discounts the addition of large amounts of hematite to the strata, either as cement or as detrital input. Compositional trends are associated with VRR cross-cut stratigraphy, indicating postdepositional processes. Higher Si and Al and lower Ti, Fe, and Mn than the underlying mudstone, particularly within distinct patches of gray/blue bedrock, are consistent with the addition of Si and Al. Lateral and vertical compositional variations suggest enhanced element mobility and fluid flow (possibly via multiple events) through VRR, increasing toward the top of the ridge, consistent with the action of warm (~50-100°C), locally acidic saline fluids as inferred from the mineralogy of drilled samples. Plain Language Summary Curiosity has explored the resistant Vera Rubin ridge (VRR) at the base of Mount Sharp, Gale crater, Mars, owing to (1) its position within the 5 km layered rocks of Mount Sharp, which record changes in Mars environment through time, and (2) the detection of hematite from orbit. The Alpha Particle X-ray Spectrometer (APXS) measures the elemental composition of rocks. APXS analyses indicate that VRR has a similar composition to underlying mudstones, consistent with continued deposition in a lake. Lower iron discounts the addition of large amounts of hematite, holding together mineral grains either as cement or as detrital grains. Other elemental trends cut across layering, indicating postdepositional processes. Lateral and vertical compositional variations suggest enhanced element mobility and fluid flow (possibly via multiple events) through VRR, particularly at the top of the ridge and within gray/blue patches of bedrock, consistent with the action of warm (~50-100°C), acidic saline fluids inferred from the mineralogy of drilled samples.

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.

Sedimentary facies within Glen Torridon record a transition from low-energy lacustrine mudstones to higher-energy fluvial sandstones.  Glen Torridon hosts the highest clay mineral abundances observed thus far by MSL while remaining in family with the Mount Sharp group.  Glen Torridon drill samples contain the greatest diversity of organic compounds yet detected by the MSL mission.

The Alpha Particle X-ray Spectrometer (APXS) on the Mars rover Curiosity analyzed more than 700 targets during its first 2,301 martian solar days after landing (sols; 1 sol = 24.7 h). The primary objectives of APXS investigations are to determine the elemental composition of geologic materials in Gale crater to infer the geologic and climate history, establish whether or not liquid water was stable for extended periods, constrain the geochemical conditions of ancient liquids, and deduce the provenance of sedimentary materials

 Subaerial and subaqueous facies were identified within stratigraphy of the Hartmann's Valley and Karasburg members of the Murray formation.  Sedimentologic, stratigraphic, and geochemical evidence suggests that facies formed in a landscape capable of sustaining water.  These members extend the range of facies and environments identified within the Murray formation.

Aeolian processes on Mars have contributed significantly to surface modification and the development of aeolian bedforms on the planet. Previous studies under Earth conditions have shown that surface winds can loft dust into suspension, and drive coarser particles (sand) across the surface in bouncing trajectories, a process known as saltation (e.g., see summaries in Bagnold (1941), andKok et al. (2012)). Saltating grains that descend at relatively high speeds from the atmospheric boundary layer splash other surface grains at each bounce. A flat, sandy bed under saltation bombardment is unstable and quickly develops ripples that grow by consolidation into mature ripple morphologies reflecting atmospheric characteristics, gravity, formative wind strength, and grain size-frequency of the locally available sediment supply (e.g.,

For the last 2 years, the Mars Science Laboratory (MSL) mission, Curiosity rover has been exploring a geomorphic trough on the lower slopes of Mount Sharp referred to as "Glen Torridon" by the MSL team. The trough overlaps with an area identified from orbit as clay-bearing (Anderson & Bell, 2010;B. J. Thomson et al., 2011;Fraeman et al., 2016;Milliken et al., 2010) and is delineated by Vera Rubin (formerly hematite) ridge (VRR) to the north and the Greenheugh pediment and sulfate-bearing unit to the south (also identified from orbit; Milliken et al., 2014) (Figures 1 and 2). Thus, this region potentially records the end of a "wetter" environment on Mars, before a transition to more arid conditions and deposition of the overlying sulfate-bearing strata. The region is therefore one of the primary exploration targets of the MSL mission (