The origin and implications of clay minerals from Yellowknife Bay, Gale crater, Mars

Experimental Identification of Potential Martian Biosignatures in Open and Closed Systems

NASA's Perseverance and ESA's Rosalind Franklin rovers have the scientific goal of searching for evidence of ancient life on Mars. Geochemical biosignatures that form because of microbe-mineral interactions could play a key role in achieving this, as they can be preserved for millions of years on Earth, and the same could be true for Mars. Previous laboratory experiments have explored the formation of biosignatures under closed systems, but these do not represent the open systems that are found in natural martian environments, such as channels and lakes. In this study, we have conducted environmental simulation experiments using a global regolith simulant (OUCM-1), a thermochemically modelled groundwater, and an anaerobic microbial community to explore the formation of geochemical biosignatures within plausible open and closed systems on Mars. This initial investigation showed differences in the diversity of the microbial community developed after 28 days. In an open-system simulation (flow-through experiment), the acetogenic Acetobacterium (49% relative abundance) and the sulfate reducer Desulfosporomusa (43% relative abundance) were the dominant genera. Whereas in the batch experiment, the sulfate reducers Desulfovibrio, Desulfomicrobium, and Desulfuromonas (95% relative abundance in total) were dominant. We also found evidence of enhanced mineral dissolution within the flow-through experiment, but there was little evidence of secondary deposits in the presence of biota. In contrast, SiO2 and Fe deposits formed within the batch experiment with biota but not under abiotic conditions. The results from these initial experiments indicate that different geochemical biosignatures can be generated between open and closed systems, and therefore, biosignature formation in open systems warrants further investigation.

Ancient Siliciclastic-Evaporites as Seen by Remote Sensing Instrumentation with Implications for the Rover-Scale Exploration of Sedimentary Environments on Mars

Accurate interpretation of the martian sedimentary rock record-and by extension that planet's paleoenvironmental history and potential habitability-relies heavily on rover-based acquisition of textural and compositional data and researchers to properly interpret those data. However, the degree to which this type of remotely sensed information can be unambiguously resolved and accurately linked to geological processes in ancient sedimentary systems warrants further study. In this study, we characterize Mars-relevant siliciclastic-evaporite samples by traditional laboratory-based geological methods (thin section petrography, X-ray diffraction [XRD], backscattered electron imaging, microprobe chemical analyses) and remote sensing methods relevant to martian rover payloads (visible-near-mid infrared reflectance spectroscopy, X-ray fluorescence mapping, XRD). We assess each method's ability to resolve primary and secondary sedimentologic features necessary for the accurate interpretation of paleoenvironmental processes. While the most dominant textures and associated compositions (i.e., bedded gypsum evaporite) of the sample suite are readily identified by a combination of remote sensing techniques, equally important, although more subtle, components (i.e., interbedded windblown silt, meniscus cements) are not resolved unambiguously in bulk samples. However, rover-based techniques capable of coordinating spatially resolved compositional measurements with textural imaging reveal important features not readily detected using traditional assessments (i.e., subtle clay-organic associations, microscale diagenetic nodules). Our findings demonstrate the improved generational capacity of rovers to explore ancient sedimentary environments on Mars while also highlighting the complexities in extracting comprehensive paleoenvironmental information when limited to currently available rover-based techniques. Complete and accurate interpretation of ancient martian sedimentary environments, and by extension the habitability of those environments, likely requires sample return or in situ human exploration. Plain Language Summary Only when correctly translated can the ancient martian sedimentary rock record reveal the environmental evolution of the planet's surface through time. In this case study, we characterize Mars-relevant sedimentary rocks and evaluate the degree to which a comprehensive geological picture can be resolved unambiguously when limited to microscale remote sensing methods relevant to rovers on Mars. While the most dominant textural features and associated compositions of the sample suite are readily identified by a combination of remote sensing techniques, equally important but more subtle components are not resolved unambiguously in bulk samples. However, rover-based techniques capable of coordinating spatially resolved compositional measurements with textural imaging, such as Perseverance Rover's Planetary Instrument for X-Ray Lithochemistry instrument, reveal important features not readily detected by more traditional methods. We demonstrate that rovers have, generationally, improved in their capacity to resolve a true geological picture in ancient sedimentary environments, likely owing to an improved ability to coordinate spatially resolved compositional measurements with textural imaging at the microscale. However, our work also highlights the complexities involved in extracting subtle environmental information when limited to currently available rover-based techniques and suggests that comprehensive interpretation of ancient martian sedimentary systems likely requires sample return or in situ human exploration. Key Points Mars-relevant samples are characterized using both traditional laboratory and microscale rover-based remote sensing techniques to assess each method's ability to recognize features necessary for accurate paleoenvironmental process interpretation. While some key paleoenvironmental processes can reasonably be inferred via remote sensing methods, others cannot be resolved unambiguously. Perseverance Rover's Planetary Instrument for X-Ray Lithochemistry instrument reveals diagenetic features that would otherwise remain unseen by traditional thin section petrography.

Habitability and Biosignature Formation in Simulated Martian Aqueous Environments

Water present on early Mars is often assumed to have been habitable. In this study, experiments were performed to investigate the habitability of well-defined putative martian fluids and to identify the accompanying potential formation of biosignatures. Simulated martian environments were developed by combining martian fluid and regolith simulants based on the chemistry of the Rocknest sand shadow at Gale Crater. The simulated chemical environment was inoculated with terrestrial anoxic sediment from the Pyefleet mudflats (United Kingdom). These enrichments were cultured for 28 days and subsequently subcultured seven times to ensure that the microbial community was solely grown on the defined, simulated chemistry. The impact of the simulated chemistries on the microbial community was assessed by cell counts and sequencing of 16S rRNA gene profiles. Associated changes to the fluid and precipitate chemistries were established by using ICP-OES, IC, FTIR, and NIR. The fluids were confirmed as habitable, with the enriched microbial community showing a reduction in abundance and diversity over multiple subcultures relating to the selection of specific metabolic groups. The final community comprised sulfate-reducing, acetogenic, and other anaerobic and fermentative bacteria. Geochemical characterization and modeling of the simulant and fluid chemistries identified clear differences between the biotic and abiotic experiments. These differences included the elimination of sulfur owing to the presence of sulfate-reducing bacteria and more general changes in pH associated with actively respiring cells that impacted the mineral assemblages formed. This study confirmed that a system simulating the fluid chemistry of Gale Crater could support a microbial community and that variation in chemistries under biotic and abiotic conditions can be used to inform future life-detection missions.

The Curiosity Rover's Exploration of Glen Torridon, Gale Crater, Mars: An Overview of the Campaign and Scientific Results

The Mars Science Laboratory rover, Curiosity, explored the clay mineral-bearing Glen Torridon region for 1 Martian year between January 2019 and January 2021, including a short campaign onto the Greenheugh pediment. The Glen Torridon campaign sought to characterize the geology of the area, seek evidence of habitable environments, and document the onset of a potentially global climatic transition during the Hesperian era. Curiosity roved 5 km in total throughout Glen Torridon, from the Vera Rubin ridge to the northern margin of the Greenheugh pediment. Curiosity acquired samples from 11 drill holes during this campaign and conducted the first Martian thermochemolytic-based organics detection experiment with the Sample Analysis at Mars instrument suite. The lowest elevations within Glen Torridon represent a continuation of lacustrine Murray formation deposits, but overlying widespread cross bedded sandstones indicate an interval of more energetic fluvial environments and prompted the definition of a new stratigraphic formation in the Mount Sharp group called the Carolyn Shoemaker formation. Glen Torridon hosts abundant phyllosilicates yet remains compositionally and mineralogically comparable to the rest of the Mount Sharp group. Glen Torridon samples have a great diversity and abundance of sulfur-bearing organic molecules, which are consistent with the presence of ancient refractory organic matter. The Glen Torridon region experienced heterogeneous diagenesis, with the most striking alteration occurring just below the Siccar Point unconformity at the Greenheugh pediment. Results from the pediment campaign show that the capping sandstone formed within the Stimson Hesperian aeolian sand sea that experienced seasonal variations in wind direction.

Organic carbon concentrations in 3.5-billion-year-old lacustrine mudstones of Mars

This work presents the first quantification of bulk organic carbon in Mars surface sedimentary rocks, enabled by a stepped combustion experiment performed by the Curiosity Rover in Gale crater, Mars. The mudstone sample analyzed by Curiosity represents a previously habitable lacustrine environment and a depositional environment favorable for preservation of organics formed in situ and/or transported from a wide catchment area. Here we present the abundance of bulk organic carbon in these mudstone samples and discuss the contributions from various carbon reservoirs on Mars.

The Sample Analysis at Mars instrument stepped combustion experiment on a Yellowknife Bay mudstone at Gale crater, Mars revealed the presence of organic carbon of Martian and meteoritic origins. The combustion experiment was designed to access refractory organic carbon in Mars surface sediments by heating samples in the presence of oxygen to combust carbon to CO₂. Four steps were performed, two at low temperatures (less than ∼550 °C) and two at high temperatures (up to ∼870 °C). More than 950 μg C/g was released at low temperatures (with an isotopic composition of δ¹³C = +1.5 ± 3.8‰) representing a minimum of 431 μg C/g indigenous organic and inorganic Martian carbon components. Above 550 °C, 273 ± 30 μg C/g was evolved as CO₂ and CO (with estimated δ¹³C = −32.9‰ to −10.1‰ for organic carbon). The source of high temperature organic carbon cannot be definitively confirmed by isotopic composition, which is consistent with macromolecular organic carbon of igneous origin, meteoritic infall, or diagenetically altered biomass, or a combination of these. If from allochthonous deposition, organic carbon could have supported both prebiotic organic chemistry and heterotrophic metabolism at Gale crater, Mars, at ∼3.5 Ga.

Mineral Matrix Effects on Pyrolysis Products of Kerogens Infer Difficulties in Determining Biological Provenance of Macromolecular Organic Matter at Mars

Ancient martian organic matter is likely to take the form of kerogen-like recalcitrant macromolecular organic matter (MOM), existing in close association with reactive mineral surfaces, especially iron oxides. Detecting and identifying a biological origin for martian MOM will therefore be of utmost importance for life-detection efforts at Mars. We show that Type I and Type IV kerogens provide effective analogues for putative martian MOM of biological and abiological (meteoric) provenances, respectively. We analyze the pyrolytic breakdown products when these kerogens are mixed with mineral matrices highly relevant for the search for life on Mars. We demonstrate that, using traditional thermal techniques as generally used by the Sample Analysis at Mars and Mars Organic Molecule Analyser instruments, even the breakdown products of highly recalcitrant MOM are transformed during analysis in the presence of reactive mineral surfaces, particularly iron. Analytical transformation reduces the diagnostic ability of this technique, as detected transformation products of both biological and abiological MOM may be identical (low molecular weight gas phases and benzene) and indistinguishable. The severity of transformational effects increased through calcite < kaolinite < hematite < nontronite < magnetite < goethite. Due to their representation of various habitable aqueous environments and the preservation potential of organic matter by iron, it is not advisable to completely avoid iron-rich strata. We conclude that hematite-rich localities, with evidence of extensive aqueous alteration of originally reducing phases, such as the Vera Rubin Ridge, may be relatively promising targets for identifying martian biologically sourced MOM.

Astrobiological Potential of Fe/Mg Smectites with Special Emphasis on Jezero Crater, Mars 2020 Landing Site

Life is known to adapt in accordance with its surrounding environment and sustainable resources available to it. Since harsh conditions would have precluded any possible aerobic evolution of life at the martian surface, it is plausible that martian life, should it exist, would have evolved in such a way as to derive energy from more optimum resources. Iron is one of the most abundant elements present in the martian crust and occurs at about twice the amount present on Earth. Clay minerals contribute to about half the iron found in soils and sediments. On Earth, clay acts as an electron donor as well as an acceptor in the carbon cycles and thereby supports a wide variety of metabolic reactions. In this context, we consider the potential of Fe/Mg smectites, one of the most widely reported hydrated minerals on Mars, for preservation of macro- and microscopic biosignatures. We proceed by understanding the environmental conditions during the formation of smectites and various microbes and metabolic processes associated with them as indicated in Earth-based studies. We also explore the possibility of biosignatures and their identification within the Mars 2020 landing site (Jezero Crater) by using the astrobiological payloads on board the Perseverance rover.

Mission Overview and Scientific Contributions from the Mars Science Laboratory Curiosity Rover After Eight Years of Surface Operations

NASA's Mars Science Laboratory mission, with its Curiosity rover, has been exploring Gale crater (5.4° S, 137.8° E) since 2012 with the goal of assessing the potential of Mars to support life. The mission has compiled compelling evidence that the crater basin accumulated sediment transported by marginal rivers into lakes that likely persisted for millions of years approximately 3.6 Ga ago in the early Hesperian. Geochemical and mineralogical assessments indicate that environmental conditions within this timeframe would have been suitable for sustaining life, if it ever were present. Fluids simultaneously circulated in the subsurface and likely existed through the dry phases of lake bed exposure and aeolian deposition, conceivably creating a continuously habitable subsurface environment that persisted to less than 3 Ga in the early Amazonian. A diversity of organic molecules has been preserved, though degraded, with evidence for more complex precursors. Solid samples show highly variable isotopic abundances of sulfur, chlorine, and carbon. In situ studies of modern wind-driven sediment transport and multiple large and active aeolian deposits have led to advances in understanding bedform development and the initiation of saltation. Investigation of the modern atmosphere and environment has improved constraints on the timing and magnitude of atmospheric loss, revealed the presence of methane and the crater's influence on local meteorology, and provided measurements of high-energy radiation at Mars' surface in preparation for future crewed missions. Rover systems and science instruments remain capable of addressing all key scientific objectives. Emphases on advance planning, flexibility, operations support work, and team culture have allowed the mission team to maintain a high level of productivity in spite of declining rover power and funding.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11214-022-00882-7.

Clays and the Origin of Life: The Experiments

There are three groups of scientists dominating the search for the origin of life: the organic chemists (the Soup), the molecular biologists (RNA world), and the inorganic chemists (metabolism and transient-state metal ions), all of which have experimental adjuncts. It is time for Clays and the Origin of Life to have its experimental adjunct. The clay data coming from Mars and carbonaceous chondrites have necessitated a review of the role that clays played in the origin of life on Earth. The data from Mars have suggested that Fe-clays such as nontronite, ferrous saponites, and several other clays were formed on early Mars when it had sufficient water. This raised the question of the possible role that these clays may have played in the origin of life on Mars. This has put clays front and center in the studies on the origin of life not only on Mars but also here on Earth. One of the major questions is: What was the catalytic role of Fe-clays in the origin and development of metabolism here on Earth? First, there is the recent finding of a chiral amino acid (isovaline) that formed on the surface of a clay mineral on several carbonaceous chondrites. This points to the formation of amino acids on the surface of clay minerals on carbonaceous chondrites from simpler molecules, e.g., CO₂, NH₃, and HCN. Additionally, there is the catalytic role of small organic molecules, such as dicarboxylic acids and amino acids found on carbonaceous chondrites, in the formation of Fe-clays themselves. Amino acids and nucleotides adsorb on clay surfaces on Earth and subsequently polymerize. All of these observations and more must be subjected to strict experimental analysis. This review provides an overview of what has happened and is now happening in the experimental clay world related to the origin of life. The emphasis is on smectite-group clay minerals, such as montmorillonite and nontronite.

Merging Perspectives on Secondary Minerals on Mars: A Review of Ancient Water-Rock Interactions in Gale Crater Inferred from Orbital and In-Situ Observations

Phyllosilicates, sulfates, and Fe oxides are the most prevalent secondary minerals detected on Mars from orbit and the surface, including in the Mars Science Laboratory Curiosity rover’s field site at Gale crater. These records of aqueous activity have been investigated in detail in Gale crater, where Curiosity’s X-ray diffractometer allows for direct observation and detailed characterization of mineral structure and abundance. This capability provides critical ground truthing to better understand how to interpret Martian mineralogy inferred from orbital datasets. Curiosity is about to leave behind phyllosilicate-rich strata for more sulfate-rich terrains, while the Mars 2020 Perseverance rover is in its early exploration of ancient sedimentary strata in Jezero crater. It is thus an appropriate time to review Gale crater’s mineral distribution from multiple perspectives, utilizing the range of chemical, mineralogical, and spectral measurements provided by orbital and in situ observations. This review compares orbital predictions of composition in Gale crater with higher fidelity (but more spatially restricted) in situ measurements by Curiosity, and we synthesize how this information contributes to our understanding of water-rock interaction in Gale crater. In the context of combining these disparate spatial scales, we also discuss implications for the larger understanding of martian surface evolution and the need for a wide range of data types and scales to properly reconstruct ancient geologic processes using remote methods.

A Review of the Phyllosilicates in Gale Crater as Detected by the CheMin Instrument on the Mars Science Laboratory, Curiosity Rover

Curiosity, the Mars Science Laboratory (MSL) rover, landed on Mars in August 2012 to investigate the ~3.5-billion-year-old (Ga) fluvio-lacustrine sedimentary deposits of Aeolis Mons (informally known as Mount Sharp) and the surrounding plains (Aeolis Palus) in Gale crater. After nearly nine years, Curiosity has traversed over 25 km, and the Chemistry and Mineralogy (CheMin) X-ray diffraction instrument on-board Curiosity has analyzed 30 drilled rock and three scooped soil samples to date. The principal strategic goal of the mission is to assess the habitability of Mars in its ancient past. Phyllosilicates are common in ancient Martian terrains dating to ~3.5–4 Ga and were detected from orbit in some of the lower strata of Mount Sharp. Phyllosilicates on Earth are important for harboring and preserving organics. On Mars, phyllosilicates are significant for exploration as they are hypothesized to be a marker for potential habitable environments. CheMin data demonstrate that ancient fluvio-lacustrine rocks in Gale crater contain up to ~35 wt. % phyllosilicates. Phyllosilicates are key indicators of past fluid–rock interactions, and variation in the structure and composition of phyllosilicates in Gale crater suggest changes in past aqueous environments that may have been habitable to microbial life with a variety of possible energy sources.

Brine-driven destruction of clay minerals in Gale crater, Mars

Mars' sedimentary rock record preserves information on geological (and potential astrobiological) processes that occurred on the planet billions of years ago. The Curiosity rover is exploring the lower reaches of Mount Sharp, in Gale crater on Mars. A traverse from Vera Rubin ridge to Glen Torridon has allowed Curiosity to examine a lateral transect of rock strata laid down in a martian lake ~3.5 billion years ago. We report spatial differences in the mineralogy of time-equivalent sedimentary rocks <400 meters apart. These differences indicate localized infiltration of silica-poor brines, generated during deposition of overlying magnesium sulfate-bearing strata. We propose that destabilization of silicate minerals driven by silica-poor brines (rarely observed on Earth) was widespread on ancient Mars, because sulfate deposits are globally distributed.

Quantifying Preservation Potential: Lipid Degradation in a Mars-Analog Circumneutral Iron Deposit

Comparisons between the preservation potential of Mars-analog environments have historically been qualitative rather than quantitative. Recently, however, laboratory-based artificial maturation combined with kinetic modeling techniques have emerged as a potential means by which the preservation potential of solvent-soluble organic matter can be quantified in various Mars-analog environments. These methods consider how elevated temperatures, pressures, and organic-inorganic interactions influence the degradation of organic biomarkers post-burial. We used these techniques to investigate the preservation potential of deposits from a circumneutral iron-rich groundwater system. These deposits are composed of ferrihydrite (Fe₅HO₈ · 4H₂O), an amorphous iron hydroxide mineral that is a common constituent of rocks found in ancient lacustrine environments on Mars, such as those observed in Gale Crater. Both natural and synthetic ferrihydrite samples were subjected to hydrous pyrolysis to observe the effects of long-term burial on the mineralogy and organic content of the samples. Our experiments revealed that organic-inorganic interactions in the samples are dominated by the transformation of iron minerals. As amorphous ferrihydrite transforms into more crystalline species, the decrease in surface area results in the desorption of organic matter, potentially rendering them more susceptible to degradation. We also find that circumneutral iron-rich deposits provide unfavorable conditions for the preservation of solvent-soluble organic matter. Quantitative comparisons between preservation potentials as calculated when using kinetic parameters show that circumneutral iron-rich deposits are ∼25 times less likely to preserve solvent-soluble organic matter compared with acidic, iron-rich environments. Our results suggest that circumneutral iron-rich deposits should be deprioritized in favor of acidic iron- and sulfur-rich deposits when searching for evidence of life with solvent extraction techniques.

Thermodynamic Constraints on Smectite and Iron Oxide Formation at Gale Crater, Mars: Insights into Potential Free Energy from Aerobic Fe Oxidation in Lake Water–Groundwater Mixing Zone

The presence of saponite and iron oxides in Sheepbed mudstone of Yellowknife Bay at Gale crater on Mars is considered as evidence of a habitable fluvio-lacustrine environment for chemolithoautotrophy. However, the energetic availability for metabolic reactions is poorly constrained. Herein, we propose the possible mixing of surface water and groundwater that (i) explains the formation of magnetite and hematite detected in Sheepbed mudstone and (ii) may work as a potential habitable zone for aerobic Fe2+-oxidizing microbes. Our thermodynamic modeling of water–rock reactions revealed that the formation of abundant saponite in Sheepbed mudstone may occur under various conditions of water-to-rock mass ratios, temperatures (5–200 °C), and initial fluid compositions. In contrast, the formation of iron oxides in the mudstone can be explained only by the mixing of Fe2+-rich groundwater and more oxidized surface waters, where the Fe2+-rich groundwater can be generated by the low-temperature water–rock reactions with a CO2-bearing initial fluid. The calculated bioavailable energy of aerobic Fe2+ oxidation in the fluid-mixing zone on Mars is similar to that estimated for a fluid-mixing zone on Earth actually inhabited by aerobic Fe2+-oxidizing microbes. The findings will contribute to a better understanding of potential habitability on Mars.

Source-to-Sink Terrestrial Analogs for the Paleoenvironment of Gale Crater, Mars

In the Late Noachian to Early Hesperian period, rivers transported detritus from igneous source terrains to a downstream lake within Gale crater, creating a stratified stack of fluviolacustrine rocks that is currently exposed along the slopes of Mount Sharp. Controversy exists regarding the paleoclimate that supported overland flow of liquid water at Gale crater, in large part because little is known about how chemical and mineralogical paleoclimate indicators from mafic-rock dominated source-to-sink systems are translated into the rock record. Here, we compile data from basaltic terrains with varying climates on Earth in order to provide a reference frame for the conditions that may have prevailed during the formation of the sedimentary strata in Gale crater, particularly focusing on the Sheepbed and Pahrump Hills members. We calculate the chemical index of alteration for weathering profiles and fluvial sediments to better constrain the relationship between climate and chemical weathering in mafic terrains, a method that best estimates the cooler limit of climate conditions averaged over time. We also compare X-ray diffraction patterns and mineral abundances from fluvial sediments in varying terrestrial climates and martian mudstones to better understand the influence of climate on secondary mineral assemblages in basaltic terrains. We show that the geochemistry and mineralogy of most of the fine-grained sedimentary rocks in Gale crater display first-order similarities with sediments generated in climates that resemble those of present-day Iceland, while other parts of the stratigraphy indicate even colder baseline climate conditions. None of the lithologies examined at Gale crater resemble fluvial sediments or weathering profiles from warm (temperate to tropical) terrestrial climates.

Constraining the preservation of organic compounds in Mars analog nontronites after exposure to acid and alkaline fluids

The presence of organic matter in lacustrine mudstone sediments at Gale crater was revealed by the Mars Science Laboratory Curiosity rover, which also identified smectite clay minerals. Analogue experiments on phyllosilicates formed under low temperature aqueous conditons have illustrated that these are excellent reservoirs to host organic compounds against the harsh surface conditions of Mars. Here, we evaluate whether the capacity of smectites to preserve organic compounds can be influenced by a short exposure to different diagenetic fluids. We analyzed the stability of glycine embedded within nontronite samples previously exposed to either acidic or alkaline fluids (hereafter referred to as “treated nontronites”) under Mars-like surface conditions. Analyses performed using multiple techniques showed higher photodegradation of glycine in the acid-treated nontronite, triggered by decarboxylation and deamination processes. In constrast, our experiments showed that glycine molecules were preferably incorporated by ion exchange in the interlayer region of the alkali-treated nontronite, conferring them a better protection against the external conditions. Our results demonstrate that smectite previously exposed to fluids with different pH values influences how glycine is adsorbed into their interlayer regions, affecting their potential for preservation of organic compounds under contemporary Mars surface conditions.

Widespread hematite at high latitudes of the Moon

Hematite (Fe₂O₃) is a common oxidization product on Earth, Mars, and some asteroids. Although oxidizing processes have been speculated to operate on the lunar surface and form ferric iron-bearing minerals, unambiguous detections of ferric minerals forming under highly reducing conditions on the Moon have remained elusive. Our analyses of the Moon Mineralogy Mapper data show that hematite, a ferric mineral, is present at high latitudes on the Moon, mostly associated with east- and equator-facing sides of topographic highs, and is more prevalent on the nearside than the farside. Oxygen delivered from Earth's upper atmosphere could be the major oxidant that forms lunar hematite. Hematite at craters of different ages may have preserved the oxygen isotopes of Earth's atmosphere in the past billions of years. Future oxygen isotope measurements can test our hypothesis and may help reveal the evolution of Earth's atmosphere.

Evidence for Multiple Diagenetic Episodes in Ancient Fluvial-Lacustrine Sedimentary Rocks in Gale Crater, Mars

The Curiosity rover's exploration of rocks and soils in Gale crater has provided diverse geochemical and mineralogical data sets, underscoring the complex geological history of the region. We report the crystalline, clay mineral, and amorphous phase distributions of four Gale crater rocks from an 80-m stratigraphic interval. The mineralogy of the four samples is strongly influenced by aqueous alteration processes, including variations in water chemistries, redox, pH, and temperature. Localized hydrothermal events are evidenced by gray hematite and maturation of amorphous SiO2 to opal-CT. Low-temperature diagenetic events are associated with fluctuating lake levels, evaporative events, and groundwater infiltration. Among all mudstones analyzed in Gale crater, the diversity in diagenetic processes is primarily captured by the mineralogy and X-ray amorphous chemistry of the drilled rocks. Variations indicate a transition from magnetite to hematite and an increase in matrix-associated sulfates suggesting intensifying influence from oxic, diagenetic fluids upsection. Furthermore, diagenetic fluid pathways are shown to be strongly affected by unconformities and sedimentary transitions, as evidenced by the intensity of alteration inferred from the mineralogy of sediments sampled adjacent to stratigraphic contacts.

First Detections of Dichlorobenzene Isomers and Trichloromethylpropane from Organic Matter Indigenous to Mars Mudstone in Gale Crater, Mars: Results from the Sample Analysis at Mars Instrument Onboard the Curiosity Rover

Chromatographic analysis of the Cumberland mudstone in Gale crater by the Sample Analysis at Mars (SAM) instrument revealed the detection of two to three isomers of dichlorobenzene. Their individual concentrations were estimated to be in the 0.5-17 ppbw range relative to the sample mass. We also report the first detection of trichloromethylpropane and the confirmation of the detection of chlorobenzene previously reported. Supporting laboratory experiments excluded the SAM internal background as the source of those compounds, thus confirming the organic carbon and chlorine of the newly detected chlorohydrocarbons are indigenous to the mudstone sample. Laboratory experiments also demonstrated that the chlorohydrocarbons were mainly produced from chemical reactions occurring in the SAM ovens between organic molecules and oxychlorines contained in the sample. The results we obtained show that meteoritic organics and tested chemical species (a polycyclic aromatic hydrocarbon, an amino acid, and a carboxylic acid) were plausible organic precursors of the chlorinated aromatic molecules detected with SAM, thus suggesting that they could be among the organic molecules present in the mudstone. Results from this study coupled with previously reported detections of chlorinated aromatics (<300 ppbw) indigenous to the same mudstone highlight that organics can be preserved from the harsh surface conditions even at shallow depth. The detection of new chlorohydrocarbons with SAM confirms that organic molecules should have been available in an environment favorable to life forms, strengthening the habitability aspect of Gale crater.

Organic Records of Early Life on Mars: The Role of Iron, Burial, and Kinetics on Preservation

Samples that are likely to contain evidence of past life on Mars must have been deposited when and where environments exhibited habitable conditions. Mars analog sites provide the opportunity to study how life could have exploited such habitable conditions. Acidic iron- and sulfur-rich streams are good geochemical analogues for the late Noachian and early Hesperian, periods of martian history where habitable conditions were widespread. Past life on Mars would have left behind fossilized microbial organic remains. These are often-sought diagnostic evidence, but they must be shielded from the harsh radiation flux at the martian surface and its deleterious effect on organic matter. One mechanism that promotes such preservation is burial, which raises questions about how organic biomarkers are influenced by the postburial effects of diagenesis. We investigated the kinetics of organic degradation in the subsurface of Mars. Natural mixtures of acidic iron- and sulfur-rich stream sediments and their associated microbial populations and remains were subjected to hydrous pyrolysis, which simulated the increased temperatures and pressures of burial alongside any promoted organic/mineral interactions. Calculations were made to extrapolate the observed changes over martian history. Our experiments indicate that low carbon contents, high water-to-rock ratios, and the presence of iron-rich minerals combine to provide unfavorable conditions for the preservation of soluble organic matter over the billions of years necessary to produce present-day organic records of late Noachian and early Hesperian life on Mars. Successful sample selection strategies must therefore consider the pre-, syn-, and postburial histories of sedimentary records on Mars and the balance between the production of biomass and the long-term preservation of organic biomarkers over geological time.

Semiarid climate and hyposaline lake on early Mars inferred from reconstructed water chemistry at Gale

Salinity, pH, and redox states are fundamental properties that characterize natural waters. These properties of surface waters on early Mars reflect palaeoenvironments, and thus provide clues on the palaeoclimate and habitability. Here we constrain these properties of pore water within lacustrine sediments of Gale Crater, Mars, using smectite interlayer compositions. Regardless of formation conditions of smectite, the pore water that last interacted with the sediments was of Na-Cl type with mild salinity (~0.1-0.5 mol/kg) and circumneutral pH. To interpret this, multiple scenarios for post-depositional alterations are considered. The estimated Na-Cl concentrations would reflect hyposaline, early lakes developed in 104-106-year-long semiarid climates. Assuming that post-depositional sulfate-rich fluids interacted with the sediments, the redox disequilibria in secondary minerals suggest infiltration of oxidizing fluids into reducing sediments. Assuming no interactions, the redox disequilibria could have been generated by interactions of upwelling groundwater with oxidized sediments in early post-depositional stages.

Paleo-Rock-Hosted Life on Earth and the Search on Mars: A Review and Strategy for Exploration

Here we review published studies on the abundance and diversity of terrestrial rock-hosted life, the environments it inhabits, the evolution of its metabolisms, and its fossil biomarkers to provide guidance in the search for life on Mars. Key findings are (1) much terrestrial deep subsurface metabolic activity relies on abiotic energy-yielding fluxes and in situ abiotic and biotic recycling of metabolic waste products rather than on buried organic products of photosynthesis; (2) subsurface microbial cell concentrations are highest at interfaces with pronounced chemical redox gradients or permeability variations and do not correlate with bulk host rock organic carbon; (3) metabolic pathways for chemolithoautotrophic microorganisms evolved earlier in Earth's history than those of surface-dwelling phototrophic microorganisms; (4) the emergence of the former occurred at a time when Mars was habitable, whereas the emergence of the latter occurred at a time when the martian surface was not continually habitable; (5) the terrestrial rock record has biomarkers of subsurface life at least back hundreds of millions of years and likely to 3.45 Ga with several examples of excellent preservation in rock types that are quite different from those preserving the photosphere-supported biosphere. These findings suggest that rock-hosted life would have been more likely to emerge and be preserved in a martian context. Consequently, we outline a Mars exploration strategy that targets subsurface life and scales spatially, focusing initially on identifying rocks with evidence for groundwater flow and low-temperature mineralization, then identifying redox and permeability interfaces preserved within rock outcrops, and finally focusing on finding minerals associated with redox reactions and associated traces of carbon and diagnostic chemical and isotopic biosignatures. Using this strategy on Earth yields ancient rock-hosted life, preserved in the fossil record and confirmable via a suite of morphologic, organic, mineralogical, and isotopic fingerprints at micrometer scale. We expect an emphasis on rock-hosted life and this scale-dependent strategy to be crucial in the search for life on Mars.

Draft Genome Sequence of Clostridium sp. Strain E02, Isolated from an Estuarine Environment

Here, we report the draft genome sequence of a strain of Clostridium isolated from sediment collected from an estuarine environment. The strain was isolated using a minimal medium designed to select for chemoautotrophic microorganisms. The strain may represent a novel species within the genus Clostridium, and this genome sequence enables further investigation into the genetic and metabolic diversity of this organism.

Metabolic Processes Preserved as Biosignatures in Iron-Oxidizing Microorganisms: Implications for Biosignature Detection on Mars

Iron-oxidizing bacteria occupy a distinct environmental niche. These chemolithoautotrophic organisms require very little oxygen (when neutrophilic) or outcompete oxygen for access to Fe(II) (when acidophilic). The utilization of Fe(II) as an electron donor makes them strong analog organisms for any potential life that could be found on Mars. Despite their importance to the elucidation of early life on, and potentially beyond, Earth, many details of their metabolism remain unknown. By using on-line thermochemolysis and gas chromatography-mass spectrometry (GC-MS), a distinct signal for a low-molecular-weight molecule was discovered in multiple iron-oxidizing isolates as well as several iron-dominated environmental samples, from freshwater and marine environments and in both modern and older iron rock samples. This GC-MS signal was neither detected in organisms that did not use Fe(II) as an electron donor nor present in iron mats in which organic carbon was destroyed by heating. Mass spectral analysis indicates that the molecule bears the hallmarks of a pterin-bearing molecule. Genomic analysis has previously identified a molybdopterin that could be part of the electron transport chain in a number of lithotrophic iron-oxidizing bacteria, suggesting one possible source for this signal is the pterin component of this protein. The rock samples indicate the possibility that the molecule can be preserved within lithified sedimentary rocks. The specificity of the signal to organisms requiring iron in their metabolism makes this a novel biosignature with which to investigate both the evolution of life on ancient Earth and potential life on Mars.

Catalytic/Protective Properties of Martian Minerals and Implications for Possible Origin of Life on Mars

Minerals might have played critical roles for the origin and evolution of possible life forms on Mars. The study of the interactions between the "building blocks of life" and minerals relevant to Mars mineralogy under conditions mimicking the harsh Martian environment may provide key insight into possible prebiotic processes. Therefore, this contribution aims at reviewing the most important investigations carried out so far about the catalytic/protective properties of Martian minerals toward molecular biosignatures under Martian-like conditions. Overall, it turns out that the fate of molecular biosignatures on Mars depends on a delicate balance between multiple preservation and degradation mechanisms, often regulated by minerals, which may take place simultaneously. Such a complexity requires more efforts in simulating realistically the Martian environment in order to better inspect plausible prebiotic pathways and shed light on the nature of the organic compounds detected both in meteorites and on the surface of Mars through in situ analysis.

Biosignature Preservation Potential in Playa Evaporites: Impacts of Diagenesis and Implications for Mars Exploration

Assessing biosignature preservation potential (BPP) in ancient habitable environments on Mars is a top NASA priority. We address this goal through the study of Miocene-Pliocene evaporites of the Verde Formation (central Arizona). We assessed the effects of diagenesis on BPP, integrating outcrop-scale observations with six lab analyses: thin-section petrography, X-ray diffraction, Raman spectroscopy, total organic carbon (TOC), electron probe microanalysis (EPMA), and visible to near-infrared (VNIR) reflectance spectroscopy. We recognized five facies and their diagenetic pathways. Two facies included mudstones which contain clusters of displacive growth gypsum (DGG). Early DGG was altered during diagenesis by dissolution forming crystal cavities and later underwent recrystallization, cation substitution, and sulfate dehydration. Another facies was identified by lenticular beds dominated by halite and late diagenetic thenardite (Na₂SO₄). These pods are overlain by a sequence of interbedded gray and red mudstones which record cyclic oxidation and Fe-oxide cementation. During the Pleistocene, a lacustrine environment developed, accompanied by magnesite cementation of playa mudstones. TOC analyses were used as a proxy for inferring the BPP in each facies. The highest BPP was associated with both red and gray mudstone facies. This study provides a taphonomic framework for playa environments on Earth that record the impacts of diagenesis on BPP, with potential applications to Mars sample return (MSR) missions.

Methane on Mars and Habitability: Challenges and Responses

Recent measurements of methane (CH4) by the Mars Science Laboratory (MSL) now confront us with robust data that demand interpretation. Thus far, the MSL data have revealed a baseline level of CH4 (∼0.4 parts per billion by volume [ppbv]), with seasonal variations, as well as greatly enhanced spikes of CH4 with peak abundances of ∼7 ppbv. What do these CH4 revelations with drastically different abundances and temporal signatures represent in terms of interior geochemical processes, or is martian CH4 a biosignature? Discerning how CH4 generation occurs on Mars may shed light on the potential habitability of Mars. There is no evidence of life on the surface of Mars today, but microbes might reside beneath the surface. In this case, the carbon flux represented by CH4 would serve as a link between a putative subterranean biosphere on Mars and what we can measure above the surface. Alternatively, CH4 records modern geochemical activity. Here we ask the fundamental question: how active is Mars, geochemically and/or biologically? In this article, we examine geological, geochemical, and biogeochemical processes related to our overarching question. The martian atmosphere and surface are an overwhelmingly oxidizing environment, and life requires pairing of electron donors and electron acceptors, that is, redox gradients, as an essential source of energy. Therefore, a fundamental and critical question regarding the possibility of life on Mars is, "Where can we find redox gradients as energy sources for life on Mars?" Hence, regardless of the pathway that generates CH4 on Mars, the presence of CH4, a reduced species in an oxidant-rich environment, suggests the possibility of redox gradients supporting life and habitability on Mars. Recent missions such as ExoMars Trace Gas Orbiter may provide mapping of the global distribution of CH4. To discriminate between abiotic and biotic sources of CH4 on Mars, future studies should use a series of diagnostic geochemical analyses, preferably performed below the ground or at the ground/atmosphere interface, including measurements of CH4 isotopes, methane/ethane ratios, H2 gas concentration, and species such as acetic acid. Advances in the fields of Mars exploration and instrumentation will be driven, augmented, and supported by an improved understanding of atmospheric chemistry and dynamics, deep subsurface biogeochemistry, astrobiology, planetary geology, and geophysics. Future Mars exploration programs will have to expand the integration of complementary areas of expertise to generate synergistic and innovative ideas to realize breakthroughs in advancing our understanding of the potential of life and habitable conditions having existed on Mars. In this spirit, we conducted a set of interdisciplinary workshops. From this series has emerged a vision of technological, theoretical, and methodological innovations to explore the martian subsurface and to enhance spatial tracking of key volatiles, such as CH4.

Magnetite Authigenesis and the Warming of Early Mars

The Curiosity rover has documented lacustrine sediments at Gale Crater, but how liquid water became physically stable on the early Martian surface is a matter of significant debate. To constrain the composition of the early Martian atmosphere during sediment deposition, we experimentally investigated the nucleation and growth kinetics of authigenic Fe-minerals in Gale Crater mudstones. Experiments show that pH variations within anoxic basaltic waters trigger a series of mineral transformations that rapidly generate magnetite and H2(aq). Magnetite continues to form through this mechanism despite high PCO2 and supersaturation with respect to Fe-carbonate minerals. Reactive transport simulations that incorporate these experimental data show that groundwater infiltration into a lake equilibrated with a CO2-rich atmosphere can trigger the production of both magnetite and H2(aq) in the mudstones. H2(aq), generated at concentrations that would readily exsolve from solution, is capable of increasing annual mean surface temperatures above freezing in CO2-dominated atmospheres. We therefore suggest that magnetite authigenesis could have provided a short-term feedback for stabilizing liquid water, as well as a principal feedstock for biologically relevant chemical reactions, at the early Martian surface.

The Microbial Community of a Terrestrial Anoxic Inter-Tidal Zone: A Model for Laboratory-Based Studies of Potentially Habitable Ancient Lacustrine Systems on Mars

Evidence indicates that Gale crater on Mars harboured a fluvio-lacustrine environment that was subjected to physio-chemical variations such as changes in redox conditions and evaporation with salinity changes, over time. Microbial communities from terrestrial environmental analogues sites are important for studying such potential habitability environments on early Mars, especially in laboratory-based simulation experiments. Traditionally, such studies have predominantly focused on microorganisms from extreme terrestrial environments. These are applicable to a range of Martian environments; however, they lack relevance to the lacustrine systems. In this study, we characterise an anoxic inter-tidal zone as a terrestrial analogue for the Gale crater lake system according to its chemical and physical properties, and its microbiological community. The sub-surface inter-tidal environment of the River Dee estuary, United Kingdom (53°21′15.40″ N, 3°10′24.95″ W) was selected and compared with available data from Early Hesperian-time Gale crater, and temperature, redox, and pH were similar. Compared to subsurface ‘groundwater’-type fluids invoked for the Gale subsurface, salinity was higher at the River Dee site, which are more comparable to increases in salinity that likely occurred as the Gale crater lake evolved. Similarities in clay abundance indicated similar access to, specifically, the bio-essential elements Mg, Fe and K. The River Dee microbial community consisted of taxa that were known to have members that could utilise chemolithoautotrophic and chemoorganoheterotrophic metabolism and such a mixed metabolic capability would potentially have been feasible on Mars. Microorganisms isolated from the site were able to grow under environment conditions that, based on mineralogical data, were similar to that of the Gale crater’s aqueous environment at Yellowknife Bay. Thus, the results from this study suggest that the microbial community from an anoxic inter-tidal zone is a plausible terrestrial analogue for studying habitability of fluvio-lacustrine systems on early Mars, using laboratory-based simulation experiments.

Organic matter preserved in 3-billion-year-old mudstones at Gale crater, Mars

Establishing the presence and state of organic matter, including its possible biosignatures, in martian materials has been an elusive quest, despite limited reports of the existence of organic matter on Mars. We report the in situ detection of organic matter preserved in lacustrine mudstones at the base of the ~3.5-billion-year-old Murray formation at Pahrump Hills, Gale crater, by the Sample Analysis at Mars instrument suite onboard the Curiosity rover. Diverse pyrolysis products, including thiophenic, aromatic, and aliphatic compounds released at high temperatures (500° to 820°C), were directly detected by evolved gas analysis. Thiophenes were also observed by gas chromatography–mass spectrometry. Their presence suggests that sulfurization aided organic matter preservation. At least 50 nanomoles of organic carbon persists, probably as macromolecules containing 5% carbon as organic sulfur molecules.

Clay mineral diversity and abundance in sedimentary rocks of Gale crater, Mars

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

Detecting Kerogen as a Biosignature Using Colocated UV Time-Gated Raman and Fluorescence Spectroscopy

The Mars 2020 mission will analyze samples in situ and identify any that could have preserved biosignatures in ancient habitable environments for later return to Earth. Highest priority targeted samples include aqueously formed sedimentary lithologies. On Earth, such lithologies can contain fossil biosignatures as aromatic carbon (kerogen). In this study, we analyzed nonextracted kerogen in a diverse suite of natural, complex samples using colocated UV excitation (266 nm) time-gated (UV-TG) Raman and laser-induced fluorescence spectroscopies. We interrogated kerogen and its host matrix in samples to (1) explore the capabilities of UV-TG Raman and fluorescence spectroscopies for detecting kerogen in high-priority targets in the search for possible biosignatures on Mars; (2) assess the effectiveness of time gating and UV laser wavelength in reducing fluorescence in Raman spectra; and (3) identify sample-specific issues that could challenge rover-based identifications of kerogen using UV-TG Raman spectroscopy. We found that ungated UV Raman spectroscopy is suited to identify diagnostic kerogen Raman bands without interfering fluorescence and that UV fluorescence spectroscopy is suited to identify kerogen. These results highlight the value of combining colocated Raman and fluorescence spectroscopies, similar to those obtainable by SHERLOC on Mars 2020, to strengthen the confidence of kerogen detection as a potential biosignature in complex natural samples. Key Words: Raman spectroscopy-Laser-induced fluorescence spectroscopy-Mars Sample Return-Mars 2020 mission-Kerogen-Biosignatures. Astrobiology 18, 431-453.

Surface clay formation during short-term warmer and wetter conditions on a largely cold ancient Mars

The ancient rock record for Mars has long been at odds with climate modelling. The presence of valley networks, dendritic channels and deltas on ancient terrains points towards running water and fluvial erosion on early Mars¹, but climate modelling indicates that long-term warm conditions were not sustainable². Widespread phyllosilicates and other aqueous minerals on the Martian surface^3-6 provide additional evidence that an early wet Martian climate resulted in surface weathering. Some of these phyllosilicates formed in subsurface crustal environments⁵, with no association with the Martian climate, while other phyllosilicate-rich outcrops exhibit layered morphologies and broad stratigraphies⁷ consistent with surface formation. Here, we develop a new geochemical model for early Mars to explain the formation of these clay-bearing rocks in warm and wet surface locations. We propose that sporadic, short-term warm and wet environments during a generally cold early Mars enabled phyllosilicate formation without requiring long-term warm and wet conditions. We conclude that Mg-rich clay-bearing rocks with lateral variations in mixed Fe/Mg smectite, chlorite, talc, serpentine and zeolite occurrences formed in subsurface hydrothermal environments, whereas dioctahedral (Al/Fe³⁺-rich) smectite and widespread vertical horizonation of Fe/Mg smectites, clay assemblages and sulphates formed in variable aqueous environments on the surface of Mars. Our model for aluminosilicate formation on Mars is consistent with the observed geological features, diversity of aqueous mineralogies in ancient surface rocks and state-of-the-art palaeoclimate scenarios.

Smectite formation in the presence of sulfuric acid: Implications for acidic smectite formation on early Mars

The excess of orbital detection of smectite deposits compared to carbonate deposits on the martian surface presents an enigma because smectite and carbonate formations are both favored alteration products of basalt under neutral to alkaline conditions. We propose that Mars experienced acidic events caused by sulfuric acid (H2SO4) that permitted phyllosilicate, but inhibited carbonate, formation. To experimentally verify this hypothesis, we report the first synthesis of smectite from Mars-analogue glass-rich basalt simulant (66 wt% glass, 32 wt% olivine, 2 wt% chromite) in the presence of H2SO4 under hydrothermal conditions (~200 °C). Smectites were analyzed by X-ray diffraction, Mossbauer spectroscopy, visible and near-infrared reflectance spectroscopy and electron microprobe to characterize mineralogy and chemical composition. Solution chemistry was determined by Inductively Coupled Plasma Mass Spectrometry. Basalt simulant suspensions in 11-42 mM H2SO4 were acidic with pH ≤ 2 at the beginning of incubation and varied from acidic (pH 1.8) to mildly alkaline (pH 8.4) at the end of incubation. Alteration of glass phase during reaction of the basalt simulant with H2SO4 led to formation of the dioctahedral smectite at final pH ~3 and trioctahedral smectite saponite at final pH ~4 and higher. Anhydrite and hematite formed in the final pH range from 1.8 to 8.4 while natroalunite was detected at pH 1.8. Hematite was precipitated as a result of oxidative dissolution of olivine present in Adirondack basalt simulant. Formation of secondary phases, including smectite, resulted in release of variable amounts of Si, Mg, Na and Ca while solubilization of Al and Fe was low. Comparison of mineralogical and solution chemistry data indicated that the type of smectite (i.e., dioctahedral vs trioctahedral) was likely controlled by Mg leaching from altering basalt and substantial Mg loss created favorable conditions for formation of dioctahedral smectite. We present a model for global-scale smectite formation on Mars via acid-sulfate conditions created by the volcanic outgassing of SO2 in the Noachian and early Hesperian.

Chemistry, mineralogy, and grain properties at Namib and High dunes, Bagnold dune field, Gale crater, Mars: A synthesis of Curiosity rover observations

The Mars Science Laboratory Curiosity rover performed coordinated measurements to examine the textures and compositions of aeolian sands in the active Bagnold dune field. The Bagnold sands are rounded to subrounded, very fine to medium sized (~45-500 μm) with ≥6 distinct grain colors. In contrast to sands examined by Curiosity in a dust-covered, inactive bedform called Rocknest and soils at other landing sites, Bagnold sands are darker, less red, better sorted, have fewer silt-sized or smaller grains, and show no evidence for cohesion. Nevertheless, Bagnold mineralogy and Rocknest mineralogy are similar with plagioclase, olivine, and pyroxenes in similar proportions comprising >90% of crystalline phases, along with a substantial amorphous component (35% ± 15%). Yet Bagnold and Rocknest bulk chemistry differ. Bagnold sands are Si enriched relative to other soils at Gale crater, and H₂O, S, and Cl are lower relative to all previously measured Martian soils and most Gale crater rocks. Mg, Ni, Fe, and Mn are enriched in the coarse-sieved fraction of Bagnold sands, corroborated by visible/near-infrared spectra that suggest enrichment of olivine. Collectively, patterns in major element chemistry and volatile release data indicate two distinctive volatile reservoirs in Martian soils: (1) amorphous components in the sand-sized fraction (represented by Bagnold) that are Si-enriched, hydroxylated alteration products and/or H₂O- or OH-bearing impact or volcanic glasses and (2) amorphous components in the fine fraction (<40 μm; represented by Rocknest and other bright soils) that are Fe, S, and Cl enriched with low Si and adsorbed and structural H₂O.

Oxidative alteration of ferrous smectites and implications for the redox evolution of early Mars

Surface conditions on early Mars were likely anoxic, similar to early Earth, but the timing of the evolution to oxic conditions characteristic of contemporary Mars is unresolved. Ferrous trioctahedral smectites are the thermodynamically predicted products of anoxic basalt weathering, but orbital analyses of Noachian-aged terrains find primarily Fe3+-bearing clay minerals. Rover-based detection of Fe2+-bearing trioctahedral smectites at Gale Crater suggest that ferrous smectites are the unoxidized progenitors of orbitally-detected ferric smectites. To assess this pathway, we conducted ambient-temperature oxidative alteration experiments on four synthetic ferrous smectites having molar Fe/(Mg+Fe) from 1.00 to 0.33. Smectite suspension in air-saturated solutions produced incomplete oxidation (24-38% Fe3+/ΣFe). Additional smectite oxidation occurred upon re-exposure to air-saturated solutions after anoxic hydrothermal recrystallization, which accelerated cation and charge redistribution in the octahedral sheet. Oxidation was accompanied by contraction of the octahedral sheet (d(060) decreased from 1.53-1.56 Å to 1.52 Å), consistent with a shift towards dioctahedral structure. Ferrous smectite oxidation by aqueous hydrogen peroxide solutions resulted in nearly complete Fe2+ oxidation but also led to partial Fe3+ ejection from the structure, producing nanoparticulate hematite. Reflectance spectra of oxidized smectites were characterized by (Fe3+,Mg)2-OH bands at 2.28-2.30 μm, consistent with oxidative formation of dioctahedral nontronite. Accordingly, ferrous smectites are plausible precursors to observed ferric smectites on Mars, and their presence in late-Noachian sedimentary units suggests that anoxic conditions may have persisted on Mars beyond the Noachian.

Chemistry, mineralogy, and grain properties at Namib and High dunes, Bagnold dune field, Gale crater, Mars: A synthesis of Curiosity rover observations

The Mars Science Laboratory Curiosity rover performed coordinated measurements to examine the textures and compositions of aeolian sands in the active Bagnold dune field. The Bagnold sands are rounded to subrounded, very fine to medium sized (~45-500 μm) with ≥6 distinct grain colors. In contrast to sands examined by Curiosity in a dust-covered, inactive bedform called Rocknest and soils at other landing sites, Bagnold sands are darker, less red, better sorted, have fewer silt-sized or smaller grains, and show no evidence for cohesion. Nevertheless, Bagnold mineralogy and Rocknest mineralogy are similar with plagioclase, olivine, and pyroxenes in similar proportions comprising >90% of crystalline phases, along with a substantial amorphous component (35% ± 15%). Yet Bagnold and Rocknest bulk chemistry differ. Bagnold sands are Si enriched relative to other soils at Gale crater, and H2O, S, and Cl are lower relative to all previously measured Martian soils and most Gale crater rocks. Mg, Ni, Fe, and Mn are enriched in the coarse-sieved fraction of Bagnold sands, corroborated by visible/near-infrared spectra that suggest enrichment of olivine. Collectively, patterns in major element chemistry and volatile release data indicate two distinctive volatile reservoirs in Martian soils: (1) amorphous components in the sand-sized fraction (represented by Bagnold) that are Si-enriched, hydroxylated alteration products and/or H2O- or OH-bearing impact or volcanic glasses and (2) amorphous components in the fine fraction (<40 μm; represented by Rocknest and other bright soils) that are Fe, S, and Cl enriched with low Si and adsorbed and structural H2O.

Remote Detection of Clay Minerals

Spectral remote sensing in the visible/near-infrared (VNIR) and mid-IR (MIR) regions has enabled detection and characterisation of multiple clays and clay minerals on Earth and in the Solar System. Remote sensing on Earth poses the greatest challenge due to atmospheric absorptions that interfere with detection of surface minerals. Still, a greater variety of clay minerals have been observed on Earth than other bodies due to extensive aqueous alteration on our planet. Clay minerals have arguably been mapped in more detail on the planet Mars because they are not masked by vegetation on that planet and the atmosphere is less of a hindrance. Fe/Mg-smectite is the most abundant clay mineral on the surface of Mars and is also common in meteorites and comets where clay minerals are detected.

Atmospheric Constraints on the Surface UV Environment of Mars at 3.9 Ga Relevant to Prebiotic Chemistry

Recent findings suggest that Mars may have been a clement environment for the emergence of life and may even have compared favorably to Earth in this regard. These findings have revived interest in the hypothesis that prebiotically important molecules or even nascent life may have formed on Mars and been transferred to Earth. UV light plays a key role in prebiotic chemistry. Characterizing the early martian surface UV environment is key to understanding how Mars compares to Earth as a venue for prebiotic chemistry. Here, we present two-stream, multilayer calculations of the UV surface radiance on Mars at 3.9 Ga to constrain the surface UV environment as a function of atmospheric state. We explore a wide range of atmospheric pressures, temperatures, and compositions that correspond to the diversity of martian atmospheric states consistent with available constraints. We include the effects of clouds and dust. We calculate dose rates to quantify the effect of different atmospheric states on UV-sensitive prebiotic chemistry. We find that, for normative clear-sky CO₂-H₂O atmospheres, the UV environment on young Mars is comparable to young Earth. This similarity is robust to moderate cloud cover; thick clouds (τ_cloud ≥ 100) are required to significantly affect the martian UV environment, because cloud absorption is degenerate with atmospheric CO₂. On the other hand, absorption from SO₂, H₂S, and dust is nondegenerate with CO₂, meaning that, if these constituents build up to significant levels, surface UV fluence can be suppressed. These absorbers have spectrally variable absorption, meaning that their presence affects prebiotic pathways in different ways. In particular, high SO₂ environments may admit UV fluence that favors pathways conducive to abiogenesis over pathways unfavorable to it. However, better measurements of the spectral quantum yields of these pathways are required to evaluate this hypothesis definitively. Key Words: Radiative transfer-Origin of life-Mars-UV radiation-Prebiotic chemistry. Astrobiology 17, 687-708.

Habitability on Early Mars and the Search for Biosignatures with the ExoMars Rover

The second ExoMars mission will be launched in 2020 to target an ancient location interpreted to have strong potential for past habitability and for preserving physical and chemical biosignatures (as well as abiotic/prebiotic organics). The mission will deliver a lander with instruments for atmospheric and geophysical investigations and a rover tasked with searching for signs of extinct life. The ExoMars rover will be equipped with a drill to collect material from outcrops and at depth down to 2 m. This subsurface sampling capability will provide the best chance yet to gain access to chemical biosignatures. Using the powerful Pasteur payload instruments, the ExoMars science team will conduct a holistic search for traces of life and seek corroborating geological context information. Key Words: Biosignatures-ExoMars-Landing sites-Mars rover-Search for life. Astrobiology 17, 471-510.

Redox stratification of an ancient lake in Gale crater, Mars

In 2012, NASA's Curiosity rover landed on Mars to assess its potential as a habitat for past life and investigate the paleoclimate record preserved by sedimentary rocks inside the ~150-kilometer-diameter Gale impact crater. Geological reconstructions from Curiosity rover data have revealed an ancient, habitable lake environment fed by rivers draining into the crater. We synthesize geochemical and mineralogical data from lake-bed mudstones collected during the first 1300 martian solar days of rover operations in Gale. We present evidence for lake redox stratification, established by depth-dependent variations in atmospheric oxidant and dissolved-solute concentrations. Paleoclimate proxy data indicate that a transition from colder to warmer climate conditions is preserved in the stratigraphy. Finally, a late phase of geochemical modification by saline fluids is recognized.

Low Hesperian PCO2 constrained from in situ mineralogical analysis at Gale Crater, Mars

Carbon dioxide is an essential atmospheric component in martian climate models that attempt to reconcile a faint young sun with planetwide evidence of liquid water in the Noachian and Early Hesperian. In this study, we use mineral and contextual sedimentary environmental data measured by the Mars Science Laboratory (MSL) Rover Curiosity to estimate the atmospheric partial pressure of CO₂ (P_CO2) coinciding with a long-lived lake system in Gale Crater at ∼3.5 Ga. A reaction-transport model that simulates mineralogy observed within the Sheepbed member at Yellowknife Bay (YKB), by coupling mineral equilibria with carbonate precipitation kinetics and rates of sedimentation, indicates atmospheric P_CO2 levels in the 10s mbar range. At such low P_CO2 levels, existing climate models are unable to warm Hesperian Mars anywhere near the freezing point of water, and other gases are required to raise atmospheric pressure to prevent lake waters from being lost to the atmosphere. Thus, either lacustrine features of Gale formed in a cold environment by a mechanism yet to be determined, or the climate models still lack an essential component that would serve to elevate surface temperatures, at least locally, on Hesperian Mars. Our results also impose restrictions on the potential role of atmospheric CO₂ in inferred warmer conditions and valley network formation of the late Noachian.

The complex relationship between olivine abundance and thermal inertia on Mars

We examine four olivine-bearing regions at a variety of spatial scales with thermal infrared, visible to near-infrared, and visible imagery data to investigate the hypothesis that the relationship between olivine abundance and thermal inertia (i.e., effective particle size) can be used to infer the occurrence of olivine chemical alteration during sediment production on Mars. As in previous work, Nili Fossae and Isidis Planitia show a positive correlation between thermal inertia and olivine abundance in Thermal Emission Spectrometer (TES) and Thermal Emission Imaging System (THEMIS) data, which could be interpreted as indicating olivine chemical weathering. However, geomorphological analysis reveals that relatively olivine-poor sediments are not derived from adjacent olivine-rich materials, and therefore, chemical weathering cannot be inferred based on the olivine-thermal inertia relationship alone. We identify two areas (Terra Cimmeria and Argyre Planitia) with significant olivine abundance and thermal inertias consistent with sand, but no adjacent rocky (parent) units having even greater olivine abundances. More broadly, global analysis with TES reveals that the most typical olivine abundance on Mars is ~5-7% and that olivine-bearing (5-25%) materials have a wide range of thermal inertias, commonly 25-600 J m^-2 K^-1 s^-1/2. TES also indicates that the majority of olivine-rich (>25%) materials have apparent thermal inertias less than 400 J m^-2 K^-1 s^-1/2. In summary, we find that the relationship between thermal inertia and olivine abundance alone cannot be used in infer olivine weathering in the examined areas, that olivine-bearing materials have a large range of thermal intertias, and therefore that a complex relationship between olivine abundance and thermal inertia exists on Mars.

Mineralogy, provenance, and diagenesis of a potassic basaltic sandstone on Mars: CheMin X-ray diffraction of the Windjana sample (Kimberley area, Gale Crater)

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

Weathering Profiles in Phosphorus-Rich Rocks at Gusev Crater, Mars, Suggest Dissolution of Phosphate Minerals into Potentially Habitable Near-Neutral Waters

Abundant evidence indicates that significant surface and near-surface liquid water has existed on Mars in the past. Evaluating the potential for habitable environments on Mars requires an understanding of the chemical and physical conditions that prevailed in such aqueous environments. Among the geological features that may hold evidence of past environmental conditions on Mars are weathering profiles, such as those in the phosphorus-rich Wishstone-class rocks in Gusev Crater. The weathering profiles in these rocks indicate that a Ca-phosphate mineral has been lost during past aqueous interactions. The high phosphorus content of these rocks and potential release of phosphorus during aqueous interactions also make them of astrobiological interest, as phosphorus is among the elements required for all known life. In this work, we used Mars mission data, laboratory-derived kinetic and thermodynamic data, and data from terrestrial analogues, including phosphorus-rich basalts from Idaho, to model a conceptualized Wishstone-class rock using the reactive transport code CrunchFlow. Modeling results most consistent with the weathering profiles in Wishstone-class rocks suggest a combination of chemical and physical erosion and past aqueous interactions with near-neutral waters. The modeling results also indicate that multiple Ca-phosphate minerals are likely in Wishstone-class rocks, consistent with observations of martian meteorites. These findings suggest that Gusev Crater experienced a near-neutral phosphate-bearing aqueous environment that may have been conducive to life on Mars in the past.

KEY WORDS

Mars-Gusev Crater-Wishstone-Reactive transport modeling-CrunchFlow-Aqueous interactions-Neutral pH-Habitability.