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We have used Mars Exploration Rover Opportunity data to investigate the origin and alteration of lithic types along the western rim of Noachian‐aged Endeavour crater on Meridiani Planum. Two geologic units are identified along the rim: the Shoemaker and Matijevic formations. The Shoemaker formation consists of two types of polymict impact breccia: clast‐rich with coarser clasts in upper units; clast‐poor with smaller clasts in lower units. Comparisons with terrestrial craters show that the lower units represent more distal ejecta from at least two earlier impacts, and the upper units are proximal ejecta from Endeavour crater. Both are mixtures of target rocks of basaltic composition with subtle compositional variations caused by differences in post‐impact alteration. The Matijevic formation and lower Shoemaker units represent pre‐Endeavour geology, which we equate with the regional Noachian subdued cratered unit. An alteration style unique to these rocks is formation of smectite and Si‐ and Al‐rich vein‐like structures crosscutting outcrops. Post‐Endeavour alteration is dominated by sulfate formation. Rim‐crossing fracture zones include regions of alteration that produced Mg‐sulfates as a dominant phase, plausibly closely associated in time with the Endeavour impact. Calcium‐sulfate vein formation occurred over extended time, including before the Endeavour impact and after the Endeavour rim had been substantially degraded, likely after deposition of the Burns formation that surrounds and embays the rim. Differences in Mg, Ca and Cl concentrations on rock surfaces and interiors indicate that mobilization of salts by transient water has occurred recently and may be ongoing. Plain Language Summary: Data returned by the Mars Exploration Rover Opportunity was used to investigate rock origins along the western rim of Endeavour crater on Meridiani Planum, Mars. The Shoemaker formation consists of impact‐formed breccia of two types: coarser‐grained upper subunits and finer‐grained lower subunits. The lower units represent ejecta from at least two older, more distant craters, while the upper units are ejecta from Endeavour crater. Subtle compositional differences are caused by differences in post‐impact alteration along the crater rim. The lower Shoemaker units represent part of the pre‐Endeavour rock sequence. An alteration style unique to these rocks is formation of Si‐ and Al‐rich structures crosscutting bedrock. Post‐Endeavour alteration is dominated by sulfate formation. Fracture zones in the rim include regions of alteration that produced Mg‐sulfates as a dominant phase, plausibly closely associated in time with the Endeavour impact. Calcium‐sulfate vein formation occurred over extended time, some before the Endeavour impact and some much later, likely after deposition of the sulfate‐rich sandstones of Meridiani Planum. Differences in composition of rock surfaces and interiors indicate that mobilization of salts by transient water has occurred recently and may be ongoing on Mars. Key Points: Clast‐poor impact breccias on Endeavour crater rim are distal ejecta from at least two earlier impactsEnrichments in Si and Al, and smectite formation are unique to pre‐Endeavour rocks; post‐Endeavour alteration formed Ca‐ and Mg‐sulfatesMagnesium, Ca and Cl have been mobilized by transient thin films of water acting on salts very recently, and could be ongoing [ABSTRACT FROM AUTHOR]

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 rocks, veins, and concretions. We show that the elemental characteristics are consistent with bedrock alteration by acidic as well as neutral and/or alkaline waters. These results are key to the goal of Curiosity's mission to determine if Gale crater had an environment where microbial life could have emerged. Key Points: Results and stratigraphic context for APXS targets from the first 2,301 sols of the Curiosity Mars rover mission are presentedGale crater sediment has a diverse provenance that is primarily basalticAlteration and diagenetic features demonstrate widespread aqueous activity [ABSTRACT FROM AUTHOR]

Abstract: We report the results of geological studies by the Opportunity Mars rover on the Endeavour Crater rim. Four major units occur in the region (oldest to youngest): the Matijevic, Shoemaker, Grasberg, and Burns formations. The Matijevic formation, consisting of fine‐grained clastic sediments, is the only pre‐Endeavour‐impact unit and might be part of the Noachian etched units of Meridiani Planum. The Shoemaker formation is a heterogeneous polymict impact breccia; its lowermost member incorporates material eroded from the underlying Matijevic formation. The Shoemaker formation is a close analog to the Bunte Breccia of the Ries Crater, although the average clast sizes are substantially larger in the latter. The Grasberg formation is a thin, fine‐grained, homogeneous sediment unconformably overlying the Shoemaker formation and likely formed as an airfall deposit of unknown areal extent. The Burns formation sandstone overlies the Grasberg, but compositions of the two units are distinct; there is no evidence that the Grasberg formation is a fine‐grained subfacies of the Burns formation. The rocks along the Endeavour Crater rim were affected by at least four episodes of alteration in the Noachian and Early Hesperian: (i) vein formation and alteration of preimpact Matijevic formation rocks, (ii) low‐water/rock alteration along the disconformity between the Matijevic and Shoemaker formations, (iii) alteration of the Shoemaker formation along fracture zones, and (iv) differential mobilization of Fe and Mn, and CaSO4‐vein formation in the Grasberg and Shoemaker formations. Episodes (ii) and (iii) possibly occurred together, but (i) and (iv) are distinct from either of these. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Alpha Particle X-ray Spectrometer (APXS) onboard the Mars rover Curiosity conducts high-precision in situ chemical measurements of rocks and regolith. Target surfaces are not always flat and thus can pose issues for interpretations that assume such. Here, we investigate when variable target relief is an important effect to consider for APXS targets. We provide operational recommendations on how to most efficiently study targets of significant changes in surface relief with the rover-arm-mounted APXS and Mars Hand Lens Imager camera. Additionally, we deconvolve the chemistry of heterogeneous targets of significant vertical relief encountered along Curiosity's traverse, providing the chemical composition of visible endmembers in the process. Specifically, presented here are the deconvolved endmember compositions of two recent targets. The first is a manganese-rich, calcium-rich, and zinc-rich dark-toned vein at Garden City that is chemically and visually unique compared with the intermixed light-toned Ca-sulfate-dominated vein. The other target is a weathered Mg-sulfate-rich nodule that is unique compared with a previously encountered nodule that had significant nickel enrichment. The distinctly different chemistry both at Garden City and with the Mg-sulfate-rich nodules along the traverse provides further evidence for either multiple fluid events or an evolving fluid on the surface of ancient Mars. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The Alpha Particle X-Ray Spectrometer (APXS) determines the chemical composition of Martian rocks and soils on-board both active Aeronautics and Space (NASA) rovers using X-ray emission spectroscopy through complementary particle-induced X-ray emission (PIXE) and X-ray fluorescence (XRF) excitation methods. A single APXS spectrum represents the sum of the signals from within the instrument's field of view (FOV). In the past, features smaller than the FOV have been investigated through repeated measurements with stepwise lateral offsets. These lateral offsets allow for empirically extracting, through elemental correlations, distinct compositions of different features. Here, we present a novel analytical method for deconvolving the endmember chemistry of visually distinct components through oversampling and the integrated analysis of the elemental data and supporting images. We discuss specifically the method's application to three targets investigated by the Mars Science Laboratory rover Curiosity during its traverse, as well as the added information that can be gained from this method in the future. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In this work, we studied the x‐ray energy dependence of x‐ray beam diameter focused by polycapillary optics. A quantitative beam diameter–energy relation enables more accurate estimation of the element‐specific interrogation area of a sample using the compositional maps produced by a micro‐XRF system. This improves upon our ability to visualize individual beam‐diameter sized mineral grains and in turn directly benefits Planetary Instrument for X‐ray Lithochemistry (PIXL) analyses of martian soil in addition to benefitting other micro‐focused x‐ray fluorescence (XRF) systems. The spatial distribution of an array of characteristic XRF emission lines was measured by sampling via a knife‐edge approach with small motor stepping of the beam across target edges. Data taken as part of this effort, from the Planetary Flight Model (PFM), were limited to only seven beam energies corresponding to the elements Ni, Cu, Se, Ta, Au, Ti and Ba. Hence, we conducted additional analysis using JPL's lab‐based breadboard (LBB) micro‐XRF system, a system that emulates PIXL's functionality where we measured beam diameter corresponding to 18 elements: Na, Mg, Al, Si, Cl, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Ge, Se, Sr and Mo. The experimental results were also compared with Monte Carlo simulations. The beam diameter (y)–energy (x) relation that we obtained for LBB was y = 185.79 exp(−0.078x) whose exponential component was then used to get a more accurate relation for the PFM even with the limited data set: y = 227.53 exp(−0.078x). The difference in the two coefficients for the PFM and LBB stems mainly from the difference in the polycapillary optic design, and this work establishes x‐ray beam diameter versus energy relation quantitatively for both the systems. [ABSTRACT FROM AUTHOR]