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Understanding the genesis of Martian soils is important to constrain the hydrogeologic history of the planet. Soils have the potential to record paleoenvironmental conditions, through the nature of secondary minerals formed during weathering. In situ X‐ray diffraction analyses in Gale crater have revealed that about one third of each soil sample is composed of amorphous materials containing hydrated phases. Here, we use the geochemical data from the ChemCam instrument to investigate the nature and origin of the hydrated amorphous phases. We report for the first time with ChemCam clues for the presence of sulfates within the amorphous component of soils. We show that sulfates are the main carrier of the soil hydration and possibly explain the nature of hydrogen and sulfur measured from orbit. These sulfates and the apparent lack of significant Al‐bearing weathering products are consistent with a model of soil formation including weathering of olivine in water‐limited acidic conditions. Plain Language Summary: The study of Martian soils is of considerable interest as the nature of the mineral phases they contain, formed by the action of water for some of them, can give information on the past environments of the planet. Mineralogical analyses in Gale crater have shown that about one third of soils are composed of poorly crystalline materials whose nature remains unclear, and that soil hydration could be associated with these phases. Here, we use the chemical analyses from the ChemCam instrument to investigate the composition of the hydrogen‐bearing products, and we report for the first time the presence of sulfates in soils with this instrument. We demonstrate that sulfates are the main contributor to the water content of soils and are probably the source of the hydrogen and sulfur measured from orbit. The presence of sulfates and the lack of significant other secondary materials, especially enriched in aluminum, suggest that soils have probably undergone an acidic aqueous alteration with a low quantity of water, favoring the dissolution of olivine as the precursor to sulfates. Key Points: The amorphous component of Gale crater soils contains hydrated sulfatesThe Eolian dust deposits are not the carrier of the identified hydrated sulfatesWater‐limited acidic conditions may have led to the formation of these phases [ABSTRACT FROM AUTHOR]

Results from the Sample Analysis at Mars‐evolved gas analyzer on board the Mars Science Laboratory Curiosity rover constrained the alteration history and habitability potential of rocks sampled across the Siccar Point unconformity in Gale crater. The Glasgow member (Gm) mudstone just below the unconformity had evidence of acid sulfate or Si‐poor brine alteration of Fe‐smectite to Fe amorphous phases, leaching loss of Fe‐Mg‐sulfate and exchange of unfractionated sulfur 34S (δ34S = 2‰ ± 7‰) with enriched 34S (20‰ ± 5‰, Vienna Cañon Diablo Troilite). Carbon abundances did not significantly change (322–661 μgC/g) consistent with carbon stabilization by amorphous Al‐ and Fe‐hydroxide phases. The Gm mudstone had no detectable oxychlorine and extremely low nitrate. Nitrate (0.06 wt.% NO3), oxychlorine (0.13 wt.% ClO4), high C (1,472 μg C/g), and low Fe/Mg‐sulfate concentration (0.24 wt.% SO3) depleted in 34S (δ34S = −27‰ ± 7), were detected in the Stimson formation (Sf) eolian sandstone above the unconformity. Redox disequilibrium through the detections of iron sulfide and sulfate supported limited aqueous processes in the Sf sandstone. Si‐poor brines or acidic fluids altered the Gm mudstone just below the unconformity but did not alter underlying Gm mudstones further from the contact. Chemical differences between the Sf and Gm rocks suggested that fluid interaction was minimal between the Sf and Gm rocks. These results suggested that the Gm rocks were altered by subsurface fluids after the Sf placement. Aqueous processes along the unconformity could have provided habitable conditions and in some cases, C and N levels could have supported heterotrophic microbial populations. Plain Language Summary: The Curiosity Rover investigated the chemistry and mineralogy of rocks in the Glen Torridon region of Gale crater, Mars. Rocks sampled across an unconformity (representative of a time gap between when two sedimentary rocks were deposited) gave insight into how they were altered over time. The Glasgow mudstones, below the unconformity, were formed in a lake environment while the younger Stimson sandstones above the unconformity were formed from ancient sand dunes. Using the Sample Analysis at Mars Evolved Gas Analysis (SAM‐EGA) experiment, solid powdered samples from these rocks were heated and released gases (e.g., water, carbon dioxide, and sulfur dioxide) were tracked. These gases revealed a complex history of alteration through the rocks. The Glasgow mudstones were characterized by acidic or Si‐poor brine alteration, variable sulfur, low amounts of carbon, and no nitrate or oxychlorine. SAM‐EGA results from the Stimson sandstone showed rocks that were less altered, had higher levels of carbon, different sulfur compounds and sources, and some nitrate/oxychlorine. Together, these results suggested that the Glasgow rocks were altered by subsurface fluids after Stimson sandstone placement. Aqueous processes along the unconformity could have provided habitable conditions and in some cases, C and N levels could have supported heterotrophic microbial populations. Key Points: Subsurface silica‐poor brines or acidic fluids altered mudstones just below the Siccar Point group unconformityStimson formation sandstone above the unconformity was exposed to limited aqueous alterationAqueous alteration processes along the unconformity could have provided habitable conditions and in some cases, sufficient microbial C and N [ABSTRACT FROM AUTHOR]

The Curiosity rover in Gale crater is investigating a mineral transition observed from orbit—an older "clay unit" to a younger "sulfate unit"—hypothesized to reflect the aridification of Mars' climate. Below this transition, the rover detected crystalline Ca‐sulfates with minor Fe‐sulfates but also found that some fraction of a rock's bulk SO3 is often in the poorly constrained X‐ray amorphous component. Here, we characterize the abundances and compositions of the X‐ray amorphous sulfur‐bearing phases in 19 drilled samples using a mass balance approach, and in a subset of 5 samples using evolved SO2 gas measured using the SAM instrument. We find that ∼20–90 wt% of a sample's bulk SO3 is in the X‐ray amorphous state and that X‐ray amorphous sulfur‐bearing phase compositions are consistent with mixtures of Mg‐S, Fe‐S, and possibly Ca‐S phases, likely sulfates or sulfites. These phases reside in the bedrock, perhaps as cementing agents deposited with detrital sediments or during early diagenesis, and in diagenetic alteration halos deposited after lithification during late diagenesis. The likely presence of highly soluble Mg‐sulfates in the rocks suggests negligible fluid flow through the bedrock post‐Mg‐sulfate deposition. The X‐ray amorphous sulfur‐bearing phases probably became amorphous through dehydration in the current Martian atmosphere or inside the CheMin instrument. X‐ray amorphous sulfur‐bearing materials likely contribute to orbital spectral detections of sulfates, and so our results help form multiple hypotheses to be tested in the sulfate unit and are important for understanding the evolution of the Martian surface environment at Gale crater. Plain Language Summary: The Curiosity rover in Gale crater, Mars is investigating a mineral transition observed from orbit—older clay‐rich to younger sulfur‐rich rocks—that likely reflects a change from a wetter to a drier Martian climate. In many of the sedimentary rocks investigated below the mineral transition, rover instruments identified different sulfur‐bearing phase assemblages. Inconsistencies indicate the presence of poorly constrained sulfur‐bearing phases that are undetected by the CheMin X‐ray diffraction instrument (X‐ray amorphous). To fully appreciate the significance of the transition to more sulfate‐rich rocks, we investigate the abundance and composition of X‐ray amorphous sulfur‐bearing phases at 19 drill sites below the transition. We find that most rocks have large fractions of X‐ray amorphous sulfur‐bearing phases that are likely mixtures of Mg‐, Fe‐, and possibly Ca‐sulfates or sulfites. In some rocks, these sulfur‐bearing phases might have precipitated from solutions delivered to the lake early on, acting to cement sediment grains. In other locations, X‐ray amorphous sulfur‐bearing phases are associated with zones where fluids altered the rocks long after they were cemented. These results help form multiple hypotheses to be tested in the sulfate‐rich rock unit and are important for understanding the evolution of the Martian surface environment at Gale crater. Key Points: A large percentage of the bulk SO3 in Gale crater sedimentary rocks is in the X‐ray amorphous state (20%–90%)X‐ray amorphous S‐bearing phases are likely mixtures of Mg, Fe, Ca, and other cation sulfates present as cement and in diagenetic featuresIn situ detections of X‐ray amorphous SO3 likely contribute to orbital spectral detections of sulfates in lower Mount Sharp [ABSTRACT FROM AUTHOR]

The CheMin instrument on the Mars Science Laboratory rover Curiosity detected ubiquitous high abundances (∼15–70 wt%) of X‐ray amorphous components (AmCs) in ancient sedimentary rocks of Gale crater. Mechanisms and timing of formation for the AmCs are poorly constrained, and could include volcanic, impact, or aqueous processes. We explore trends in AmC composition and abundance, and look for systematic compositional variation between sites within Gale crater. AmC compositions were estimated indirectly based on bulk chemistry and the nature and abundance of the crystalline phases for 19 sedimentary rock samples. AmC abundances positively correlate with AmC SiO2 contents, and a mixing relationship appears to exist between SiO2‐rich and FeOT‐rich AmC endmembers. Endmember compositions are inconsistent with volcanic or impact glass alone, and so we conclude that the SiO2 and FeOT contents formed largely through aqueous processes. Cross‐cutting relationships and geologic context provide evidence that the most SiO2‐rich AmCs observed in Gale crater thus far may result from interactions with localized fluids during late diagenesis. AmCs with moderate to low SiO2 contents likely formed earlier (before or soon after sediment deposition). Thus, the AmC SiO2 and FeOT contents in Gale crater rocks represent mixtures of sedimentary materials formed over most of the sedimentary history of Gale crater, starting before the first sediments were deposited in the crater (late Noachian), and ending well after the youngest sediments were lithified (at least mid‐Hesperian). However, it remains unclear how these metastable minerals have persisted through billions of years of diagenesis in Gale crater sediments. Plain Language Summary: The CheMin instrument on the Mars Science Laboratory rover Curiosity detected ubiquitous high abundances (∼15–70 wt%) of amorphous materials in ancient sedimentary rocks of Gale crater. It is uncertain what geologic processes these materials represent (volcanic eruptions, impacts, or interactions with water), and when these materials were formed. Here we explore these possible formation mechanisms and their timing by estimating the chemical compositions of the amorphous materials, and looking for trends and variations in their compositions along the rover traverse. We find that amorphous iron‐ and silica‐rich materials formed mostly through interactions with water, and likely represent mixtures of chemical weathering products, chemical sediments, and cements that accumulated over time. We also find that some of the amorphous materials formed early on in the history of the crater, while others formed long after sediments were deposited and turned into rock, indicating that water persisted at Gale crater for ∼1 billion years. On Earth, it is thought that amorphous materials are readily converted into more crystalline materials in the presence of water, but it is unclear why this is not the case for Mars. Key Points: X‐ray amorphous abundances are positively correlated with SiO2 content, and amorphous SiO2 and FeOT contents are anticorrelatedX‐ray amorphous SiO2 and FeOT contents in Gale crater sedimentary rocks largely represent multiple aqueous processes occurring over timeIt is unclear how such metastable materials have persisted for billions of years [ABSTRACT FROM AUTHOR]

The ubiquitous eolian dust on Mars plays important roles in the current sedimentary and atmospheric processes of the planet. The ChemCam instrument retrieves a consistent eolian dust composition at the submillimeter scale from every first laser shot on Mars targets. Its composition presents significant differences with the Aeolis Palus soils and the Bagnold dunes as it contains lower CaO and higher SiO2. The dust FeO and TiO2 contents are also higher, probably associated with nanophase oxide components. The dust spectra show the presence of volatile elements (S and Cl), and the hydrogen content is similar to Bagnold sands but lower than Aeolis Palus soils. Consequently, the dust may be a contributor to the amorphous component of soils, but differences in composition indicate that the two materials are not equivalent. Plain Language Summary: Eolian dust on Mars is very fine dust that covers the entire surface of the planet, gives it its typical red hue, and is mobilized by wind. It plays a significant role in the current rock cycle of the planet and for the temperature of the atmosphere. ChemCam uses a series of pulsed laser shots to analyze the chemical composition of target materials. Each first laser shot by ChemCam gives the composition of the deposited dust. These measurements have been constant over the duration of the Mars Science Laboratory mission. The dust is homogeneous at the millimeter scale (approximately the size of the ChemCam analysis spot). Compared to local soils and sands at Gale crater, the dust contains higher levels of iron and titanium, associated with volatile elements like hydrogen, sulfur, and chlorine. We infer from this difference that the dust does not entirely originate locally and may be part of a separate global cycle. Key Points: The martian eolian dust chemical composition is homogeneous at the submillimeter scaleThe dust composition is different from the Aeolis Palus soils and Bagnold sands with a larger content of FeO and TiO2 and lower hydrationAlthough dust may be a contributor to the amorphous component of soils, its composition indicates that the two materials are not equivalent [ABSTRACT FROM AUTHOR]

The martian soil is of particular interest as it can help us understand the different processes that have occurred on Mars by studying the chemistry and mineralogy of its constituents as a function of grain size. The fine-grained martian soil is thought to be homogeneous across the planet and thus to represent a global component. In this study we report on the soil targets analysed by the SuperCam instrument aboard the Perseverance rover, which is currently exploring Jezero crater. A total of 343 targets were analysed. Their grain size distribution confirms the sparsity of 250–900 Â μ m particles in the martian soil, although both smaller and larger grains are present. We found that the local components, due to erosion of the local bedrock, are present not only in the very coarse grains or larger gravels of the soil, but also in the very fine ones (¡250 Â μ m). We detected some very coarse grains enriched in olivine, pyroxene and carbonate in both the crater floor and the delta front locations, whereas phyllosilicate-rich grains have been encountered only in the delta front. We have compared the Jezero fine-grained soil targets with those of Gale crater using ChemCam data. We found that those at Jezero show no evidence of Mg sulfates, in contrast to the observation at Gale. In addition, the fine-grained soil at Jezero is more hydrated than that at Gale, probably due to its higher specific surface area. • Composition of regolith targets at Jezero crater as a function of grain size and location. • Sparsity of 250–900 μ m particles in the martian regolith. • Fine-grained regolith (< 250 μ m) at Jezero is different from the one at Gale crater. • Jezero fine-grained soil regolith is more hydrated than that at Gale crater. [ABSTRACT FROM AUTHOR]

The Curiosity rover has been characterizing mineralogical and chemical compositions of Gale crater soils on Mars since 2012. Given its sub-millimeter scale of analysis, the ChemCam instrument is well suited to study the composition of soil constituents. However, the interpretation of LIBS data on soils in the martian environment is complicated by the large diversity of particle sizes (from dust to sand), combined with the unknown physical arrangement of their mineral constituents (i.e., the type of grain mixtures). For example, martian soils contain a significant amount of X-ray amorphous materials whose physical form remains unclear. In this study, we reproduced martian soil analyses in the laboratory to understand how the LIBS technique can provide specific insights into the physical and chemical properties of granular soils. For this purpose, different types of samples were studied with various ranges of grain sizes, mimicking two possible mixtures that may occur in martian soils: mechanical mixtures of two populations of grains made of distinct chemical compositions; and material forming a compositionally distinct coating at the surface of grains. Our results, also supported by in situ ChemCam data, demonstrate that both the sizes and the type of mixture of soil particles have a strong influence on the LIBS measurement. For mechanical mixtures of two populations of grains larger than 125–250 μm, the scatter of the data provides information about the chemical composition of the end-members. On the other hand, the chemistry recorded by LIBS for grains with surface coatings is fully dominated by the outer material for grains smaller than 500 μm in diameter. This is due to the small penetration depth of the laser (~0.3–1.5 μm per shot), combined with the ejection of small grains at each shot, which leads to a constant replenishment of fresh material. This experimental work will thus improve our understanding of martian soils analyzed by ChemCam, and more broadly, will benefit LIBS studies of granular materials. • Mechanical mixtures and grain coatings were analyzed using LIBS in a martian atmosphere. • Grain size and concentration were varied to study their influences on LIBS signal. • We show that mixed grains or coatings have different effects on LIBS analyses. • We provide constraints on the distribution of the amorphous component in martian soil. [ABSTRACT FROM AUTHOR]

Onboard NASA's Curiosity rover, the ChemCam LIBS instrument has provided a wealth of information on the chemistry of rocks within Gale crater. Here, we use ChemCam in order to search for carbonates among the >3500 individual targets analyzed by this instrument. Because the carbon-lines are a combination of signal from the CO 2 -rich atmosphere and possible carbon from the targets, we developed a laboratory-based univariate calibration obtained under Mars-like atmosphere. We measured different type of carbon-bearing samples (sediments, coals, carbonates) and their mixture with a basaltic powder. Based on this work, the preferred approach to qualitatively assess carbon under a CO 2 -rich atmosphere is to use a ratio to an oxygen line (777 nm) and the estimated limit of detection for carbon in a single LIBS point are found to be of 4.5 wt% and 6.9 wt% for reduced and organic carbon, respectively. Considering carbonate, this LOD correspond to about 50 wt% carbonate in the analyzed volume. Analysis of data obtained on Mars by ChemCam up to sol 3350 reveals the presence of a correlation between the intensity of carbon and oxygen lines, as observed in the laboratory, confirming that most carbon signal is related to ionization of the atmosphere. Some variability in the carbon signal appears related to the physical state of the atmosphere (density, temperature). Based on a combined analysis of carbon lines and major element compositions (Ca, Fe, Mg), there was no detection of carbonate in the ChemCam dataset up to sol 3355. Therefore, we conclude that carbonate was not present as a major constituent (>50%) in the ChemCam LIBS targets, and that soils are not enriched in carbon beyond the limit of detection. The dominant salts present are sulfate, chlorides, and the lack of carbonates in Gale, while observed in Jezero, may at least partly be related to a difference in protolith. • Carbon may be detected with LIBS on Mars using a combination of C and O lines • The LODs are found to be 4.5 and 6.9 wt% for reduced and organic C, respectively • No pure carbonates were found in the CCAM dataset up to sol 3350 [ABSTRACT FROM AUTHOR]