The Curiosity rover explored the region between the orbitally defined phyllosilicate-bearing Glen Torridon trough and the overlying layered sulfate-bearing unit, called the “clay-sulfate transition region.” Samples were drilled from the top of the fluviolacustrine Glasgow member of the Carolyn Shoemaker formation (CSf) to the eolian Contigo member of the Mirador formation (MIf) to assess in situ mineralogical changes with stratigraphic position. The Sample Analysis at Mars-Evolved Gas Analysis (SAM-EGA) instrument analyzed drilled samples within this region to constrain their volatile chemistry and mineralogy. Evolved H₂O consistent with nontronite was present in samples drilled in the Glasgow and Mercou members of the CSf but was generally absent in stratigraphically higher samples. SO₂ peaks consistent with Fe sulfate were detected in all samples, and SO₂ evolutions consistent with Mg sulfate were observed in most samples. CO₂ and CO evolutions were variable between samples and suggest contributions from adsorbed CO₂, carbonates, simple organic salts, and instrument background. The lack of NO and O₂ in the data suggest that oxychlorines and nitrates were absent or sparse, and evolved HCl was consistent with the presence of chlorides in all samples. The combined rover data sets suggest that sediments in the upper CSf and MIf may represent similar source material and were deposited in lacustrine and eolian environments, respectively. Rocks were subsequently altered in briny solutions with variable chemical compositions that resulted in the precipitation of sulfates, carbonates, and chlorides. The results suggest that the clay-sulfate transition records progressively drier surface depositional environments and saline diagenetic fluid, potentially impacting habitability.

Key Points

Seven rock samples were drilled in the clay-sulfate transition region and analyzed using the Sample Analysis at Mars (SAM) instrument
SAM evolved gas data are consistent with a decrease in phyllosilicates and the presence of Mg sulfate in the clay-sulfate transition region
SAM data suggest rocks in the clay-sulfate transition region formed in progressively drier conditions, potentially impacting habitability

Plain Language Summary

The Mars Science Laboratory Curiosity rover investigated the “clay-sulfate transition” in Gale crater, Mars, with its suite of science instruments in order to understand the geologic history and environmental changes recorded in sedimentary rock. This region is significant because orbital data showed that clay minerals, which require abundant liquid water to form, were present in the lower older rocks. Orbital data showed that hydrated Mg-sulfates, which generally form under evaporitic conditions, were present in the overlying younger rocks. Rocks were drilled within the clay-sulfate transition region and analyzed with the Sample Analysis at Mars (SAM) instrument, which analyzes gases that are produced when minerals break down during heating. SAM data indicate the geologically older rocks contain clay minerals, consistent with formation in an aqueous environment. SAM data from the geologically younger rocks lack evidence for clay minerals but reveal that Mg sulfates are more common, suggesting that these rocks formed in a more arid environment. SAM data are consistent with the presence of Fe carbonate, simple organic salts, oxidized organic matter, and chlorides. SAM data suggest that younger rocks in the clay-sulfate transition region formed under drier and more saline conditions that were less conducive for the formation of clay minerals.

1 Introduction

The Mars Science Laboratory (MSL) Curiosity rover landed in Gale crater, Mars in 2012 with the mission goals of understanding the crater site's geologic history, evaluating its potential to have hosted past habitable environments, and investigating the evolution of climate (e.g., Grotzinger et al., 2012; Vasavada, 2022). Curiosity has fulfilled these goals by searching for evidence of past water, potential energy sources for past microbial life (e.g., organic compounds, nitrates), and the elements that are the building blocks for life on Earth (e.g., carbon, hydrogen, sulfur, nitrogen, phosphorus) (e.g., Eigenbrode et al., 2018; Grotzinger et al., 2015; Sutter et al., 2017). Curiosity's exploration targets within Gale crater have included the Glen Torridon (GT) topographic trough and the overlying Layered Sulfate-bearing unit (LSu), which were identified in orbital data due to their distinct mineralogical signatures of phyllosilicates and hydrated Mg sulfates, respectively (Figure S1 in Supporting Information S1, Figure 1) (Grotzinger et al., 2012; Milliken et al., 2010). The region in between the phyllosilicate-bearing GT trough and the LSu, called the “clay-sulfate transition region,” was inferred to potentially record a major change in environmental conditions, possibly linked to a transition from a warm and wet climate to a colder and more arid climate (Figure 1, Figure S1 in Supporting Information S1) (Milliken et al., 2010). Similar mineralogical transitions have been observed globally across Mars by the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) instrument on the Mars Express spacecraft (Bibring et al., 2006) and by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on the Mars Reconnaissance Orbiter (MRO) (Murchie et al., 2009). Therefore, exploration of this region by Curiosity offers a unique opportunity for in situ examination of a mineralogical transition that may be global in scale.

Details are in the caption following the image — **Figure 1**
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Vertically exaggerated High Resolution Imaging Science Experiment (HiRISE) image of the Layered Sulfate-bearing unit (LSu), the Glen Torridon (GT) trough, and the region in between (the clay-sulfate transition region). CRISM sulfate and phyllosilicate detections are overlain, showing a transition from phyllosilicate-dominated surface materials to hydrated sulfate-dominated surface materials. The color of the drill hole name (Bardou to Canaima) represents if it was drilled in the GT trough, the clay-sulfate transition region, or the LSu. Drill holes are represented with yellow stars and the rover traverse is shown with a white line.

Curiosity's suite of science instruments analyzed rocks within and surrounding the clay-sulfate transition region between Sols 3050 and 3625 of the MSL mission (March 2021–October 2022) over an elevation change of ∼190 vertical meters and traverse distance of ∼3.9 km (Figures 1 and 2; Figure S1 in Supporting Information S1). Seven drill samples were acquired during the clay-sulfate transition region exploration campaign and their volatile-bearing mineralogy was assessed through evolved gas analysis (EGA) experiments using the Sample Analysis at Mars (SAM) instrument suite. SAM-EGA's capabilities were especially important in this region because of the ability to detect trace phases, volatiles in X-ray amorphous phases, and organic molecules. SAM-EGA data, along with bulk mineralogical data from the Chemistry and Mineralogy (CheMin) X-ray diffractometer (XRD) (e.g., Blake et al., 2012), elemental data from the Alpha Particle X-ray Spectrometer (APXS) (e.g., Gellert et al., 2015) and the Chemistry and Camera (ChemCam) Laser Induced Breakdown Spectroscopy (LIBS) instrument (e.g., Maurice et al., 2012), and imagery from onboard cameras, were used to ground-truth orbital data. The overall goals of the work were to use SAM-EGA results to provide constraints on the volatile-bearing mineralogy of strata in the clay-sulfate transition region and to interpret ancient depositional environments and alteration conditions in Gale crater. The specific objectives of this work were to (a) test the hypothesis that rocks in this region record a progression to increasingly arid conditions and (b) assess past alteration conditions (e.g., early and late diagenetic conditions) experienced by rocks within and surrounding the clay-sulfate transition region.

1.1 Geologic Context

The orbitally defined clay-sulfate transition region is situated between regions where MRO's CRISM instrument observed strong clay mineral absorptions (i.e., the phyllosilicate-bearing GT topographic trough) and Mg-sulfate absorptions (i.e., the Layered Sulfate-bearing unit) (Fraeman et al., 2016). For Curiosity's exploration, this region approximated to strata present above the fluvial Mercou member of the Carolyn Shoemaker formation (CSf) to the Contigo member of the Mirador formation (MIf). The drill holes Nontron (NT) and Bardou (BD) were drilled in the phyllosilicate-bearing GT trough and the drill hole Canaima (CA) was drilled in the Layered Sulfate-bearing unit. The samples drilled in between these endmembers (Pontours (PT), Maria Gordon (MG), Zechstein (ZE), and Avanavero (AV)) were drilled within the clay-sulfate transition region (Figure S1 in Supporting Information S1). Detailed descriptions of the stratigraphy and geologic context of the clay-sulfate transition region are provided in Edgar et al. (2024) and Meyer et al. (2024). The drill samples discussed in this paper were obtained in the stratigraphic units outlined below.

1.1.1 Carolyn Shoemaker Formation (CSf)

The Glasgow member of the CSf, where the NT sample was drilled, contained gray-toned mudstones with millimeter-scale laminations consistent with deposition in a lacustrine environment (Fedo et al., 2022). The Glasgow member strata contained clusters of northward-dipping fractures that sometimes obscured primary depositional features (Fedo et al., 2022). The top of the Glasgow member is located at the base of Mont Mercou, a ∼7-m-high escarpment interpreted as fluvial barform deposits in an alluvial or shoreline environment (Figure 3a) (Cardenas et al., 2022). Mont Mercou represents the Mercou member of the CSf, and in general it is consistent with the overall up-section transition from deposition in a low-energy lacustrine environment to deposition in a higher energy fluvial/lake margin environment (Bennet et al., 2023; Fedo et al., 2022; McAdam et al., 2022; Thorpe et al., 2022). The Mercou member, where BD was drilled, is located near the boundary where CRISM detected spectral evidence of both clay mineral and hydrated sulfate signatures (Bennet et al., 2023; Fraeman et al., 2016; Sheppard et al., 2022).

Strata above Mont Mercou, in the Pontours member of the CSf, corresponded to weaker orbital signatures of clay minerals and somewhat stronger signatures of Mg sulfate (Figure 1) (Fraeman et al., 2016). The Pontours member, where the PT sample was drilled, consists of limited examples of planar lamination and is mostly dominated by products of diagenetic overprinting (i.e., nodules and abundant veins) that obscure grain size and primary sedimentology (Figure 3b) (Meyer et al., 2024). Though difficult to discern due to this overprinting, strata in the Pontours member may be most similar to those in the Glasgow member and thus may have formed in a marginal lake environment (Edgar et al., 2024; Meyer et al., 2024).

1.1.2 Mirador Formation

Strata above the CSf are mineralogically and lithologically distinct, leading to the designation of a new formation—the Mirador formation (MIf). The CSf to MIf transition represents a shift from marginal lacustrine/fluvial to an ancient eolian depositional environment, as evidenced by the presence of large-scale cross-bedded sandstones (Edgar et al., 2024; Meyer et al., 2024). Strata of the lowest member, the Dunnideer member (where MG was drilled), consist of meter-scale cross-stratified sandstones representative of a dry eolian dune environment, though the lowermost several meters exhibit a strong diagenetic overprint (Figure 3c) (Edgar et al., 2024; Gupta et al., 2023; Meyer et al., 2024). Overlying strata of the Port Logan member of the MIf (where ZE was drilled) are characterized by a lower degree of diagenetic overprinting, providing clearer visibility of cross-stratified sandstones indicative of an eolian dune environment (Figure 3d) (Edgar et al., 2024; Gupta et al., 2022; Meyer et al., 2024). Similarly, the Contigo member of the MIf (where AV and CA were drilled) consists of cross-stratified sandstones, but this member also exhibits laterally extensive lenses of thinly bedded and locally ripple cross-laminated sandstones (Edgar et al., 2024). This suggests sediment deposition in an eolian dune environment that was interrupted by periods of shallow interdune water bodies (Figures 3e and 3f) (Edgar et al., 2024). Orbital signatures of hydrated Mg sulfates became more distinct midway through the Contigo member in an area called the Marker Band Valley (MBV) (Figure S1 in Supporting Information S1), and this marks the beginning of the orbitally defined Layered Sulfate-bearing unit (Figure 1).

1.1.3 Mineralogy and Chemistry

Quantitative mineralogy derived from CheMin XRD patterns revealed changes in the abundances and compositions of phyllosilicates, sulfates, and iron-oxides that were important for interpreting changes in mode of deposition and subsequent diagenesis, especially when placed in the context of variations in sedimentology. Samples drilled from the Glasgow member (CSf) to the Contigo member (MIf) contained hematite, plagioclase, potassium feldspar, pyroxene, calcium sulfates, and minor quartz. Goethite (FeO(OH)) was detected for the first time in the mission at the top of the Glasgow member in the NT drill sample (Rampe et al., 2023, 2024). Goethite abundances generally increased up-section from the Glasgow member (CSf) to the Contigo member (MIf), and was as high as 4.1 wt.% in the AV drill sample (Contigo member) (Rampe et al., 2023). Ca sulfates that were detected included anhydrite (CaSO₄), bassanite (Ca(SO₄)·0.5H₂O), and gypsum (CaSO₄·2H₂O), with an especially high gypsum abundance (∼18 wt.%) in the ZE drill target (Port Logan member) due to a cross-cutting Ca sulfate vein (Rampe et al., 2023). CheMin detected crystalline Mg sulfate (starkeyite; MgSO₄·4H₂O) for the first time in the mission in the CA sample, which was drilled in the Contigo member (Chipera et al., 2023). Amorphous Mg sulfate was also present in the CA drill target (Chipera et al., 2023), consistent with indirect elemental evidence for X-ray amorphous Mg sulfate lower in the MIf (e.g., Meyer et al., 2024). Trace amounts of halite (NaCl) were detected in the PT drill sample (Pontours member) (Rampe et al., 2023).

CheMin-derived phyllosilicate abundances were as high as 34 wt.% in the GT trough (Thorpe et al., 2022) but decreased in the clay-sulfate transition region. CheMin detected a phyllosilicate with 10 Å basal spacing and 020 reflections at 4.5 Å, consistent with collapsed nontronite (Na_0.3Fe³⁺₂Si₃AlO₁₀(OH)₂•4(H₂O)), similar to previous detections in the GT trough, in the Glasgow and Mercou members of the CSf (18 wt.% in NT and 12 wt.% in the BD drill sample) (Rampe et al., 2023). Notably, phyllosilicates were absent or below CheMin's detection limit in all overlying samples (Rampe et al., 2023). All the drilled samples discussed in this study contained a significant component of X-ray amorphous material, ranging from 40 ± 10 wt.% (in NT) to 61 ± 15 wt.% (in CA).

In contrast to the mineralogy, the bulk elemental compositions of bedrock within the clay-sulfate transition exhibited little variation. Bulk chemical compositions determined by APXS did not reflect most of the mineralogical changes observed within and surrounding the clay-sulfate transition (Berger et al., 2023), nor did bulk bedrock chemistry derived from ChemCam LIBS data (Meyer et al., 2024). Bedrock from the Glasgow member (CSf) to the Contigo member (MIf) had low variability in major and minor elements and concentrations did not change systematically with elevation (Berger et al., 2023). One exception was the increase in the average CaO and SO₃, consistent with 30% higher Ca sulfate in the bedrock matrix, relative to the underlying Mt. Sharp group bedrock (Berger et al., 2023). Additionally, APXS measurements provided the first evidence of ∼5–10 wt.% addition of Mg sulfate in the Marker Band Valley, with this distinct elemental change occurring over a narrow stratigraphic interval of <5-m (−3,889 to −3,884 m elevation, between the AV and CA drill holes (Figure 2; Figure S1 in Supporting Information S1)) (Berger et al., 2023). With the exception of the change in the Ca- and Mg-sulfate assemblages, the average bedrock composition in the MIf (up to where CA was drilled) was the same as stratigraphically lower samples of the CSf, which generally had the same bulk chemical characteristics as the Murray formation samples above the Pahrump Hills member (Berger et al., 2023). Below the Marker Band Valley region, Mg sulfate enrichments as seen by APXS had only been evident in larger, ∼5–20 mm nodules, although APXS measurements of nodules do not provide a systematic sampling of the features (e.g., Berger et al., 2020; VanBommel et al., 2016).

With the exception of Ca and Mg sulfates, the bulk chemical composition of bedrock within and surrounding the clay-sulfate transition was effectively isochemical. However, parts of the stratigraphic section have been significantly affected by diagenetic processes as a result of several distinct episodes of water-rock interaction (Meyer et al., 2024). Integrating SAM data with these and other independent mineralogical, chemical, and sedimentological observations allows for a more nuanced understanding of the abundances and hosts of various volatiles in this critical stratigraphic section of Mt. Sharp. This, in turn, provides a more detailed understanding of the style, variability, and relative timing of water-rock interactions in Gale crater that correspond to a distinct and widely observed change in mineralogy across Mars.

2 Materials and Methods

2.1 The SAM Instrument Suite

The purpose of the SAM instrument suite is to measure the abundances and isotopic compositions of volatiles evolved during the heating of martian surface materials and to sample directly from the atmosphere (Mahaffy et al., 2012). The SAM quadrupole mass spectrometer has detected H₂O, O₂, SO₂, HCl, CO₂, CO, and NO, as well as organic fragments in the rocks and sediment samples collected in Gale crater (e.g., Eigenbrode et al., 2018; Leshin et al., 2013; McAdam et al., 2022; Millan et al., 2022; Ming et al., 2014; Sutter et al., 2017; Szopa et al., 2020). The parent phases of these volatiles (e.g., phyllosilicates, oxychlorines, sulfates) were identified based on the compositions, relative abundances, and evolution temperatures of volatiles observed in each sample. SAM-EGA supports mineral detections by the CheMin instrument and can also infer the presence of amorphous phases or phases present at abundances below CheMin's detection limit (∼1–2 wt.%). SAM-EGA can also constrain the types of phyllosilicates (e.g., dioctahedral or trioctahedral smectite) or sulfates (e.g., Mg or Fe sulfate) present in a sample based on the temperature of gas (e.g., H₂O, SO₂) evolutions.

The SAM instrument suite is also capable of detecting organic molecules in samples from drilled rocks (e.g., Eigenbrode et al., 2018; Freissinet et al., 2015; Millan et al., 2022; Szopa et al., 2020). SAM-EGA specifically can reveal potential associations between organic compounds and minerals. SAM organic detections have included chlorine and sulfur-bearing compounds, fragments from alkyl and aromatic compounds, and alkanes in mudstones and sandstones (Millan et al., 2022).

2.2 SAM Analyses

The SAM instrument suite is composed of two ovens and three gas analysis instruments: a quadrupole mass spectrometer (QMS), six gas chromatography columns, and a tunable laser spectrometer (TLS) (Mahaffy et al., 2012). This paper focused on SAM's evolved gas analysis (EGA) mode, which is when solid samples are heated and the evolved gases are analyzed directly by the QMS. SAM's gas chromatography-mass spectrometry (GCMS) system is used to separate out evolved volatiles sampled over specific temperature ranges (termed “temperature cuts”) during SAM-EGA experiments, and SAM-TLS is used to analyze the amounts and isotopic compositions of specific volatiles (H₂O, CO₂, and CH₄) in SAM-EGA temperature cuts. EGA data are thus obtained any time a TLS run or GCMS run is completed on a solid sample.

During SAM-EGA experiments, drill fines are delivered to quartz cups and heated to ∼900°C at a rate of ∼35°C/min in one of the two SAM ovens (e.g., McAdam et al., 2022; Sutter et al., 2017). Gases produced during the heating of samples are then directed to the QMS using a He carrier gas (∼0.8 standard cubic centimeters per minute, ∼25 mbar) for EGA (McAdam et al., 2022), and to the TLS.

During the clay-sulfate transition campaign, after initial characterization by SAM-EGA-TLS experiments, second portions of the NT, MG, and CA samples were analyzed by SAM-EGA-GCMS to obtain more sensitive organic carbon detections. In SAM's GCMS-mode, organic molecule identification is based on both their retention time in GC columns and their mass spectra (Freissinet et al., 2015; Millan et al., 2022).

2.3 Sample Drilling and Delivery

All samples drilled in the clay-sulfate transition region were collected using Curiosity's rotary percussive drill and transferred to the CheMin and SAM inlets for analysis. Samples were collected using feed-extended drilling (FED) and feed-extended sample transfer (FEST), which were first implemented on Sol 1977 of the MSL mission (February 2018) after a drill feed mechanism anomaly on Sol 1536 (December 2016) (Fraeman et al., 2020). During FED, the rover's arm extended the drill bit into the surface rather than the drill feed. The sample was then delivered to the SAM inlet using the FEST method, which hovered the drill bit over the SAM inlet and rotated the bit in reverse to release the sample which accumulated in a sleeve around the drill bit.

Seven rocks were drilled from the Glasgow member (CSf) to the Contigo member (MIf) with the goal of observing changes in volatile chemistry and mineralogy in the clay-sulfate transition (Figures 1, 2, and 4). The stratigraphically lowest drill sample examined in this study, NT, was drilled at the base of Mont Mercou to characterize the volatile chemistry before Curiosity entered the clay-sulfate transition region. The BD drill sample was acquired at the top of the Mont Mercou escarpment (representing the bottom of the Mercou member of the CSf) and the NT-BD pair offers a unique opportunity to investigate the mineralogy and chemistry of two samples at the bottom and top of an exposed rock outcrop with a rover mission. Beyond BD, drill sampling occurred at ∼25-m vertical intervals in order to have consistent measurements of changes in mineralogy and volatile chemistry as a function of stratigraphic position.

In addition to the seven drill samples acquired during the clay-sulfate transition campaign, a cup with the post-heat residue of the Windjana sample (Sutter et al., 2017) was analyzed on Sol 3487 (May 2022) between the analyses of ZE and AV. This sample, herein called the “post-Zechstein blank” (post-ZE blank), was the first SAM blank sample analyzed since 2015, when a sample cup containing the post-heat residue of the Greenhorn drill sample (Sutter et al., 2017) was analyzed by a SAM-EGA-GCMS experiment. One major motivation for running the post-ZE blank was a recent SAM experiment that utilized the thermochemolysis agent tetramethylammonium hydroxide (TMAH) in methanol to search for organics in the Mary Anning drill sample lower in the CSf (McAdam et al., 2022). That experiment had the potential to add new background compounds to the SAM data from subsequent samples. The post-ZE blank facilitated determinations of whether detected volatiles were derived from inorganic and organic compounds indigenous to the sample or whether they were due to reactions involving derivatization or thermochemolysis agents or their byproducts.

The post-ZE blank contained a triple-portion of the Windjana drill sample that had been heated multiple times. A previously heated sample was chosen as a blank rather than an empty sample cup because it could show background from gas adsorbed onto particles and from the absorption and reaction of SAM background organics once the sample was introduced into the oven. During the blank experiment, the sample cup containing the Windjana drill sample residue was first subjected to a preconditioning step, where it was heated to the same maximum temperature reached during the preconditioning step preceding regular SAM-EGA experiments (∼900°C). After the preconditioning step, the cup was reheated using the same procedures as regular SAM analyses, and evolved gases were directed to the QMS and the GCMS.

2.4 Evolved Gas Corrections

The signals for certain evolved gases (O₂, NO, CO) were corrected to eliminate contributions from fragments of the same mass to charge ratio (m/z) derived from other gas species. Correction methods are described in detail in Sutter et al. (2022) but are briefly reviewed here for clarity.

Correction factors were derived from a combination of pre-flight SAM analyses, the National Institute of Standards and Technology (NIST) Standard Reference Database (SRD), and the SAM-EGA data themselves. Pre-flight SAM analyses of calcite (CaCO₃) were used to derive factors to correct for contributions of CO₂ fragments to m/z 28 (CO) (Sutter et al., 2022). Mass spectra from the NIST SRD were used to estimate contributions from isotopologues with the same m/z.

Correction factors were determined using the SAM-EGA data in Origin Pro® graphing software. Data from contributing fragments or isotopologues with the same m/z as the gas of interest were multiplied by their correction factors and plotted against the raw, uncorrected data from the gas species of interest (e.g., O₂ (m/z 32)). The initial correction factors derived from NIST ratios were slightly altered to determine if the summation of contributions from fragments and isotopologues matched the profile of the raw uncorrected data from the gas species of interest (e.g., O₂ (m/z 32); Table S1 in Supporting Information S1). The corrected data (uncorrected data minus total contribution from fragments and isotopologues) were also plotted. In cases where there were no “real” peaks (i.e., peaks that were related to isotopologues or fragments of the gas species of interest), the corrected data signal was near zero. Correction factors for O₂ varied between samples (Equation 1; Table S1 in Supporting Information S1), whereas the correction factors for NO and CO were solely based on NIST ratios and were the same between samples (Equations 2 and 3).

O₂ (main m/z of 32) was corrected for contributions from fragments of H₂O, SO₂, and O₂ from sulfate decomposition, and from methanol. Methanol (m/z 31), related to the SAM TMAH experiment, was found to contribute to the SAM background (Sutter et al., 2022; Williams et al., 2021). Drill samples analyzed after Mary Anning 3, including all the samples presented in this study, were corrected for methanol contributions.

{\mathrm{O}}_{2}=\mathrm{m}/\mathrm{z}\,32\mbox{--}(\mathrm{a}\times \mathrm{m}/\mathrm{z}\,64)\mbox{--}(\mathrm{b}\times \mathrm{m}/\mathrm{z}\,19\,\text{or}\,\mathrm{m}/\mathrm{z}\,17)\mbox{--}(\mathrm{c}\times \mathrm{m}/\mathrm{z}\,31)

(1)

where a, b, and c are correction factors (see Table S1 in Supporting Information S1).

Nitrate (NO, m/z 30) was corrected for contributions from the thermal decomposition products of the derivatization agents methyl-N-(tert-butyldimethylsilyl) trifluoroacetamide (MTBSTFA) and dimethylformamide (DMF), and the thermochemolysis agent TMAH in methanol. MTBSTFA and DMF, which were used in SAM-GCMS wet chemistry experiments, leaked from at least one of several sealed cups and contributed to the SAM background (Freissinet et al., 2015; Glavin et al., 2013). TMAH and methanol were introduced to the SAM cups during Mary Anning 3 run (as discussed above). MTBSTFA/DMF/TMAH byproducts that contributed to m/z 30 included formaldehyde (m/z 29) and ethane (m/z 25 used for correction), and methanol (m/z 31) was present with the TMAH thermochemolysis agent. Prior to correcting NO, formaldehyde was corrected for the contributions from ethane, methyl propene (m/z 39 used for correction), and methanol.

\text{Formaldehyde}=\mathrm{m}/\mathrm{z}\,29\mbox{--}(0.24\times \mathrm{m}/\mathrm{z}\,39)\mbox{--}(6.15\times \mathrm{m}/\mathrm{z}\,25)\mbox{--}(0.46\times \mathrm{m}/\mathrm{z}\,31)

(2)

\text{Nitrate}\,(\text{NO})=\mathrm{m}/\mathrm{z}\,30\mbox{--}(0.8\times \text{Formaldehyde})\mbox{--}(7.18\times \mathrm{m}/\mathrm{z}\,25)\,(0.0649\times \mathrm{m}/\mathrm{z}\,31)

(3)

CO was corrected for MTBSTFA byproducts (methyl propene, formaldehyde, ethane, and CO₂), methanol from TMAH, and CO₂ from the sample. The same formaldehyde correction equation was used for the NO and CO corrections.

\text{CO}=\mathrm{m}/\mathrm{z}\,28\mbox{--}(3.2\times \mathrm{m}/\mathrm{z}\,45)\mbox{--}(0.49\times \mathrm{m}/\mathrm{z}\,39)\mbox{--}(0.24\times \text{Formaldehyde})\mbox{--}(28.6\times \mathrm{m}/\mathrm{z}\,25)\mbox{--}(0.05\times \mathrm{m}\times \mathrm{z}\,31)

(4)

2.5 Sample Delivery Masses and Evolved Gas Abundance Calculations

Sample delivery masses prior to Sol 1536 were estimated at 45 ± 18 mg per portion based on pre-flight testing of MSL testbed hardware (Fraeman et al., 2020) and evolved gas abundance calculations were performed using methods described in Archer et al. (2014). Samples collected after Sol 1536 used the FED/FEST collection and sample delivery method, which bypassed the Collection and Handling for Interior Martian Rock Analysis tool (CHIMRA) and prevented portioning and sieving (Fraeman et al., 2020). Sample masses could no longer be estimated based on pre-flight testing data but a new method of sample delivery mass estimation was developed that relied upon the weight percentages of certain volatile-bearing minerals (e.g., gypsum, bassanite, smectites) measured by CheMin and the total amounts of evolved gases associated with that particular phase (e.g., water) (McAdam et al., 2022; Sutter et al., 2022). The greater uncertainty associated with the estimated delivery mass caused an increased error associated with calculated concentrations (reported in wt.% or nmol/mg) for each evolved gas species (Tables S2–S9 in Supporting Information S1). However, errors associated with the total evolved gas detections (reported in nmol or μmol) were unaffected. Additionally, interpretations of sample mineralogy based on the evolved gas profiles were independent of abundance calculations.

3 Results

3.1 H₂O

Samples from the upper Glasgow member (CSf) to the Contigo member (MIf) evolved between 0.87 wt.% and 7.74 wt.% H₂O (Figure 5, Table S2 in Supporting Information S1), with contributions from hydrated salts (e.g., sulfates), amorphous phases, iron oxyhydroxides, and phyllosilicates. The two Maria Gordon drill samples (MG1, MG2) and the Zechstein sample (ZE)—all in the MIf—contained the highest abundances of water out of all samples collected thus far in the mission, ranging from 5.35 to 7.74 wt.% (Table S2 in Supporting Information S1). These water abundances were higher than the phyllosilicate-rich samples drilled in the GT trough (McAdam et al., 2022) and were likely due to the high abundance of gypsum in ZE (18.2 wt.%) (Rampe et al., 2023, 2024) and possibly due to adsorbed water released from the relatively high amorphous component in MG (54 ± 13 wt.% (Rampe et al., 2023; Rampe et al., 2024). Samples in this study evolved H₂O between approximately 100°C and 750°C, with multiple distinct peaks within that temperature range (Figure 5). H₂O was not detected in the post-ZE blank (Table S2 in Supporting Information S1).

All samples evolved similar low-temperature (<250°C) water releases that were attributed to adsorbed water from the martian atmosphere (Zent & Quinn, 1995), hydrated salts, and phases within the amorphous component (Figure 5). Water that evolved at temperatures less than 250°C was consistent with the dehydration of bassanite, which was detected by CheMin in all samples (Rampe et al., 2023, 2024). The broad, low-temperature water release in CA was consistent with, although not unique to, the detection of starkeyite by CheMin (Chipera et al., 2023) and laboratory EGA studies of synthetic starkeyite (Clark et al., 2023). Amorphous materials, which ranged from 40 to 62 wt.% in these samples, can also include phases that evolve low-temperature water such as hydrated sulfates, opaline silica, nanophase iron oxyhydroxides, and poorly crystalline silicates and aluminosilicates (e.g., McAdam et al., 2022).

A H₂O peak at ∼270°C was observed in all samples except for ZE and CA (Figure 5) and was attributed to goethite. Goethite was detected by CheMin in all samples with the exception of CA and BD (Rampe et al., 2023, 2024), which may explain the absence of the ∼270°C water peak in CA and the relatively small peak in BD (Figure 5). Any water peaks at 270°C in ZE may have been obscured by the large, lower temperature water release derived from gypsum dehydration (18.2 wt.% gypsum in ZE (Rampe et al., 2023; Rampe et al., 2024)).

Mid-temperature H₂O peaks ∼360–430°C were observed in NT, BD, and AV and were most consistent with the dehydroxylation of nontronite. Although the total water abundances did not decrease with increasing elevation, mid-temperature water releases generally decreased in relative intensity with elevation (Figure 5). The decrease in relative intensity of mid-temperature water peaks with elevation was consistent with the decrease in phyllosilicate abundances as detected by CheMin (Figure 5) (Rampe et al., 2023, 2024).

AV evolved a minor high-temperature H₂O peak at approximately 700°C that was consistent with montmorillonite (Figure 5). However, CheMin did not detect phyllosilicates in AV (Rampe et al., 2023, 2024), so if phyllosilicates were present they would be below the detection limit of CheMin (∼2 wt.% for smectites (Achilles et al., 2012)). BD, PT, and MG evolved water releases with minor shoulders at temperatures above 600°C, however CheMin did not detect montmorillonite in these samples (Rampe et al., 2023, 2024).

3.2 SO₂

The broad mid-temperature SO₂ evolutions between approximately 320 and 750°C were observed in all samples collected from the upper Glasgow member (CSf) to the Contigo member (MIf) and were attributed to the thermal decomposition of ferric sulfate or mixed-cation sulfates (Pandey et al., 2024). The SO₂ evolutions in NT1 and NT2 occurred over a narrower temperature range compared with stratigraphically higher samples, suggesting that the Fe sulfate in NT had a higher degree of crystallinity or it did not contain mixed-cation sulfates. The mid-temperature SO₂ releases in BD, PT, MG, ZE, and CA were broad and generally similar in profile (Figure 6). The SO₂ peak in AV occurred at a temperature of 683°C, which was higher than that of the other samples and was possibly caused by mixed-cation Fe sulfates. CheMin did not detect Fe sulfates in these samples, suggesting that they were below the instrumental detection limit of CheMin (∼1–2 wt.%) or were in the amorphous fraction. Mass balance calculations using APXS and CheMin XRD data (Achilles et al., 2020; Smith et al., 2021) showed that FeO and SO₃ were present in the CheMin amorphous fraction of all samples, although work to determine their relationship to each other, and the other major amorphous-forming oxides is ongoing (Simpson et al., 2024).

High-temperature (>750°C) SO₂ releases attributed to Mg sulfate were present in all samples except for BD, and the fraction of integrated SO₂ attributed to Mg sulfate to total integrated SO₂ (Mg sulfate and Fe sulfate) generally increased with elevation (Figures 6 and 7). This suggests that the stratigraphically higher sample generally contained more Mg sulfate than stratigraphically lower samples (Figure 7), although it is important to note that SAM SO₂ data do not account for sulfates that decompose above the instrument temperature range (e.g., Ca sulfate). Laboratory EGA studies have demonstrated that Mg sulfate begins thermal decomposition ∼800°C when mixed with other phases such as halite (Clark et al., 2024). The SO₂ peaks above 750°C were due to the decrease in the oven heating rate at the end of the temperature ramp, which slowed and eventually stopped the thermal decomposition of the sulfate, resulting in decreased SO₂ production. The fraction of SO₂ released above ∼750°C was higher in CA than in any other sample drilled in the clay-sulfate transition region (Figures 6 and 7). This was consistent with the detection of starkeyite (MgSO₄·4H₂O) in CA, which was the first CheMin detection of crystalline Mg sulfate on the mission (Chipera et al., 2023). The Mg sulfate detected by SAM-EGA in other samples may have been below CheMin's detection limit or be present in an X-ray amorphous state. ChemCam data also showed that certain nodules in the clay-sulfate transition region contained amorphous Mg sulfate (Meyer et al., 2024).

Subsamples of the same drill hole (NT1 and NT2, MG1, and MG2) produced slightly different SO₂ release patterns (Figure 6), which may be caused by heterogeneity in the types of sulfate phases within the same drill hole. Sulfate heterogeneity between subsamples may arise from differences in the amount of diagenetic/nodular material in each aliquot delivered to SAM. Diagenetic and nodular material in the clay-sulfate transition region has been shown to be enriched in Ca and Mg sulfate (Meyer et al., 2024). Variations in SO₂ release patterns between subsamples may also arise from differences in the amount of phases that catalyze sulfate thermal decomposition or differences in crystallinity. For example, the lower temperature SO₂ peak in NT2 (452°C) compared to NT1 (545°C) may be due to a higher abundance of phases that catalyze Fe sulfate thermal decomposition (e.g., chlorides, Fe-phases) or the Fe sulfate in NT1 may have had a higher degree of crystallinity. The high-temperature SO₂ release in NT2 (790°C), which was less prominent or absent in NT1, could be caused by the presence of more Mg sulfate in that subsample. Alternatively, Mg sulfate could have been present in both subsamples, but there may have been a higher abundance of phases that catalyze Mg sulfate decomposition to a lower temperature in NT2.

SO₃ abundances derived from SAM-EGA SO₂ peak integrations ranged from 0.22 ± 0.13 wt.% to 2.19 ± 1.39 wt.% (Table S3 in Supporting Information S1) and did not systematically increase with elevation (Figure 6). Additionally, SAM-EGA SO₂-derived SO₃ abundances were not higher in samples drilled in the clay-sulfate transition region than in samples drilled in the GT trough (0.3 ± 0.21 wt.% to 1.43 ± 0.70 wt.%) (McAdam et al., 2022). The highest SAM-derived SO₃ abundances were in subsamples from the Maria Gordon drill hole, which also had the highest density of sulfate-bearing nodules (Meyer et al., 2024).

APXS SO₃ abundances from drill tailings and drill bit assembly dump piles ranged from 4.77 ± 0.1 wt. % to 19.21 ± 0.22 wt. % (Table S3 in Supporting Information S1) and were higher than the SO₃ abundances obtained from SAM-EGA data (Figure 6) (Berger, 2024; Berger et al., 2023). The discrepancy between SO₃ abundances obtained from SAM-EGA data and measured by APXS was caused primarily by the presence of Ca sulfates that do not thermally decompose within the operating temperature of SAM. Mg sulfate, as discussed above, also does not completely thermally decompose within the SAM operating temperature range. Therefore, SO₃ abundances obtained from the SAM-EGA data do not account for sulfates that evolve SO₂ above the maximum operational temperature of SAM.

3.3 O₂ and NO

Drill samples acquired during the clay-sulfate transition region campaign, including the post-ZE blank, did not evolve O₂ during SAM-EGA analyses (Figure S2 in Supporting Information S1). O₂ evolutions from samples analyzed earlier in the mission were attributed to the thermal decomposition of perchlorates and chlorates (Hogancamp et al., 2018; Ming et al., 2014; Sutter et al., 2017, 2022) or known organic compounds inside SAM (i.e., MTBSTFA, TMAH). The lack of evolved O₂ in these samples suggested that perchlorates or chlorates were absent or that oxychlorines were present below the ClO₄ detection limit (<0.001 wt.%) of SAM. Alternatively, if minor amounts of perchlorates and chlorates were present in the samples, their evolved O₂ could potentially be consumed by Fe-bearing phases present in the sample (e.g., iron oxides), preventing their detection (Hogancamp et al., 2018; Sutter et al., 2015). This explanation is less likely, however, because the stratigraphically lower samples that evolved O₂ also contained Fe-bearing phases (Rampe et al., 2019; Sutter et al., 2017; Treiman et al., 2016; Yen et al., 2017) and the amount of O₂ released would need to be very minimal in order to be completed consumed by Fe-bearing phases (Archer et al., 2024). Overall, the absence of evidence for perchlorates and chlorates in samples drilled in the clay-sulfate transition region was similar to what was observed in stratigraphically lower samples drilled in the GT trough (McAdam et al., 2022).

Evolved O₂ can also be derived from the thermal decomposition of Mn oxides, where Mn is in the 3+ or 4+ state (e.g., Mn⁴⁺O₂) (Clark et al., 2021). The detection of MnO in drill material by APXS (0.21–0.43 wt.%) (Berger, 2024; Berger et al., 2023) and the lack of evolved O₂ by SAM suggested that Mn²⁺O, which does not evolve O₂, was the likely Mn species present in these samples.

NO was undetectable (<0.001 wt.%) or minor in samples drilled within and surrounding the clay-sulfate transition region (Figure S3 and Table S4 in Supporting Information S1). The only sample with detectable NO was BD, which produced a broad NO release with a peak ∼700°C. The last potential detection of nitrate salts was in the GT trough and in the Rock Hall sample that was drilled on the topographic Vera Rubin ridge (McAdam et al., 2020, 2022).

3.4 HCl

All samples drilled from the upper Glasgow member of the CSf to the Contigo member of the MIf region evolved strikingly similar broad HCl releases with peaks at approximately 550–600°C (Figure 8). A relatively minor low-temperature (<200°C) HCl release was detected in the post-ZE blank (not visible in Figure 8), likely from residual HCl in the system from previous runs. This low-temperature HCl release was also observed in all drilled samples and was not included in HCl abundance calculations (Table S5 in Supporting Information S1). HCl releases were likely derived from chlorides (e.g., NaCl) and not oxychlorines due to the lack of evolved O₂. The broad HCl releases observed in all samples were attributed to the reaction between melting chlorides and evolved water (Equation 5), which has been demonstrated in laboratory SAM-analog experiments (Clark et al., 2020, 2024; McAdam et al., 2014; Robinson, 1872).

{\mathrm{H}}_{2}{\mathrm{O}}_{(\mathrm{g})}+{\text{NaCl}}_{(\mathrm{s},\mathrm{l})}\rightleftharpoons 2{\text{HCl}}_{(\mathrm{g})}+{\text{Na}}_{2}{\mathrm{O}}_{(\mathrm{s})}

(5)

SAM-EGA derived Cl abundances from drilled material and APXS Cl abundances of drill tailings or dumped samples did not systematically increase or decrease with elevation (Figure 8). Cl abundances measured by the APXS were higher than Cl abundances derived from SAM-EGA HCl peak integrations, which ranged from 0.11 wt.% to 0.65 wt.% (Table S5 in Supporting Information S1; Figure 8). This suggests that Cl-bearing phases in the samples did not volatilize or react to completion during heating. Alternatively, the material sampled by SAM and APXS may have varied in Cl abundances and Cl-bearing mineralogy. Halite was only detected by CheMin in PT (0.6 wt.% NaCl (Rampe et al., 2023; Rampe et al., 2024)). However, chlorides may have been present below the detection limit of CheMin (∼1–2 wt.% for most minerals) or were X-ray amorphous (Tables S6 and S7 in Supporting Information S1) (Simpson et al., 2024).

3.5 CO₂

Samples drilled from the upper Glasgow member (CSf) to the Contigo member (MIf) evolved between 96 and 1668 μg/g C, with most of the CO₂ evolved between 150 and 550°C (Figure 9; Table S8 in Supporting Information S1). Samples that evolved lower amounts of CO₂ may have contained carbon-bearing phases that were not readily volatilized or lower amounts of carbon-bearing phases in general (organic or inorganic). Some CO₂ profiles and peak temperatures were different between aliquots of the same drill sample (e.g., NT1 vs. NT2), suggesting heterogeneity in carbon phases between subsamples from the same drillhole as well as in the region as a whole.

Low-temperature (<250°C) CO₂ releases observed in all samples may have been caused by adsorbed CO₂ from the atmosphere. CO₂ comprises 95.32% of the atmosphere at the surface of Mars and may be present as an adsorbed layer on particle surfaces (Zent & Quinn, 1995). BD and AV evolved low-temperature CO₂ that is not readily visible in Figure 9 due to the scale of the mid-temperature CO₂ releases.

Mid-temperature CO₂ peaks (∼295–375°C) were observed in most samples, especially in NT2, PT, MG, and CA (Figure 9). These mid-temperature CO₂ releases were attributed to organic carbon oxidized by H₂O or SO₂ or simple organic salts such as oxalates (Lewis et al., 2021; Ming et al., 2014). Mid-temperature CO₂ releases that coevolved with CO (NT2, PT, MG1, and CA2 (Figure S4 in Supporting Information S1)) suggest these peaks may have resulted from the decomposition of simple organic salts (Lewis et al., 2021; Ming et al., 2014) or instrument background as opposed to carbonates that do not evolve CO.

NT1, BD, ZE, and AV exhibited relatively sharp mid-temperature CO₂ peaks (∼330°C–430°C) that did not overlap with CO (Figure S4 in Supporting Information S1) and were therefore most consistent with the presence of a carbonate such as siderite (FeCO₃) (Archer et al., 2020) though contributions from oxidized organics cannot be excluded. BD and AV evolved CO₂ releases with broad bases, suggesting that these releases had contributions from other carbon-bearing phases in addition to Fe carbonates. CheMin only detected ∼1 wt.% ankerite (Ca(Mg,Fe²⁺, Mn) (CO₃)₂) in NT and BD (Rampe et al., 2023, 2024), and siderite was previously detected by CheMin in stratigraphically lower samples in the GT region (Thorpe et al., 2022). The detection of Fe carbonate by SAM-EGA in NT1, BD, ZE, and AV is inconsistent with CheMin's findings and suggests carbonate heterogeneity within the drillhole. Additionally, SAM has lower detection limits for volatile carbon-bearing phases than CheMin and can detect both crystalline and amorphous volatile phases.

SAM-TLS data from the mid-temperature CO₂ release in BD showed that it was enriched in ¹³C relative to other carbon-bearing materials on Mars (Burtt et al., 2024). This was attributed to a combination of mechanisms including Rayleigh distillation and evaporitic effects, possibly related to cryogenic precipitation of carbonates (Burtt et al., 2024).

CO₂ releases with peaks ∼600–700°C were observed in most samples and were consistent with the presence of a Ca-bearing carbonate such as ankerite, although CheMin only detected ankerite in NT and BD (Rampe et al., 2024). Ankerite was previously suggested to be present in the Mary Anning 3 and Groken drill samples in the GT region based on distinct (but weak) peaks in CheMin XRD data (Thorpe et al., 2022).

The post-ZE blank evolved minor amounts of CO₂ (0.2 ± 0.1 µmols), where no significant peaks in the evolved gas data were present when compared to data from the drilled samples (Table S8 in Supporting Information S1; Figure 9). Evolved CO₂ in the post-ZE blank was caused by byproducts of derivatization or thermochemolysis agents (MTBSTFA and TMAH) (Glavin et al., 2013; Sutter et al., 2022).

3.6 CO

The majority of CO was evolved between 230 and 525°C in all samples and ranged from 0.25 to 1.5 μmol. Low (<230°C) and mid-temperature (230–525°C) corrected CO releases were generally extremely similar between samples (Figure 10; Figure S5 in Supporting Information S1) and thus may likely be derived from an instrumental source. Mid-temperature CO releases that coevolved with CO₂ (NT2, PT, MG1, and CA2 (Figure S4 in Supporting Information S1)) may have resulted from the decomposition of organic salts, such as Fe(II) oxalates (Lewis et al., 2021).

Several samples, most notably CA, exhibited CO peaks at approximately 800°C followed by an increase in CO counts above 850°C (Figure 10). Results from the post-ZE blank indicated a high-temperature CO source in the instrumental background (Figure 10). Calculated CO abundances therefore did not include contributions from high-temperature CO releases that were also observed in the blank (Table S9 in Supporting Information S1).

4 Discussion

4.1 Ground-Truthing Orbital Detections of Phyllosilicates and Sulfates With In-Situ Data

CRISM Vis-NIR reflectance spectra predicted that the phyllosilicate-bearing GT trough would contain in situ evidence of Fe-rich smectite and the overlying clay-sulfate transition region and Layered Sulfate-bearing unit would contain Mg sulfates (Anderson & Bell III, 2010; Fraeman et al., 2016; Milliken et al., 2010; Sheppard et al., 2022). As described above, CheMin-derived mineralogy confirms these predictions, and SAM-EGA H₂O data from the Glasgow and Mercou members of the CSf (NT and BD) are also consistent with the presence of nontronite. The mid-temperature H₂O release consistent with nontronite dehydroxylation generally decreased in magnitude with elevation (Figure 5), suggesting a decrease in phyllosilicate abundances with increasing stratigraphic position. CheMin data were consistent with the findings from SAM and showed that NT and BD contained 18 wt.% and 12 wt.% phyllosilicates, respectively (Rampe et al., 2023, 2024). Samples above BD generally did not evolve H₂O in a manner consistent with phyllosilicate dehydroxylation and the corresponding CheMin data did not contain clear evidence of phyllosilicates. The one exception to this trend was the AV drill sample, which exhibited a minor mid-temperature H₂O peak but for which CheMin did not detect phyllosilicates.

The fraction of high-temperature (>750°C) SO₂ detected by SAM-EGA generally increased with elevation (Figures 6 and 7), suggesting that Mg sulfates were present in the stratigraphically higher samples in the clay-sulfate transition region as well as in the NT2 sample at the base of Mont Mercou. This confirms orbital detections of Mg sulfates in the clay-sulfate transition region. CheMin only detected crystalline Mg sulfate in the stratigraphically highest sample, CA, meaning that Mg sulfates in NT2, PT, MG, ZE, and AV were present at abundances below CheMin's detection limit or were X-ray amorphous. This is consistent with the calculated amorphous compositions of the drilled samples, which indicate that all samples contained sulfates in the X-ray amorphous component (Simpson et al., 2024). The presence of Mg sulfates in those samples, in an amorphous state, is also consistent with interpretations based on elemental data of various diagenetic features in those stratigraphic sections (e.g., Meyer et al., 2024).

4.2 Implications for Depositional Environments and Subsequent Alteration

The hypothesis that the clay-sulfate transition region records a shift in conditions from those favoring clay formation to those favoring sulfate formation—generally wetter to drier—is supported by minimal or absent mid-temperature H₂O releases attributable to smectite (e.g., nontronite) above the BD drill sample. Relatively minor high-temperature and mid-temperature water releases, consistent with small amounts of smectite clay minerals, were observed in the AV sample data, suggesting that there may have been intermittent periods where conditions were favorable to clay formation. The intermittent occurrence of clays is also consistent with analysis of CRISM data by Sheppard et al. (2022) that showed patchy distributions of clay-bearing zones within the sulfate unit of Mt. Sharp as a whole.

However, the SAM-EGA data are consistent with the general rover observations that show the clay-sulfate transition region records a shift from aqueous (lacustrine/fluvial) to dry (eolian) surface conditions. Lithology and sedimentary structures of the upper CSf (NT, BD, PT) record a lacustrine/alluvial environment (Cardenas et al., 2022; Edgar et al., 2024), and SAM-EGA data confirm the presence of clays. In contrast, samples drilled in the overlying MIf (MG, ZE, AV, CA) represent areas with lithologic features consistent with a more arid eolian environment (e.g., sandstones with large-scale cross-beds), and SAM-EGA data confirm a paucity of clays and an increase in sulfate phases. All rocks in the Mt. Sharp Group have experienced diagenesis to some degree, and diagenetic products, including sulfate and Fe-oxides, are particularly prevalent in some members of the MIf. This further highlights the complex depositional and diagenetic changes that are recorded in the clay-sulfate transition region, as described below and illustrated in Figure 11. The proposed sequence of events to explain the sedimentological and compositional observations in the clay-sulfate transition is as follows:

Rocks in the upper CSf had a basaltic sediment source (Rampe et al., 2023, 2024). The likely basaltic source material was transported to and deposited within a lacustrine and/or marginal lake environment (Figure 11). It is possible that nontronite, indicated by the presence of mid-temperature water evolutions in SAM-EGA data, formed syn-depositionally or post-depositionally in an environment with a relatively high water to rock (W/R) ratio. Previous studies that focused on clay mineralogy in the GT region have suggested authigenic formation (Bristow et al., 2021; Thorpe et al., 2022), as did (Meyer et al., 2024) for upper CSf samples. Surface and near-surface availability of water decreased as the lake dried and the shoreline retreated, which may have limited phyllosilicate formation in the uppermost CSf, explaining the lack of phyllosilicates in the Pontours drill sample. Additionally, the presence of Fe³⁺-rich dioctahedral smectites in NT and BD and the lack of trioctahedral (e.g., Mg²⁺, saponitic) phyllosilicates indicate these materials may have formed in a more open geochemical alteration system (Mangold et al., 2019).
Subsequently, aqueous processes ceased and the clay-sulfate transition region transitioned to an eolian environment where basaltic material was deposited in dunes or sand sheets. Meyer et al. (2024) proposed that there is a regional unconformity at the lowermost part of the MIf based on orbital and in situ evidence, including changes in the abundance and composition of nodules. The proposed unconformity suggests that fluviolacustrine depositional environments, as recorded in the upper CSf, persisted for an unknown amount of time prior to non-deposition, erosion, and eolian deposition (Meyer et al., 2024).

The dry depositional environment recorded in the MIf was not conducive to the formation of phyllosilicates as indicated by minimal or absent mid-temperature H₂O releases associated with smectites (nontronite) in the SAM-EGA data (Figure 5). However, the mid-temperature and high-temperature H₂O releases in the AV sample data, consistent with the presence of smectites, are suggestive of intermittent wet periods or different diagenetic conditions (e.g., higher W/R ratio), as indicated by thinly bedded lenses that were consistent with the presence of shallow (<2 m) interdune water bodies detected between ZE and AV (Edgar et al., 2024).
Post-lithification alteration (diagenesis) of rocks in the clay-sulfate transition region is supported by the presence of abundant veins, nodules, and diagenetic overprinting that do not cross-cut or deform primary laminations (Meyer et al., 2024). Rocks were altered by briny solutions with varying chemical compositions, similar to the model proposed by Tosca and McLennan (2006). Fe-carbonates (e.g., siderite, amorphous Fe-carbonate), as determined by CO₂ evolutions in SAM-EGA data, formed heterogeneously throughout the strata in anoxic waters with sufficient dissolved inorganic carbon (Tosca et al., 2018; Tutolo et al., 2024), leaving a Fe and HCO₃-poor residual fluid. Subsequent precipitation of Ca sulfates (i.e., gypsum, bassanite) left a Ca-poor and SO₄-rich fluid, leading to conditions conducive to the precipitation of Mg sulfates (e.g., epsomite; MgSO₄·7H₂O). The post-depositional formation of halite in the clay-sulfate transition region is supported by Na-enriched veins/fractures, as detected by ChemCam (Meyer et al., 2024), and the high-temperature HCl evolutions observed in SAM-EGA data.

Fe carbonates may have decomposed into Fe oxyhydroxides heterogeneously throughout the region due to changes in diagenetic conditions and water chemistry (e.g., pH, redox state, pCO₂), which would have released CO₂ (Tutolo et al., 2024). Hematite (Fe₂O₃) was detected by CheMin in all samples drilled from the upper Glasgow member of the CSf to the Contigo member of the MIf, and goethite was detected by CheMin and SAM-EGA in several samples (Rampe et al., 2023, 2024). At least some of these Fe-oxides appear to be associated with late-stage diagenetic nodules (Meyer et al., 2024), indicating persistent subsurface Fe mobility. Fe sulfates, detected by SAM-EGA in all samples, may have formed from the interaction between acidic groundwater and Fe carbonates (Tutolo et al., 2024), but whether they are in early (e.g., cement) or late diagenetic components remains unclear.

(Meyer et al., 2024) proposed that clay minerals in the CSf may have created a hydraulic barrier for briny fluids. Additionally, the proposed unconformity may have acted as a fluid conduit for briny post-depositional fluids, leading to the formation of nodules and other diagenetic features, which were highly concentrated at the PT drill site (Meyer et al., 2024). This is supported by SAM-EGA data, which suggest the presence of clay minerals in the Glasgow and Mercou members of the CSf and increasing amounts of Mg sulfates in the MIf (Figures 5-7).
Rocks were eroded by wind, exposing strata in the CSf and the MIf.
Surface and near-surface rocks were exposed to low relative humidity (RH) air and solar heat, causing dehydration and transformation of certain minerals (e.g., partial dehydration and amorphization of Mg sulfates). Crystalline Mg sulfate (starkeyite) was detected by CheMin for the first time in the mission in the CA sample in the Marker Band Valley and its presence is supported by SAM-EGA SO₂ detections. Crystalline Mg sulfates may have formed at the surface or near sub-surface and subsequently dehydrated to lower hydration states (e.g., the starkeyite in CA) (Chipera et al., 2023). Crystalline Mg sulfate may have transformed into amorphous Mg sulfate, also detected in CA, through exposure to solar heat (Chipera et al., 2023).

Rocks in the uppermost clay-sulfate transition region (i.e., Marker Band Valley) marked a sharp change in sulfate mineralogy that was observed by SAM, CheMin, and APXS. The Mg sulfates detected by CheMin and SAM in the CA drill sample correlate with elevated MgO and SO₃ detections by APXS in the bedrock (Berger et al., 2023), suggesting it is part of the cementing agent. This is distinct from Mg sulfate in the lower portions of the MIf, which is primarily associated with various late stage (post-lithification) diagenetic nodules formed from deep circulating fluids (Meyer et al., 2024). In contrast, sediment deposited within the Marker Band Valley region, either syn- or post-lithification, appears to have been exposed to fluids rich in dissolved Mg and S that led to precipitation of higher amounts of Mg sulfates. The detection of crystalline and amorphous Mg sulfate in the bedrock within the Marker Band Valley coincides with the beginning of the LSu.

5 Conclusions

Seven samples were drilled from the top of the Glasgow member of the CSf through the top of the Contigo member of the MIf. Samples were analyzed with SAM-EGA to assess changes in volatile mineralogy, compare with chemical and mineralogical results from other rover instruments, ground-truth orbital mineral detections, and interpret past depositional environments and alteration conditions. SAM-EGA analyses were especially important for analyzing mineralogical changes in the clay-sulfate transition region because APXS data showed low variability in major and minor elements in bedrock from the Glasgow member (CSf) to the Contigo member (MIf), with the exception of CaO and SO₃ (Berger et al., 2023).

SAM-EGA results suggest that mid-temperature H₂O releases, indicative of nontronite, were more prominent in the stratigraphically lower samples (NT, BD) and were either absent or minimal in samples stratigraphically above BD. The decrease in mid-temperature H₂O above BD agree with CheMin data, which show that phyllosilicates were absent above BD. Evolved SO₂ releases are consistent with the presence of Fe sulfates in all samples in the clay-sulfate transition region, and some samples (NT2, MG, ZE, AV, and CA) evolved high-temperature SO₂ consistent with Mg sulfate decomposition. The prominent high-temperature SO₂ release in CA is consistent with the detection of starkeyite by CheMin and the detection of elevated MgO and SO₃ in the bedrock by APXS. APXS-derived SO₃ abundances exceed SAM-derived SO₃ abundances because Ca sulfate decomposes above the maximum oven temperature of SAM, making Ca sulfate undetectable in the SAM SO₂ data. SAM-EGA data suggest that nontronite was present in the upper Glasgow and Mercou members of the CSf and Mg sulfates were present in most samples.

No trends are observed in evolved CO₂, CO, NO, O₂, and HCl in samples drilled within and surrounding the clay-sulfate transition region. The presence of broad HCl releases and absence of O₂ suggests that chlorides, and not oxychlorines, were present in all samples. NO was minimal or absent in all samples, suggesting that nitrate was largely absent in samples drilled in this region. Evolved CO₂ was extremely heterogeneous throughout the region and between drill sample aliquots, and is attributed to a mixture of carbonates, simple organic salts, adsorbed CO₂, and oxidized organics (from the instrument background or indigenous to the sample). In comparison to CO₂, CO was more consistent between samples and is primarily attributed to instrument background although a contribution from simple organic salts, such as Fe(II) oxalate, was also possible.

SAM-EGA confirm the presence of several elements that are essential for life in the clay-sulfate transition region, including S (in SO₂), O (in SO₂, H₂O, and CO₂), H (in H₂O), and C (in CO and CO₂), although SAM data alone cannot be used to determine if life was possible in Gale crater. Additionally, evidence for aqueous depositional environments in the upper CSf, a necessary ingredient for life, is supported by the detection of clay minerals by SAM-EGA. Water-limited conditions that formed the MIf may have been less habitable for microbial life. Additionally, SAM-EGA indicate that nitrates were absent or sparse in the clay-sulfate transition region, another limiting factor for microbial life. Overall, SAM-EGA data provide evidence that necessary elements for life were available throughout the clay-sulfate transition although water likely became progressively less abundant in the geologically younger rocks.

Data sets generated by Curiosity's suite of analytical techniques, including SAM-EGA, suggest that material in the CSf and MIf formed in lacustrine and ancient eolian depositional environments, respectively. The lack of mid-temperature evolved water, indicative of nontronite, above the Mercou member of the CSf suggest that stratigraphically higher samples formed in relatively water-limited conditions (e.g., a dune environment). Rocks were altered by briny solutions with variable chemical composition, forming Fe carbonates, sulfates, and chlorides. Strata were eroded and Mg sulfates were dehydrated and altered by solar heat and low relative humidity. Overall, SAM-EGA data confirm orbital mineral detections of phyllosilicates and Mg sulfates and suggest that the strata in the clay-sulfate transition region formed in progressively drier conditions.

Acknowledgments

The authors are grateful to the engineers and scientists of the NASA MSL Curiosity team, who have made the mission possible and the reported data available. This study was funded by the NASA Mars Science Laboratory Project. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. The authors are grateful to the anonymous reviewers who provided detailed and insightful edits.

Open Research

Data Availability Statement

SAM and CheMin data are publicly available (MSL, 2024). Data presented in this paper are publicly available (Clark, 2024).

Supporting Information

References

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Volume129, Issue12

December 2024

e2024JE008587

Environmental Changes Recorded in Sedimentary Rocks in the Clay-Sulfate Transition Region in Gale Crater, Mars: Results From the Sample Analysis at Mars-Evolved Gas Analysis Instrument Onboard the Mars Science Laboratory Curiosity Rover

Abstract

Key Points

Plain Language Summary