Alpha Particle X-ray Spectrometers (APXS) were an integral component of the science payload that flew on the twin Mars Exploration Rovers (MER) Spirit and Opportunity. An updated version of the MER APXS instrument, further optimized for in situ geochemical analyses on Mars, is currently operational within Gale crater onboard the Mars Science Laboratory (MSL) rover Curiosity. APXS on MER and MSL were designed and calibrated for high-precision in situ analyses of geologic materials on Mars. The use of curium-244 sources provides high sensitivity to lower-Z elements. This low-Z sensitivity is important for characterizing the abundance of rock forming elements such as Na, but also enables analyses of Ar, which makes up ~2% of the Martian atmosphere, and thus ~40% all non-condensable gas species. Atmospheric dynamics on Mars are driven in large part by condensation flow. The temperature and pressure at the winter pole leads to the deposition of carbon dioxide (which makes up ~95% of the atmosphere) onto the polar cap. The following spring, carbon dioxide sublimates from the cap, a cycle which creates a pressure gradient across the planet. Non-condensable gases, such as Ar, are not deposited on the polar cap and become enriched relative to carbon dioxide. Most environmental monitoring hardware flown to Mars can measure the absolute pressure of the atmosphere, but not specifically the abundance of non-condensable species. In the case of the Sample Analysis at Mars (SAM) instrument on MSL, atmospheric constituents can be deduced with great accuracy, but not with a high frequency. We summarize efforts on MER and MSL to characterize variability in non-condensable gas density on Mars using instruments designed to measure the composition of rocks and regolith. Analyses by Spirit enabled calibration of the MER APXS for atmospheric analyses. The Opportunity mission, spanning ~5000 sols, acquired around 2250 hours of atmospheric data with its APXS. This data set revealed an annual short-lived Ar enrichment occurring around Ls 150, previously unreported in the literature and not present in climate models at that time. This phenomenon has since been regularly targeted on MSL with APXS (and SAM), with ~800 hours of atmospheric analyses conducted by APXS thus far. We report recent findings from Mars Year 37, where dedicated high-frequency APXS atmospheric campaigns were conducted, coinciding with solar conjunction and extended holiday plans, significantly improved constraints on the timing of this short-lived enrichment at Gale crater, and compare the observed results to those from Opportunity.

AbstractAlpha Particle X‐ray spectrometer (APXS) analyses of the distinct Amapari Marker Band (AMB), Gale crater, Mars reveal the highest, in situ, FeO and Zn abundances (47.51, 2.23 wt%), and elevated MnO associated with a lower rippled unit. APXS analyses also reveal a marked shift in provenance, to a generally basaltic composition, compared to the underlying Mg‐sulfate‐bearing strata, which persists into the overlying stratigraphy. The AMB also records perturbation in the MgSO4‐forming conditions present above and below. AMB chemistry could be consistent with a volcanic ash source; high metal concentrations resulting from volatile reactions within an ash cloud. Alternatively, syn‐ and/or post‐depositional precipitation processes within a primary lake setting and/or a later diagenetic event or events may have played a role. Ongoing and future work will aim to further constrain processes responsible for deposition of the AMB, the high metal concentrations and its regional and global implications.

The Mars Science Laboratory rover Curiosity encountered nodules rich in manganese and phosphorus while exploring the Knockfarril Hill member of Gale crater on Mars. Deconvolution of X-ray spectroscopy data acquired by the Alpha Particle X-ray Spectrometer (APXS) at the spectral level indicate P2O5 concentrations possibly in excess of 18 wt% and MnO exceeding 8 wt%. The nodules occur intermittently in-mm-thick layers concordant with the sedimentary laminae, extending up to-10 cm laterally. Calcium sulfate associated with the nodules is interpreted as having precipitated from fluid that infiltrated between the nodule-bearing bedding planes in a separate and subsequent fluid event. Though the Mn-and P-bearing phase(s) was(were) not definitively iden-tified by X-ray diffraction, evolved gas analyses show that the oxidation state of Mn is most likely 2+.

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.

Geochemical analyses by X-ray spectrometry and laser-induced breakdown spectroscopy (LIBS) instruments on the surface of Mars enable detailed studies of surface materials. The two techniques are utilized in concert by rovers to glean information in a complementary fashion. However, fundamental differences in how these analytical techniques function can produce perceived discrepancies in results, such as those resulting from variation in sampling volume. Here we utilize data acquired by the APXS (X-ray spectrometer) and ChemCam (LIBS) instruments on the Curiosity rover to investigate a manganese-rich surface layer, and, in the process, provide an improved chemical depth profile. We also demonstrate a method whereby current and future spacecraft capable of utilizing both techniques can potentially improve estimates of martian material near-surface density.

Chemical data acquired by Curiosity's Alpha Particle X‐ray Spectrometer during examination of the contact between the upper Mount Sharp group and overlying Stimson formation sandstones at the Greenheugh pediment reveal compositional similarities to rocks encountered earlier in the mission. Mount Sharp group strata encountered below the Basal Siccar Point group unconformity at the base and top of the section, separated by >300 m in elevation, have distinct and related compositions. This indicates enhanced post‐depositional fluid flow and alteration focused along this contact. Sandstone targets exposed immediately above the unconformity have basaltic compositions consistent with previously encountered eolian Stimson formation sandstones, except at the contact, where they show the addition of S. Resistant sandstone outcrops above the contact have higher K, Mn, and Na and lower Ni concentrations that primarily reflect changes in provenance. They are compositionally related to cap rock float blocks encountered as Curiosity climbed through the Mount Sharp group, and Bradbury group sandstone outcrops. The higher K, pediment sandstones are interpreted to have a similar provenance to some Bradbury group sandstones, further evidence for widespread, alkaline source rock within and/or in the vicinity of Gale crater. The Bradbury and Siccar Point groups may both be younger than the Mount Sharp group. Alternatively, an alkaline source area in and around Gale crater has been eroded by both water and wind at different times (both before and after deposition of the Mount Sharp group), during the evolution of the crater and its infill.

<p><strong>Introduction:</strong></p> <p><em>Curiosity</em> 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.</p> <p><img src="" alt="" width="1093" height="579" /></p> <p><strong>APXS results:</strong></p> <p>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 3xK<sub>2</sub>O 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 >K<sub>2</sub>O concentrations that fall into two groups (Figures 1,2): 1) with ~4xK<sub>2</sub>O, trending to >Na<sub>2</sub>O concentrations, and 2) with ~5xK<sub>2</sub>O, trending to <Na<sub>2</sub>O, Al<sub>2</sub>O<sub>3</sub>, and Ni, ≫Zn and Ge, and ≈MgO concentrations. The eastern GP, high-K sandstones also overlie more typical basaltic chemistry sandstones.</p> <p><img src="" alt="" width="860" height="484" /></p> <p><strong>Compositional relationship to Bradbury group and other targets previously encountered on the mission:</strong></p> <p>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.</p> <p><img src="" alt="" width="709" height="946" /></p> <p>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<strong> </strong>derived from the mixing of multiple, igneous source rocks including basaltic and more alkaline compositions [13-14].</p> <p><img src="" alt="" width="743" height="942" /></p> <p>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.</p> <p><strong><img src="" alt="" width="734" height="1259" /> </strong></p> <p><strong>Timing Implications: </strong></p> <p>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.</p> <p>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.</p> <p><img src="" alt="" width="905" height="509" /></p> <p><strong>Acknowledgements:</strong></p> <p>MSL APXS is managed and financed by the Canadian Space Agency (CSA). Thanks to the JPL engineers and MSL science team.<strong> </strong></p> <p><strong>References:</strong></p> <p>[1] Banham, S.G., et al. (2018) <em>Sedimentology, 65</em>, 993–1042. doi.org/10.1111/sed.12469</p> <p>[2] Banham, S.G., et al. (2021)<em> JGR: Planets</em>, <em>126</em> (4): e2020JE006554</p> <p>[3] Banham, S.G., et al. (submitted) <em>JGR: Planets</em></p> <p>[4] Watkins, J.A., et al. (2016) <em>47th LPSC</em>, Abstract #2939</p> <p>[5] Stolper, E.M., et al. (2013) <em>Science 341</em> (6153), 1239463. doi:10.1126/science.1239463</p> <p>[6] Schmidt, M.E., et al. (2014) <em>JGR Planets</em>, <em>119</em>(1):64-81, doi:10.1002/2013JE004481</p> <p>[7] Sautter, V., et al. (2015) <em>Geoscience, 8</em>(8), 605.</p> <p>[8] Thompson, L.M., et al. (2016) <em>JGR Planets, 121</em>(10):1981-2003, doi:10.1002/2016JE005055</p> <p>[9] Treiman, A.H., et al. (2016) <em>JGR Planets, 121</em>(1), p.75 – 106. doi:10.1002/2015JE004932</p> <p>[10] Cousin, A., et al. (2017) <em>Icarus 288</em>, 265-283. doi:10.1016/j.icarus.2017.01.014</p> <p>[11] Edwards, P.H., et al. (2017) <em>MAPS, 52</em> 2931-2410. doi.org/10.1111/maps.12953</p> <p>[12] Bedford, C.C., et al., (2019) <em>Geochimica et Cosmochimica Acta, 246</em>, 234-266. doi.org/10.1016/j.gca.2018.11.031</p> <p>[13] Thompson, L.M., et al., (submitted) <em>JGR Planets </em>2021JE007178</p> <p>[14] Thompson, L.M., et al., (2022) <em>53rd LPSC</em>, 1475.pdf</p> <p>[15] LeDeit, L., et al. (this conference)</p> <p>[16] Thorpe, M.T., et al. (Submitted) <em>JGR Planets </em>2021JE00709</p>

R.Gellert, J.A.Berger, N.I.Boyd, C.D.O’Connell-Cooper, M.McCraig, L.M.Thompson, S.J.VanBommel, A.S.Yen. Univ. of Guelph (Guelph, ON, N1G2W1, Canada; rgellert@uoguelph.ca), Johnson Space Center, Houston, TX,Univ.of New Brunswick, Fredericton, NB, Washington University in St. Louis, St. Louis, MO, Jet Propulsion Lab, Pasadena, CA Introduction: Gale Crater was selected as landing site for the Curiosity rover because of a sequence of clay, hematite and sulfate signals detected from orbit on the slopes of Mount Sharp [1]. The transition in minerals is expected to shed light in the changing environmental conditions over the first one billion years on Mars, a time frame when life arose on Earth. As of sol 2985, the rover has traversed ~23km from its landing site and climbed across the first two areas, closing in on the sulfate unit. Here, we will speculate, what the rover might encounter in that unit, based on earlier sulfate detections in previous rover missions and in-situ sulfates already seen in Gale Crater. We will discuss, how the rover can efficiently explore the sulfate unit and how optimal drill samples for SAM and Chemin might be selected, in particular by the use of the APXS [2]. Method: The APXS is an arm-mounted X-ray spectrometer using a combination of PIXE and XRF to quantify 16 standard elements, among them sulfur, with high precision, good accuracy and low detection limits. Starting with Na, elements are quantified by their characteristic X-ray peaks. Their usual oxides are normalized to 100%, assuming a homogeneous and water and carbonate free sample. The assumption that sulfur is fully oxidized as SO3 in the overwhelming number of cases can be confirmed by quantification of the invisible elements – essentially oxygen – using the APXS scatter peak method [3] or other mineralogical results like CheMin or Moessbauer. Identifying and quantifying sulfate candidates is straight forward using APXS results in many cases through mass balance or elemental correlations; however, detailed structural information, including the hydration state is not possible. These details require dedicated drill campaigns of several weeks for CheMin and SAM. We will first discuss possible settings the sulfate unit could represent, based on already encountered sulfate deposits from MSL and MER. Soils: The unconsolidated material (soil) at all landing sites, including Pathfinder and Viking, shows similarity in overall composition – interpreted as average martian crustand a significant abundance of sulfur, about 5% SO3. Sulfur correlates well with chlorine and zinc in soils (fig 1), and with the Fe/FeT content from Moessbauer on MER. These phases likely represent the fine Martian dust [4] and are usually linked to the amorphous component in CheMin XRD, since the amount of crystalline sulfates in a few soils cannot account for the SO3 abundance from APXS. Meridiani Planum: The dominant bedrock at Meridiani is the Burns Formation, a sandstone with up to 25% SO3 from APXS and hematite and jarosite from Moessbauer. Over the traverse of ~45km, the bedrock is remarkably homogeneous, with some variations with depth in impact craters, interpreted as dissolved Mg-sulfates caused by changing ground water levels. The observed 1:1 molar decrease in Mg and S is one example for the application of bulk chemistry deduced mineralogy [2]. Plotting the sulfur content against the major possible cations in fig 2 reveals that no single cation can be identified in the Burns formation; however, Mg and Fe are clearly not diluted by the addition of sulfur, although Fe is also attributable to hematite in the bedrock. Abraded interiors have the highest sulfate content, while brushed and as-is surfaces have lower values due to soil/dust cover or preferential abrasion of sulfate minerals from the rock surface, which ultimately could be a source of sulfur in the global soil. Figure 1 S vs Cl and Zn in soils 0 1 2 3 4 5 6 7 8 9 10 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

In August 2015, the Curiosity Mars rover discovered tridymite, a high‐temperature silica polymorph, in Gale crater. The existing model for its occurrence suggests erosion and detrital sedimentation from silicic volcanic rocks in the crater rim or central peak. The chemistry and mineralogy of the tridymite‐bearing rocks, however, are not consistent with silicic volcanic material. Using data from Curiosity, including chemical composition from the Alpha Particle X‐ray Spectrometer, mineralogy from the CheMin instrument, and evolved gas and isotopic analyses from the Sample Analysis at Mars instrument, we show that the tridymite‐bearing rocks exhibit similar chemical patterns with silica‐rich alteration halos which crosscut the stratigraphy. We infer that the tridymite formed in‐place through hydrothermal processes and show additional chemical and mineralogical results from Gale crater consistent with hydrothermal activity occurring after sediment deposition and lithification.

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.

Introduction: Currently on sol 2635, the MSL Rover Curiosity is climbing up Mount Sharp in Gale Crater to characterize the environmental conditions under which the encountered bedrock was laid down in the distant past [1]. Through extended imaging, remote and in-situ chemistry, as well as drill samples for mineralogy and wet chemistry, so far ~ 400 meters of stratigraphy were characterized from the foothills. In general, the bedrock was found to be very homogeneous over kilometers and in most cases fragments or pebbles clearly shared chemical similarity with the intact layers [2]. Currently the rover is in the clay bearing unit (CBU). During the mission several erratic rocks were found, either igneous floats or suspected cap rocks. Recently, a pebble was measured with the APXS [3], which shows little chemical similarity with the bedrock. Two hypotheses for its origin are discussed in this abstract. APXS results and context: The ~5 cm wide pebble dubbed GretnaGreen (GG) was found in close proximity to the Western Butte capping rock in the CBU, figure 1.

multiple diagenetic events. L. M. Thompson1 (lthompso@unb.ca), J. A. Berger2, N. I. Boyd3, R. Gellert3, M. A. McCraig3, C. O’Connell-Cooper1, M. E. Schmidt4, J. G. Spray1, S. J. VanBommel5 and A. S. Yen6 1Planetary and Space Science Centre, University of New Brunswick, Canada, 2NASA Johnson Space Center, USA, 3University of Guelph, Canada, 4Brock University, Canada, 5Washington University, USA, 6Jet Propulsion Laboratory, California Institute of Technology, USA

RESULTS THROUGH EARLY MARS YEAR 35: A COMPARISON WITH OPPORTUNITY APXS, MSL SAM, AND MARS MODEL FOR PREDICTION ACROSS SCALES. S. J. VanBommel1, Y. Lian2, R. Gellert3, J. A. Berger4, N. I. Boyd3, V. A. Flood3, J. U. Hanania3, M. McCraig3, C. D. O’Connell-Cooper5, M. I. Richardson2, L. M. Thompson5, B. J. Wilhelm3, and M. G. Trainer6, 1McDonnell Center for the Space Sciences, Department of Earth and Planetary Sciences, Washington University in Saint Louis, 2Aeolis Research, 3University of Guelph, 4NASA Johnson Space Center, 5University of New Brunswick, 6NASA Goddard.

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 similar to 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.