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The Alpha Particle X-ray Spectrometer (APXS) onboard the Mars Science Laboratory (MSL) rover Curiosity has acquired approximately 1,300 geochemical analyses since landing in 2012. The APXS utilizes a combination of X-ray fluorescence and particle-induced X-ray emission to determine the chemical composition of materials within its 15+ mm diameter field of view (FOV) [1, 2]. Diagenetic features provide a means to further understand and constrain the habitability of Curiosity's landing site, Gale crater. These features often present as veins or nodules with an areal extent on the sub-cm scale. APXS analyses of these features therefore contain a mixture of signals from the feature and host substrate.To probe the composition of these sub-FOV features, Curiosity has developed a technique whereby multiple APXS measurements are conducted in close proximity to the primary target (referred to as a raster). The data are then analyzed to not only localize APXS FOVs, mitigating arm placement uncertainty which is on the order of 1-2 cm [2], but also infer the composition of the various endmembers within the workspace. The original raster analysis method (e.g., [2, 3]) has proven useful at deconvolving the chemistry of diagenetic features from the surrounding substrate. However, this method utilizes APXS oxide data as the primary input. These data are derived assuming a homogeneous sample for the purposes of calculating and correcting for matrix effects (the attenuation of induced X-rays by other elements in the sample). In instances of clear chemical heterogeneities, these matrix corrections can result in skewed compositions of the derived endmembers, such as a vein or nodule.Here we present an improvement to this method whereby we utilize low-level data products and isolate matrix effect calculations for each individual endmember (e.g., [4]). The derived results show significant improvements (10-30%) compared to the oxide method in stoichiometric ratios when applied to calcium sulfate veins, an ideal proof-of-concept sample. Subsequent analyses of magnesium-sulfate dominated nodules hint at other potential mobile elements within the fluids present during diagenesis, such as P, Mn, Ni, and/or Zn. Similar elements were enriched in nodules at the Ayton/Groken field site, where P2O5 and MnO concentrations in the nodular material totaled over 25 wt% at a ~2:1 P:Mn molar ratio [4]. The improved analytical method will be particularly useful as Curiosity continues to explore the Marker Band and sulfate unit.[1] Gellert & Clark (2015), Elements, 11.[2] VanBommel et al. (2016), XRS, 45.[3] VanBommel et al. (2017), XRS, 46.[4] VanBommel et al. (2023), Icarus, 392.

The Alpha Particle X-ray Spectrometers (APXS) onboard the Mars Exploration Rovers (MER) Spirit and Opportunity interrogated the bedrock, soil, and regolith at Gusev crater and Meridiani Planum, respectively. The APXS derives the composition of geologic materials through a combination of particle-induced X-ray emission (PIXE) and X-ray fluorescence (XRF) spectroscopy. Each measurement results in a histogram of energies with characteristic peak areas proportional to elemental concentrations. This spectrum reflects not only the composition of a target but also varies with experimental conditions (e.g., measurement duration, mission age, standoff, temperature), which must be accounted for to accurately quantify the elements present in a spectrum. Individual APXS measurements often provide sufficient counting statistics to resolve and quantify major, minor, and select trace elements (e.g., Ni, Zn, Br) while others (e.g., Ga, Ge) are more difficult to precisely quantify due to, in part, their typical low concentrations (e.g., sub-30 µg/g).To combat the effect of statistical noise on trace element quantification, individual spectra are summed together to create a composite spectrum. We have assembled a database of target characteristics, such as target type (e.g., rock, soil), location, feature, target, formation, and degree of sample preparation (e.g., as-is, brushed, abraded), for each individual APXS spectrum. Spectra of targets with shared geological context and geochemical characteristics (e.g., ratio of Fe3+ to FeT) were summed to create meaningful combinations of individual spectra (i.e., composite spectrum). The composite spectra were fit with a simplified (i.e., Gaussian peaks with a linear background) nonlinear least squares fitting routine to identify promising composites for quantification with the fitting routine developed and utilized for the quantification of other elements within MER APXS spectra. Composite spectra were also assessed visually and quantitatively to confirm they were representative of trace element peaks within each of the individual spectra rather than outliers.At both Meridiani Planum and Gusev crater, results indicate that the concentrations of Ga and Ge in outcrops are more than an order of magnitude higher than expected from meteoritic contribution alone. The ratios of Ga to Al and Ge to Si can also be used to infer the geologic history of a region due to their similar ionic radii and charges and therefore geochemical behavior. The Ga/Al molar ratio tends to be much more consistent at Meridiani Planum compared to that of Ge/Si, which shows more variation between formations. The divergence of the behaviors of Ge and Si could be explained by high temperature diagenetic fluids, as could the consistent behaviors of Ga and Al. We conclude that the elevated concentrations of trace elements such as Ga and Ge may be sourced in part from volcanic outgassing, and regional trends in the molar ratios of Ga/Al and Ge/Si are potentially due to high temperature diagenetic fluids.

Introduction: Curiosity has encountered eolian Stimson formation, Siccar Point group sandstones [1-3] at several locations during ascent of Mount Sharp, Gale crater, at: 1) the Emerson and Naukluft plateaus; 2) on the northern Greenheugh pediment (GP); and most recently 3) on the eastern GP (Figure 1). The Siccar Point group unconformably overlies [4] the predominantly lacustrine, Mount Sharp group, which itself is considered to overlie the Bradbury group (Figure 2). Alpha Particle X-ray spectrometer(APXS) analyses acquired during recent investigations of the eastern GP have expanded the range of eolian Stimson formation compositions and provide further evidence for a relationship with Bradbury group sandstones and cap rocks encountered early in the mission. The Bradbury group includes alkaline composition sandstones, clasts within breccio-conglomerates, and float rocks interpreted to represent primary igneous and volcaniclastic rocks derived from mixing of multiple igneous source rocks [5-12]. We report the results of APXS analyses of the Stimson formation and discuss implications for provenance and timing of events within Gale crater. APXS results: Basaltic composition sandstones, related to average Mars sand and soil, were identified at all three Stimson locations (Figures 1,2). All compositions are discussed here, relative to typical basaltic Stimson chemistry. Sandstones with 3xK2O and >MnO concentrations are identified on the northern GP overlying basaltic composition sandstones (Figures 1,2) [13-14]. Most recently, on the eastern GP, APXS analyses have revealed sandstones with even >K2O concentrations that fall into two groups (Figures 1,2): 1) with ~4xK2O, trending to >Na2O concentrations, and 2) with ~5xK2O, trending to 2O, Al2O3, and Ni, ≫Zn and Ge, and ≈MgO concentrations. The eastern GP, high-K sandstones also overlie more typical basaltic chemistry sandstones. Compositional relationship to Bradbury group and other targets previously encountered on the mission: The higher-K, Stimson sandstones share many compositional characteristics with Bradbury group rocks as well as several float rocks encountered along the Mount Sharp group traverse [13-14] (Figures 3,4). The CCAM instrument has also detected similar relationships [15]. The Bradbury group targets include sandstones near the landing site, high-K Bathurst and Kimberley formation sandstones, and capping sandstones situated just prior to driving on to the Mount Sharp group. Related composition float rocks analyzed along the Mount Sharp traverse are typically associated with Stimson outcrops. The higher-K Stimson sandstones do not plot on the sorting trend defined by the varying grainsize, modern Gale crater sands, and the basaltic Stimson sandstones on an A-CN-K diagram (Figure 4b). Instead, they are offset parallel to the plagioclase–potassium feldspar join. Thus, we do not attribute the compositional differences between the basaltic and high-K pediment Stimson to be the result of eolian sorting. Instead, based on sedimentology [3], mineralogy [16], and composition [13-14], we invoke a change in provenance, with input of a more potassium feldspar-rich source. Given the compositional relationship to Bradbury group sandstones, we suggest that the high-K Stimson sandstones are also derived from the mixing of multiple, igneous source rocks including basaltic and more alkaline compositions [13-14]. Many of the Bradbury group sandstones, Mount Sharp float and higher-K, pediment sandstones are also texturally alike. They are all relatively smooth, resistant, and blockier in appearance than typical basaltic Stimson sandstone (Figure 5). The high-K pediment and Bradbury sandstones also share similar orbital characteristics; both are manifest as crater-retaining units. Timing Implications: The more alkaline composition Bradbury group sandstones near the landing site are >9 km lateral distance and 400 m of elevation from the high-K pediment sandstones. Therefore, we have found evidence along the entire traverse for the presence of alkaline igneous source rocks. These source rocks have provided sediment input to both fluvial (Bradbury) and eolian (Siccar Point) sandstones [13-14]. Based on the compositional and textural relationships between the Bradbury and Siccar Point groups we revisit the timing of events at Gale. The Bradbury group is generally considered to be older than both the Mount Sharp and Siccar Pt groups (Figure 1). This work raises the possibility that they could instead be contemporaneous, with both being younger than the Mount Sharp group. In this scenario, the Mount Sharp group represents relatively old lacustrine/fluvial deposits, which were buried, lithified, and eroded, before deposition of the Bradbury and Siccar Point groups onto that erosional surface, thus explaining differences in elevation between the two groups (Figure 6a). Alternatively, the Bradbury group alkaline sedimentary rocks are the oldest unit and would have been deposited prior to the Mount Sharp group. The Siccar Point group eolian sandstones (basaltic and alkaline) would then have been deposited on to the Mount Sharp group (Figure 6b). In this case, a more alkaline source area in the vicinity of, or within, Gale crater has been eroded by both water and wind at different times during the history of the evolution of the crater and its infill [13-14]. Both scenarios are consistent with widespread evidence at Gale crater for the presence of multiple igneous source rocks. Acknowledgements: MSL APXS is managed and financed by the Canadian Space Agency (CSA). Thanks to the JPL engineers and MSL science team. References: [1] Banham, S.G., et al. (2018) Sedimentology, 65, 993–1042. doi.org/10.1111/sed.12469 [2] Banham, S.G., et al. (2021) JGR: Planets, 126 (4): e2020JE006554 [3] Banham, S.G., et al. (submitted) JGR: Planets [4] Watkins, J.A., et al. (2016) 47th LPSC, Abstract #2939 [5] Stolper, E.M., et al. (2013) Science 341 (6153), 1239463. doi:10.1126/science.1239463 [6] Schmidt, M.E., et al. (2014) JGR Planets, 119(1):64-81, doi:10.1002/2013JE004481 [7] Sautter, V., et al. (2015) Geoscience, 8(8), 605. [8] Thompson, L.M., et al. (2016) JGR Planets, 121(10):1981-2003, doi:10.1002/2016JE005055 [9] Treiman, A.H., et al. (2016) JGR Planets, 121(1), p.75 – 106. doi:10.1002/2015JE004932 [10] Cousin, A., et al. (2017) Icarus 288, 265-283. doi:10.1016/j.icarus.2017.01.014 [11] Edwards, P.H., et al. (2017) MAPS, 52 2931-2410. doi.org/10.1111/maps.12953 [12] Bedford, C.C., et al., (2019) Geochimica et Cosmochimica Acta, 246, 234-266. doi.org/10.1016/j.gca.2018.11.031 [13] Thompson, L.M., et al., (submitted) JGR Planets 2021JE007178 [14] Thompson, L.M., et al., (2022) 53rd LPSC, 1475.pdf [15] LeDeit, L., et al. (this conference) [16] Thorpe, M.T., et al. (Submitted) JGR Planets 2021JE00709

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 geolog...

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 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 mobilization of salts by transient water that has occurred recently and may be ongoing.

Alpha Particle X-ray Spectrometers (APXS) have flown on the Mars Exploration Rovers (MER) Spirit and Opportunity as well as the Mars Science Laboratory (MSL) rover Curiosity. The APXS was designed and calibrated for in situ interrogation of solid martian samples through the use of complementary particle-induced X-ray emission and X-ray fluorescence analysis techniques. Its compact and robust design, combined with low power and data demand, further suit the APXS instrument and method for lengthy missions to the surface of rocky bodies in our solar system. Since their three respective landings, the science derived from the latest APXS instruments has been expanded beyond its original scope through the integration of computational techniques and modest changes to how the instrument is utilized on Mars. We will discuss these new methods, operational considerations, as well as the enhanced science achieved, with a particular focus on the relevance and future application on the surface of Mars.

The Dynamic Albedo of Neutrons (DAN) experiment onboard the MSL Curiosity rover performs active neutron measurements at the rover’s stops. It produces short pulses of high-energy neutrons and observes the time profile of slowed down neutrons leaking from the subsurface. Due to neutrons’ high penetrating power it is sensitive to the abundance of neutron moderating elements (mostly hydrogen) and neutron capturing elements (most significant are chlorine and iron) in approximately top 60 cm of the subsurface under the rover, a few tons of matter in total. The DAN data processing procedure is based on numerical simulations of neutron propagation, moderation and capture with the MCNPX software package and returns both the model acceptance probability and the best fit estimates for H2O mass fraction and equivalent Cl mass fraction. The latter corresponds to the total neutron absorption of the subsurface assuming a fixed Fe content. If external information on Fe content is available from other measurements, the DAN equivalent Cl mass fraction can be transformed into an estimate of the real Cl mass fraction in the subsurface.We compare the DAN data on neutron absorption in the subsurface to the data on the Cl and Fe mass fractions on the surface as measured by the APXS instrument in different regions along the Curiosity path. Our analysis shows that the DAN and APXS measurements taken at the same location are in many cases not consistent with each other as the neutron absorption corresponding to the surface concentrations of chlorine and iron measured by APXS is too high to be accepted by the DAN data.We investigate this finding in several regions along the Curiosity path. E.g., for the Glen Torridon region the DAN neutron absorption level for different measurements corresponds to the average chlorine mass fraction of 0.81% with a standard deviation of 0.18% and with typical measurement uncertainty of 0.10% (assuming the Fe mass fraction measured by APXS), while the chlorine mass fraction measured by APXS is 1.19% on average with a standard deviation of 0.43%. These two distributions are significantly different, and only 22% of the DAN measurements in the Glen Torridon region agree with the APXS-based neutron absorption for the same location.We discuss several possible causes for this inconsistency, including either differences between Cl abundances in the martian dust particles and in rocks, or differences between Cl content in the very top surface layer and in the subsurface, or possible bias in APXS target selection.

Three generations of the Alpha-Particle-X-ray-Spectrometer (APXS) have been part of the science suite on all four landed NASA Mars rovers so far. Using x-ray spectroscopy following excitation with alpha particles and x-rays from 244Cm radioactive sources, so far about 2000 samples have been investigated along the combined traverse of ~85km on the surface of Mars.The APXS reports 16 standard elements in all samples and additional trace elements like Ge, Cu, Ga, Rb, Sr, As, Se, Y and Pb if at elevated levels. The sample spot of ~ 20 mm diameter is often large enough to represent bulk content, though small enough to reveal evidence for certain minerals through element correlations when oversampled in rasters. The results from all missions revealed large scale sedimentary formations, like Murray and Burns indicating specific environmental conditions in the past. The soil was found similar at all sites, representing a well mixed global crust component. APXS geochemical data were used for important constraints of complimentary mineralogy results, ground truth for orbiters and comparison to Martian meteorites.Results from the ongoing Curiosity mission and the long living MER rovers will be discussed. Additionally, some very successful applications and investigations that were serendipitously developed after launch will be reviewed. Part of the presentation will be devoted to the unique challenges, trade-offs during design and lessons learned from the long operation of the instrument. The combination of APXS, XRD and Moesbauer results from MER and MSL with future fine scale XRF results of the soil at the Mars 2020 landing site might shed a light into the enigmatic amorphous phase, which could represent a record of the past alteration processes on Mars.

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 CaSO_4‐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.

We present APXS data for the active Bagnold dune field within the Gale impact crater (MSL mission). We derive an APXS-based Average Basaltic Soil (ABS) composition for Mars based on past and recent data from the MSL and MER missions. This represents an update to the Taylor and McLennan (2009) average martian soil, and facilitates comparison across martian datasets. 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.

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

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.

The Alpha Particle X-ray Spectrometer instrument on the Mars exploration rover Spirit determined the compositions of 222 rock and soil targets in Gusev crater. The analyses show wide ranges of volatile/mobile element (S, Cl, Zn, and Br) contents that provide evidence constraining a variety of aqueous alteration processes. Rock coatings and soil indurations have correlations of SO 3 and Zn with Cl and represent a recent alteration process that might be occurring in the present Martian climate. This process occurred at very low water/rock ratios and low temperatures. Pervasive alteration affected the older rocks and soils on the Columbia Hills and was mostly acid-sulfate alteration by fumarolic/hydrothermal activity, likely associated with volcanism. Water/rock ratios were quite variable, and temperatures were likely moderate—a few 10s to a few 100s of degrees C—leading to substantial differences in volatile/mobile element contents and alteration mineralogy. In the extreme case, either leaching by alteration fluids at high water/rock ratio or precipitation from solutions resulted in silica-rich rocks and soils (up to 91 wt% SiO 2 ). One outcrop contains abundant Mg-Mn-Fe-carbonates and might have been altered by near-neutral hydrothermal solutions; an alternative scenario—alteration at moderate- to low-temperatures in an ephemeral lake—cannot be excluded. In spite of prelanding predictions that Gusev crater had hosted a large body of standing water in the Late Noachian/Early Hesperian time frame, evidence garnered by Spirit shows that alteration was most commonly highly localized and done under conditions of low water/rock ratios, and must have occurred after draining of the Gusev crater lake.

The Opportunity rover has explored the surface of Mars in the area of southwestern Meridiani Planum since 2004. This site was specifically chosen because of orbital evidence for a watery past, evidenced by the occurrence of abundant hematite. Opportunity demonstrated that the primary source of hematite inferred from orbit is 1–6-mm concretions, observed both within regionally exposed sandstones and as residual erosional products distributed across the plains. Sandstones of the Late Noachian to Early Hesperian Burns formation also contain high concentrations of sulfate minerals, some of which are hydrated, including jarosite. The mineralogy and chemical compositions of ancient Meridiani sandstones bear witness to the alteration of basaltic protoliths, regional precipitation of evaporites, sedimentary reworking, diagenetic alteration involving groundwater interactions, oxidation, and perhaps more recent formation of alteration rinds and veneers on exposed rock surfaces. Traverses of the interiors and ejecta of successively larger impact craters have permitted Opportunity to investigate materials from different stratigraphic levels and thus different times. In particular, exploration of materials along the rim of the 22-km-diameter Endeavour Crater has revealed older Noachian rocks that bear evidence of phyllosilicate alteration. Aqueous alteration and cation/anion mobilization along fractures related to the Endeavour impact reflect ancient aqueous processes. Its fortuitous landing site and its longevity have allowed Opportunity to explore terrain and rocks spanning the Late Noachian to Early Hesperian transition on Mars, a time when an early, relatively wet Martian surface was essentially drying out.

The Alpha Particle X-ray spectrometer (APXS) onboard the Curiosity rover at the Kimberley location within Gale crater, Mars, analyzed basaltic sandstones that are characterized by potassium enrichments of two to eight times estimates for average martian crust. They are the most potassic rocks sampled on Mars to date. They exhibit elevated Fe, Mg, Mn and Zn, and depleted Na, Al and Si. These compositional characteristics are common to other potassic sedimentary rocks analyzed by APXS at Gale, but distinct from other landing sites and martian meteorites. CheMin and APXS analysis of a drilled sample indicate mineralogy dominated by sanidine, Ca-rich and Ca-poor clinopyroxene, magnetite, olivine and andesine. The anhydrous mineralogy of the Kimberley sample, and the normative mineralogy derived from APXS of other Bathurst class rocks, together indicate provenance from one or more potassium-rich magmatic or impact-generated source rocks on the rim of Gale crater or beyond. Elevated Zn, Ge and Cu suggest that a localized area of the source region(s) experienced hydrothermal alteration, which was subsequently eroded, dispersed and diluted throughout the unaltered sediment during transport and deposition. The identification of the basaltic, high potassium Bathurst class and other distinct rock compositional classes by the APXS, attests to the diverse chemistry of crustal rocks within and in the vicinity of Gale crater. We conclude that weathering, transport and diagenesis of the sediment did not occur in a warm and wet environment, but instead under relatively cold and wet conditions, perhaps more fitting with processes typical of glacial/periglacial environments.

The Curiosity rover observed high Mn abundances (>25 wt % MnO) in fracture-filling materials that crosscut sandstones in the Kimberley region of Gale crater, Mars. The correlation between Mn and trace metal abundances plus the lack of correlation between Mn and elements such as S, Cl, and C, reveals that these deposits are Mn oxides rather than evaporites or other salts. On Earth, environments that concentrate Mn and deposit Mn minerals require water and highly oxidizing conditions; hence, these findings suggest that similar processes occurred on Mars. Based on the strong association between Mn-oxide deposition and evolving atmospheric dioxygen levels on Earth, the presence of these Mn phases on Mars suggests that there was more abundant molecular oxygen within the atmosphere and some groundwaters of ancient Mars than in the present day.

In the search for evidence of past aqueous activity by the Mars Exploration Rover Opportunity, fracture-filling veins and rock coatings are prime candidates for exploration. At one location within a segment of remaining rim material surrounding Endeavour Crater, a set of “boxwork” fractures in an outcrop called Esperance are filled by a bright, hydrated, and highly siliceous (SiO2 ~ 66 wt%) material, which has overall a montmorillonite-like chemical composition. This material is partially covered by patches of a thin, dark coating that is sulfate-rich (SO3 ~ 21 wt%) but also contains significant levels of Si, Fe, Ca, and Mg. The simultaneous presence of abundant S, Si, and Fe indicates significant mineralogical complexity within the coating. This combination of vein and coating compositions is unlike previous analyses on Mars. Both materials are heterogeneously eroded, presumably by eolian abrasion. The evidence indicates at least two separate episodes of solute precipitation from aqueous fluids at this location, possibly widely separated in time. In addition to the implications for multiple episodes of alteration at the surface of the planet, aqueous chemical environments such as these would have been habitable at the time of their formation and are also favorable for preservation of organic material.

Mars Reconnaissance Orbiter HiRISE images and Opportunity rover observations of the ~22 km wide Noachian age Endeavour Crater on Mars show that the rim and surrounding terrains were densely fractured during the impact crater-forming event. Fractures have also propagated upward into the overlying Burns formation sandstones. Opportunity’s observations show that the western crater rim segment, called Murray Ridge, is composed of impact breccias with basaltic compositions, as well as occasional fracture-filling calcium sulfate veins. Cook Haven, a gentle depression on Murray Ridge, and the site where Opportunity spent its sixth winter, exposes highly fractured, recessive outcrops that have relatively high concentrations of S and Cl, consistent with modest aqueous alteration. Opportunity’s rover wheels serendipitously excavated and overturned several small rocks from a Cook Haven fracture zone. Extensive measurement campaigns were conducted on two of them: Pinnacle Island and Stuart Island. These rocks have the highest concentrations of Mn and S measured to date by Opportunity and occur as a relatively bright sulfate-rich coating on basaltic rock, capped by a thin deposit of one or more dark Mn oxide phases intermixed with sulfate minerals. We infer from these unique Pinnacle Island and Stuart Island rock measurements that subsurface precipitation of sulfate-dominated coatings was followed by an interval of partial dissolution and reaction with one or more strong oxidants (e.g., O_2) to produce the Mn oxide mineral(s) intermixed with sulfate-rich salt coatings. In contrast to arid regions on Earth, where Mn oxides are widely incorporated into coatings on surface rocks, our results demonstrate that on Mars the most likely place to deposit and preserve Mn oxides was in fracture zones where migrating fluids intersected surface oxidants, forming precipitates shielded from subsequent physical erosion.

The Alpha Particle X-Ray Spectrometer (APXS) determines the chemical composition of Martian rocks and soils on-board both active National Aeronautics and Space Administration (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.

The intensity ratio C / R between Compton and Rayleigh scatter peaks of the exciting Pu L X-rays in the alpha particle X-ray spectrometer (APXS) is strongly affected by the presence of very light elements such as oxygen which cannot be detected directly by the APXS. C / R values are determined along with element concentrations by fitting APXS spectra of geochemical reference materials (GRMs) with the GUAPX code. A quantity K is defined as the ratio between the C / R value determined by Monte Carlo simulation based on the measured element concentrations and the fitted C / R value from the spectrum. To ensure optimally accurate K values, the choice of appropriate GRMs is explored in detail, with attention paid to Rb and Sr, whose characteristic Kα X-ray peaks overlap the Pu Lα scatter peaks. The resulting relationship between the ratio K and the overall oxygen fraction is linear. This provides a calibration from which the concentration of additional light invisible constituents (ALICs) such as water may be estimated in unknown rock and conglomerate samples. Several GRMs are used as ‘unknowns’ in order to evaluate the accuracy of ALIC concentrations derived in this manner.

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.

The Alpha Particle X-ray Spectrometer (APXS), a field instrument onboard four martian rovers, measures largely unprepared, in situ samples on Mars. The APXS has high precision that enables the determination of elemental concentrations in a wide range of geologic materials. However, lack of sample preparation can lead to heterogeneous matrix effects, and understanding the associated uncertainty is essential for interpreting APXS data. Here we use Particle Induced X-ray Emission spectrometry (PIXE) to analyze a suite of geologic samples from Hawai'i as an analogue study to better understand APXS analyses of martian samples. Wavelength-Dispersive X-ray Fluorescence (WDXRF) analyses of fused glass beads establish higher-accuracy standards for the Hawaiian samples. Sulfate-silicate mixtures were made to evaluate sulfur analysis by PIXE. Results show that the PIXE concentrations for most major elements have 2–6% accuracy, which is comparable to the APXS. However, the PIXE concentrations are systematically high in Al and low in Mg, resulting in lower accuracy (13% and 20%, respectively). Olivine-phyric lavas and most of their altered products have the largest discrepancies with Al concentrations up to 25% high and Mg up to 35% low. Sulfur is systematically high (up to 30% in a basalt matrix) compared to gravimetric S concentrations in the sulfate-silicate mixtures. These systematic deviations in Mg, Al, and S are linked to heterogeneous matrix effects, because PIXE and APXS analyses assume all atoms in a sample to be homogeneously mixed on the sub-micrometer scale, which is not the case. Two key implications for APXS results are: (1) Olivine-bearing samples likely have reported concentrations of Mg that is too low and Al that is too high. Thus, olivine-phyric basalts in Gusev crater and the basaltic sand and soil at three landing sites may have Mg and Al concentrations closer to those of the olivine-phyric shergottites and modelled martian crust than previously thought. (2) Sulfate-silicate mixtures may have overestimated S concentrations reported, resulting in greater uncertainty in the stoichiometry of Ca-sulfates, which is used to deduce the geochemical associations of sulfur in samples.

Clay minerals found in Gale crater, Mars, record surficial chemical weathering and changing conditions in an ancient lake., Clay minerals provide indicators of the evolution of aqueous conditions and possible habitats for life on ancient Mars. Analyses by the Mars Science Laboratory rover Curiosity show that ~3.5–billion year (Ga) fluvio-lacustrine mudstones in Gale crater contain up to ~28 weight % (wt %) clay minerals. We demonstrate that the species of clay minerals deduced from x-ray diffraction and evolved gas analysis show a strong paleoenvironmental dependency. While perennial lake mudstones are characterized by Fe-saponite, we find that stratigraphic intervals associated with episodic lake drying contain Al-rich, Fe3+-bearing dioctahedral smectite, with minor (3 wt %) quantities of ferripyrophyllite, interpreted as wind-blown detritus, found in candidate aeolian deposits. Our results suggest that dioctahedral smectite formed via near-surface chemical weathering driven by fluctuations in lake level and atmospheric infiltration, a process leading to the redistribution of nutrients and potentially influencing the cycling of gases that help regulate climate.

International audience; Mars Science Laboratory (MSL) Curiosity rover data are used to describe the morphology of desiccation cracks observed in ancient lacustrine strata at Gale crater, Mars, and to interpret their paleoenvironmental setting. The desiccation cracks indicate subaerial exposure of lacustrine facies in the Sutton Island member of the Murray formation. In association with ripple cross-stratification and possible eolian cross-bedding, these facies indicate a transition from longer-lived perennial lakes recorded by older strata to younger lakes characterized by intermittent exposure. The transition from perennial to episodically exposed lacustrine environments provides evidence for local to regional climate change that can help constrain Mars climate models.

The Mars Science Laboratory Curiosity rover has been traversing strata at the base of Aeolis Mons (informally known as Mount Sharp) since September 2014. The Murray formation makes up the lowest exposed strata of the Mount Sharp group and is composed primarily of finely laminated lacustrine mudstone intercalated with rare crossbedded sandstone that is prodeltaic or fluvial in origin. We report on the first three drilled samples from the Murray formation, measured in the Pahrump Hills section. Rietveld refinements and FULLPAT full pattern fitting analyses of X-ray diffraction patterns measured by the MSL CheMin instrument provide mineral abundances, refined unit-cell parameters for major phases giving crystal chemistry, and abundances of X-ray amorphous materials. Our results from the samples measured at the Pahrump Hills and previously published results on the Buckskin sample measured from the Marias Pass section stratigraphically above Pahrump Hills show stratigraphic variations in the mineralogy; phyllosilicates, hematite, jarosite, and pyroxene are most abundant at the base of the Pahrump Hills, and crystalline and amorphous silica and magnetite become prevalent higher in the succession. Some trace element abundances measured by APXS also show stratigraphic trends; Zn and Ni are highly enriched with respect to average Mars crust at the base of the Pahrump Hills (by 7.7 and 3.7 times, respectively), and gradually decrease in abundance in stratigraphically higher regions near Marias Pass, where they are depleted with respect to average Mars crust (by more than an order of magnitude in some targets). The Mn stratigraphic trend is analogous to Zn and Ni, however, Mn abundances are close to those of average Mars crust at the base of Pahrump Hills, rather than being enriched, and Mn becomes increasingly depleted moving upsection. Minerals at the base of the Pahrump Hills, in particular jarosite and hematite, as well as enrichments in Zn, Ni, and Mn, are products of acid-sulfate alteration on Earth. We hypothesize that multiple influxes of mildly to moderately acidic pore fluids resulted in diagenesis of the Murray formation and the observed mineralogical and geochemical variations. The preservation of some minerals that are highly susceptible to dissolution at low pH (e.g., mafic minerals and fluorapatite) suggests that acidic events were not long-lived and that fluids may not have been extremely acidic (pH>2pH>2). Alternatively, the observed mineralogical variations within the succession may be explained by deposition in lake waters with variable Eh and/or pH, where the lowermost sediments were deposited in an oxidizing, perhaps acidic lake setting, and sediments deposited in the upper Pahrump Hills and Marias Pass were deposited lake waters with lower Eh and higher pH.

The Mars Science Laboratory (MSL) rover, Curiosity, has a titanium science observation tray (o-tray), upon which portions from drilled and scooped Martian samples can be delivered for analyses by the Alpha-Particle X-ray Spectrometer (APXS). The standard APXS calibration approach to derive elemental concentrations cannot be applied to samples on the o-tray because they (1) have a nonuniform three-dimensional distribution within the APXS field of view and (2) are thin ( 90 µm). To develop techniques for interpreting MSL-APXS o-tray measurements, we conducted laboratory measurements of thin particulate basalt samples on Ti metal with the Flight Equivalent APXS Unit. The experiments demonstrate that, relative to an “infinitely thick” sample, increasing areal coverage of particulates on a Ti metal substrate results in a proportional decrease in the Ti signal and increase in the sample signal. Count rates for heavier elements (Mn and Fe) drop with decreasing sample thickness because the mean thickness is smaller than the APXS information depth. Similar effects were seen in the MSL-APXS o-tray measurement of Rocknest fines on Martian solar day 95, an aliquot of material delivered to Sample Analysis at Mars and Chemistry and Mineralogy. The thin layer effect caused a drop in Mn and Fe signals, which cannot be quantitatively compared to the in situ Rocknest target “Portage” because sample thickness was unknown. Otherwise, Rocknest fines on the o-tray had no significant compositional differences from Portage, except for slight increases in S and Cl.

The first four rocks examined by the Mars Science Laboratory Alpha Particle X-ray Spectrometer indicate that Curiosity landed in a lithologically diverse region of Mars. These rocks, collectively dubbed the Bradbury assemblage, were studied along an eastward traverse (sols 46–102). Compositions range from Na- and Al-rich mugearite Jake_Matijevic to Fe-, Mg-, and Zn-rich alkali-rich basalt/hawaiite Bathurst_Inlet and span nearly the entire range in FeO* and MnO of the data sets from previous Martian missions and Martian meteorites. The Bradbury assemblage is also enriched in K and moderately volatile metals (Zn and Ge). These elements do not correlate with Cl or S, suggesting that they are associated with the rocks themselves and not with salt-rich coatings. Three out of the four Bradbury rocks plot along a line in elemental variation diagrams, suggesting mixing between Al-rich and Fe-rich components. ChemCam analyses give insight to their degree of chemical heterogeneity and grain size. Variations in trace elements detected by ChemCam suggest chemical weathering (Li) and concentration in mineral phases (e.g., Rb and Sr in feldspars). We interpret the Bradbury assemblage to be broadly volcanic and/or volcaniclastic, derived either from near the Gale crater rim and transported by the Peace Vallis fan network, or from a local volcanic source within Gale Crater. High Fe and Fe/Mn in Et_Then likely reflect secondary precipitation of Fe^(3+) oxides as a cement or rind. The K-rich signature of the Bradbury assemblage, if igneous in origin, may have formed by small degrees of partial melting of metasomatized mantle.

Of water and methane on Mars The Curiosity rover has been collecting data for the past 2 years, since its delivery to Mars (see the Perspective by Zahnle). Many studies now suggest that many millions of years ago, Mars was warmer and wetter than it is today. But those conditions required an atmosphere that seems to have vanished. Using the Curiosity rover, Mahaffy et al. measured the ratio of deuterium to hydrogen in clays that were formed 3.0 to 3.7 billion years ago. Hydrogen escapes more readily than deuterium, so this ratio offers a snapshot measure of the ancient atmosphere that can help constrain when and how it disappeared. Most methane on Earth has a biological origin, so planetary scientists have keenly pursued its detection in the martian atmosphere as well. Now, Webster et al. have precisely confirmed the presence of methane in the martian atmosphere with the instruments aboard the Curiosity rover at Gale crater. Science , this issue p. 412 , p. 415 ; see also p. 370

Tridymite, a low-pressure, high-temperature (>870 °C) SiO_2 polymorph, was detected in a drill sample of laminated mudstone (Buckskin) at Marias Pass in Gale crater, Mars, by the Chemistry and Mineralogy X-ray diffraction instrument onboard the Mars Science Laboratory rover Curiosity. The tridymitic mudstone has ∼40 wt.% crystalline and ∼60 wt.% X-ray amorphous material and a bulk composition with ∼74 wt.% SiO_2 (Alpha Particle X-Ray Spectrometer analysis). Plagioclase (∼17 wt.% of bulk sample), tridymite (∼14 wt.%), sanidine (∼3 wt.%), cation-deficient magnetite (∼3 wt.%), cristobalite (∼2 wt.%), and anhydrite (∼1 wt.%) are the mudstone crystalline minerals. Amorphous material is silica-rich (∼39 wt.% opal-A and/or high-SiO_2 glass and opal-CT), volatile-bearing (16 wt.% mixed cation sulfates, phosphates, and chlorides−perchlorates−chlorates), and has minor TiO_2 and Fe_2O_3T oxides (∼5 wt.%). Rietveld refinement yielded a monoclinic structural model for a well-crystalline tridymite, consistent with high formation temperatures. Terrestrial tridymite is commonly associated with silicic volcanism, and detritus from such volcanism in a “Lake Gale” catchment environment can account for Buckskin’s tridymite, cristobalite, feldspar, and any residual high-SiO_2 glass. These cogenetic detrital phases are possibly sourced from the Gale crater wall/rim/central peak. Opaline silica could form during diagenesis from high-SiO_2 glass, as amorphous precipitated silica, or as a residue of acidic leaching in the sediment source region or at Marias Pass. The amorphous mixed-cation salts and oxides and possibly the crystalline magnetite (otherwise detrital) are primary precipitates and/or their diagenesis products derived from multiple infiltrations of aqueous solutions having variable compositions, temperatures, and acidities. Anhydrite is post lithification fracture/vein fill.

Mars Exploration Rover Opportunity is exploring the rim of 22 km diameter, Noachian-aged Endeavour crater. Marathon Valley cuts through the central region of the western rim providing a window into the local lower rim stratigraphic record. Spectra from the Compact Reconnaissance Imaging Spectrometer for Mars show evidence for the occurrence of Fe-Mg smectite in this valley, indicating areally extensive and distinct lithologic units and/or styles of aqueous alteration. The Alpha Particle X-ray Spectrometer has determined the compositions of 59 outcrop targets on untreated, brushed and abraded surfaces. Rocks in the Marathon Valley region are soft breccias composed of mm- to cm-sized darker clasts set in a lighter-toned, finegrained matrix. They are basaltic in non-volatile-element composition and compositionally similar to breccias investigated elsewhere on the rim. Alteration styles recorded in the rocks include: (1) Enrichments in Si, Al, Ti and Cr in more reddish-colored rock, consistent with leaching of more soluble cations and/or precipitation of Si +/- Al, Ti, Cr from fluids. Coprecipitation of Ge-rich phases with Si occurred in the western area only; high water:rock is indicated. Pancam multispectral observations indicate higher nanophase ferric oxide contents, but the rocks have lower Fe contents. The highly localized nature of the red zones indicate they cannot be the source of the widespread smectite signature observed from orbit. (2) Outcrops separated by approximately 65 m show common compositional changes between brushed and abraded (approximately 1 mm deep) targets: increases in S and Mg; decreases in Al, Cl and Ca. These changes are likely due to relatively recent, surface-related alteration of valley rocks and formation of surface coatings under low water:rock. (3) One target, from the center of a region of strong CRISM smectite signature, shows modest differences in composition (higher Si, K; lower Mn) compared to most Marathon Valley rocks, while another target approximately 40 cm away on the same outcrop does not; a change towards smectite bulk compositions is not observed. The smectite signature likely resulted from alteration under low water:rock such that primary minerals were partially altered to phyllosilicates, but wholesale leaching of cations by fluids did not occur.

The Windjana drill sample, a sandstone of the Dillinger member (Kimberley formation, Gale Crater, Mars), was analyzed by CheMin X‐ray diffraction (XRD) in the MSL Curiosity rover. From Rietveld refinements of its XRD pattern, Windjana contains the following: sanidine (21% weight, ~Or95); augite (20%); magnetite (12%); pigeonite; olivine; plagioclase; amorphous and smectitic material (~25%); and percent levels of others including ilmenite, fluorapatite, and bassanite. From mass balance on the Alpha Proton X‐ray Spectrometer (APXS) chemical analysis, the amorphous material is Fe rich with nearly no other cations—like ferrihydrite. The Windjana sample shows little alteration and was likely cemented by its magnetite and ferrihydrite. From ChemCam Laser‐Induced Breakdown Spectrometer (LIBS) chemical analyses, Windjana is representative of the Dillinger and Mount Remarkable members of the Kimberley formation. LIBS data suggest that the Kimberley sediments include at least three chemical components. The most K‐rich targets have 5.6% K2O, ~1.8 times that of Windjana, implying a sediment component with >40% sanidine, e.g., a trachyte. A second component is rich in mafic minerals, with little feldspar (like a shergottite). A third component is richer in plagioclase and in Na2O, and is likely to be basaltic. The K‐rich sediment component is consistent with APXS and ChemCam observations of K‐rich rocks elsewhere in Gale Crater. The source of this sediment component was likely volcanic. The presence of sediment from many igneous sources, in concert with Curiosity's identifications of other igneous materials (e.g., mugearite), implies that the northern rim of Gale Crater exposes a diverse igneous complex, at least as diverse as that found in similar‐age terranes on Earth., Key Points First mineralogical analysis of sandstone on MarsWindjana sandstone very rich in sanidine, implying a trachyte source rockThe source of Gale Crater sediments is an incredibly diverse igneous terrane

The alpha-particle X-ray spectrometer (APXS) for the Mars Science Laboratory (MSL) mission was calibrated for routine analysis of: Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Br, Rb, Sr, and Y. The following elements were also calibrated, but may be too low to be measured (10s–100s ppm) for their usual abundance on Mars: V, Cu, Ga, As, Se and W. An extensive suite of geological reference materials, supplemented by pure chemical elements and compounds was used. Special attention was paid to include phyllosilicates, sulfates and a broad selection of basalts as these are predicted minerals and rocks at the Gale Crater landing site. The calibration approach is from first principles, using fundamental physics parameters and an assumed homogeneous sample matrix to calculate expected elemental signals for a given instrument setup and sample composition. Resulting concentrations for most elements accord with expected values. Deviations in elements of lower atomic number (Na, Mg, Al) indicate significant influences of mineral phases, especially in basalts, ultramafic rocks and trachytes. The systematics of these deviations help us to derive empirical, iterative corrections for different rock groups, based on a preliminary APXS analysis which assumes a homogeneous sample. These corrections have the potential to significantly improve the accuracy of APXS analyses, especially when other MSL instrument results, such as the X-ray diffraction data from CheMin, are included in the overall analysis process.

Home Plate is a layered plateau in Gusev crater on Mars. It is composed of clastic rocks of moderately altered alkali basalt composition, enriched in some highly volatile elements. A coarsegrained lower unit lies under a finer-grained upper unit. Textural observations indicate that the lower strata were emplaced in an explosive event, and geochemical considerations favor an explosive volcanic origin over an impact origin. The lower unit likely represents accumulation of pyroclastic materials, whereas the upper unit may represent eolian reworking of the same pyroclastic materials.

The Alpha Particle X-Ray Spectrometer (APXS) is a small instrument to determine the elemental composition of a given sample. For the ESA Rosetta mission, the periodical comet 67P/Churyumov-Gerasimenko was selected as the target comet, where the lander PHILAE (after landing) will carry out in-situ observations. One of the instruments onboard is the APXS to make measurements on the landing site. The APXS science goal is to provide basic compositional data of the comet surface. As comets consist of a mixture of ice and dust, the dust component can be characterized and compared with known meteoritic compositions. Various element ratios can be used to evaluate whether chemical fractionations occurred in cometary material by comparing them with known chondritic material. To enable observations of the local environment, APXS measurements of several spots on the surface and one spot as function of temperature can be made. Repetitive measurements as function of heliocentric distance can elucidate thermal processes at work. By measuring samples that were obtained by drilling subsurface material can be analyzed. The accumulated APXS data can be used to shed light on state, evolution, and origin of 67P/Churyumov- Gerasimenko.