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.

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

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 (

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: Knockfarril Hill; Glasgow member: Glasgow). This transition defines a change in depositional setting. Alpha Particle X-ray Spectrometer (APXS) results and statistical analysis reveal 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 Jura_GT and the stratigraphically equivalent Jura on Vera Rubin ridge (Jura_ VRR). The data define two geochemical facies (high-K or high-Mg), with a strong bimodal grain distribution in Jura_GT and Knockfarril Hill. The contact between Knockfarril Hill and Glasgow is marked by abrupt sedimentological changes but a similar composition for both. Away from the contact, the Knockfarril Hill and Glasgow plot discretely, suggesting a zone of common alteration at the transition and/or a gradual transition in provenance with increasing elevation in the Glasgow member. 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 in 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.

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.1