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The Eridania region in the southern highlands of Mars once contained a vast inland sea with a volume of water greater than that of all other Martian lakes combined. Here we show that the most ancient materials within Eridania are thick (>400 m), massive (not bedded), mottled deposits containing saponite, talc-saponite, Fe-rich mica (for example, glauconite-nontronite), Fe- and Mg-serpentine, Mg-Fe-Ca-carbonate and probable Fe-sulphide that likely formed in a deep water (500–1,500 m) hydrothermal setting. The Eridania basin occurs within some of the most ancient terrain on Mars where striking evidence for remnant magnetism might suggest an early phase of crustal spreading. The relatively well-preserved seafloor hydrothermal deposits in Eridania are contemporaneous with the earliest evidence for life on Earth in potentially similar environments 3.8 billion years ago, and might provide an invaluable window into the environmental conditions of early Earth. The Eridania basin on Mars was once the site of a vast inland sea. Here, the authors show that the most ancient materials in the Eridania basin were formed in a deep-water hydrothermal setting and may be an analogue for early environmental conditions on Earth.

The sulphate-rich deposits at Meridiani Planum, Mars, discovered by the rover Opportunity, were proposed to be playa evaporites that had been reworked by eolian processes. Alternative hypotheses include volcanic or impact-driven formation of the sediments. Here we argue that the cation chemistry, scale, mineralogy and structure of the Meridiani sedimentary deposits are best explained by eolian or impact-driven reworking of the sublimation residue from a large-scale deposit consisting of dust and ice. We suggest that silicate material underwent significant acid weathering inside the ice deposit when thin films of water, formed through radiant heating, enabled the reaction between silicate material and sulphate-rich aerosols deposited from the atmosphere. The massive ice deposit could have formed during a period of high obliquity or polar wander, and subsequently sublimed away when obliquity changed or the pole moved to a new location. We propose acid weathering inside massive ice deposits as an explanation for the formation of many of the sulphate-rich layered deposits on Mars, which share many characteristics, including mineralogy, structure, erosional characteristics and size, with the sediments found at Meridiani Planum. Although a number of hypotheses have been put forward to explain the sulphate deposits discovered by the Opportunity rover at Meridiani Planum, Mars, the sedimentary layers remain enigmatic. A re-analysis of the chemistry, sedimentology and geology of the deposits suggests they formed through a reworking of the sublimation residue from a large-scale deposit of ice and dust.

By the time eukaryotic life or photosynthesis evolved on Earth, the martian surface had become extremely inhospitable, but the subsurface of Mars could potentially have contained a vast microbial biosphere. Crustal fluids may have welled up from the subsurface to alter and cement surface sediments, potentially preserving clues to subsurface habitability. Here we present a conceptual model of subsurface habitability of Mars and evaluate evidence for groundwater upwelling in deep basins. Many ancient, deep basins lack evidence for groundwater activity. However, McLaughlin Crater, one of the deepest craters on Mars, contains evidence for Mg–Fe-bearing clays and carbonates that probably formed in an alkaline, groundwater-fed lacustrine setting. This environment strongly contrasts with the acidic, water-limited environments implied by the presence of sulphate deposits that have previously been suggested to form owing to groundwater upwelling. Deposits formed as a result of groundwater upwelling on Mars, such as those in McLaughlin Crater, could preserve critical evidence of a deep biosphere on Mars. We suggest that groundwater upwelling on Mars may have occurred sporadically on local scales, rather than at regional or global scales. The subsurface of Mars could potentially have contained a vast microbial biosphere. An evaluation of the possibility of groundwater upwelling, which might provide clues to subsurface habitability, reveals evidence in the deep McLaughlin crater for clays and carbonates that probably formed in an alkaline, groundwater-fed lacustrine setting.

Ongoing research on martian meteorites and a new set of observations of carbonate minerals provided by an unprecedented series of robotic missions to Mars in the past 15 years help define new constraints on the history of martian climate with important crosscutting themes including: the CO 2 budget of Mars, the role of Mg-, Fe-rich fluids on Mars, and the interplay between carbonate formation and acidity. Carbonate minerals have now been identified in a wide range of localities on Mars as well as in several martian meteorites. The martian meteorites contain carbonates in low abundances (<1 vol.%) and with a wide range of chemistries. Carbonates have also been identified by remote sensing instruments on orbiting spacecraft in several surface locations as well as in low concentrations (2–5 wt.%) in the martian dust. The Spirit rover also identified an outcrop with 16 to 34 wt.% carbonate material in the Columbia Hills of Gusev Crater that strongly resembled the composition of carbonate found in martian meteorite ALH 84001. Finally, the Phoenix lander identified concentrations of 3–6 wt.% carbonate in the soils of the northern plains. The carbonates discovered to date do not clearly indicate the past presence of a dense Noachian atmosphere, but instead suggest localized hydrothermal aqueous environments with limited water availability that existed primarily in the early to mid-Noachian followed by low levels of carbonate formation from thin films of transient water from the late Noachian to the present. The prevalence of carbonate along with evidence for active carbonate precipitation suggests that a global acidic chemistry is unlikely and a more complex relationship between acidity and carbonate formation is present.

The evolution and escape of the martian atmosphere and the planet’s water inventory can be separated into an early and late evolutionary epoch. The first epoch started from the planet’s origin and lasted ∼500 Myr. Because of the high EUV flux of the young Sun and Mars’ low gravity it was accompanied by hydrodynamic blow-off of hydrogen and strong thermal escape rates of dragged heavier species such as O and C atoms. After the main part of the protoatmosphere was lost, impact-related volatiles and mantle outgassing may have resulted in accumulation of a secondary CO 2 atmosphere of a few tens to a few hundred mbar around ∼4–4.3 Gyr ago. The evolution of the atmospheric surface pressure and water inventory of such a secondary atmosphere during the second epoch which lasted from the end of the Noachian until today was most likely determined by a complex interplay of various nonthermal atmospheric escape processes, impacts, carbonate precipitation, and serpentinization during the Hesperian and Amazonian epochs which led to the present day surface pressure.

Large Hesperian-aged (~3.7 Ga) layered deposits of sulfate-rich sediments in the equatorial regions of Mars have been suggested to be evidence for ephemeral playa environments. But early Mars may not have been warm enough to support conditions similar to what occurs in arid environments on Earth. Instead cold, icy environments may have been widespread. Under cryogenic conditions sulfate formation might be blocked, since kinetics of silicate weathering are typically strongly retarded at temperatures well below 0 °C. But cryo-concentration of acidic solutions may counteract the slow kinetics. Here we show that cryo-concentrated acidic brines rapidly chemically weather olivine minerals and form sulfate minerals at temperatures as low as −60 °C. These experimental results demonstrate the viability of sulfate formation under current Martian conditions, even in the polar regions. An ice-hosted sedimentation and weathering model may provide a compelling description of the origin of large Hesperian-aged layered sulfate deposits on Mars. Sulphate-rich sediments have been taken as evidence of surface water on Mars. Here, the authors show that cryo-concentrated brines chemically weather olivine minerals forming sulfate minerals at up to −60 °C, showing that cryogenic weathering and sulfate formation can occur under current Martian conditions.

Significance Martian meteorite ALH 84001 serves as a witness plate to the history of the Martian climate ∼4 Ga ago. This study describes ion microprobe δ ¹⁸O analyses coupled with δ ¹³C, δ ¹⁸O, and Δ ¹⁷O analyses from stepped acid dissolution of the meteorite that identifies a new carbonate phase with distinct isotope compositions. These new measurements of the oxygen isotope composition of carbonates within this meteorite reveal several episodes of aqueous activity that were strongly influenced by atmospheric chemistry. When paired with carbon isotope measurements, these data suggest that the ancient atmosphere of Mars was significantly depleted in ¹³C compared to the present day. This implies substantial enrichment in the δ ¹³C of the atmosphere since the Noachian which may have occurred through extensive atmospheric loss. Carbonate minerals provide critical information for defining atmosphere–hydrosphere interactions. Carbonate minerals in the Martian meteorite ALH 84001 have been dated to ∼3.9 Ga, and both C and O-triple isotopes can be used to decipher the planet’s climate history. Here we report Δ ¹⁷O, δ ¹⁸O, and δ ¹³C data of ALH 84001 of at least two varieties of carbonates, using a stepped acid dissolution technique paired with ion microprobe analyses to specifically target carbonates from distinct formation events and constrain the Martian atmosphere–hydrosphere–geosphere interactions and surficial aqueous alterations. These results indicate the presence of a Ca-rich carbonate phase enriched in ¹⁸O that formed sometime after the primary aqueous event at 3.9 Ga. The phases showed excess ¹⁷O (0.7‰) that captured the atmosphere–regolith chemical reservoir transfer, as well as CO ₂, O ₃, and H ₂O isotopic interactions at the time of formation of each specific carbonate. The carbon isotopes preserved in the Ca-rich carbonate phase indicate that the Noachian atmosphere of Mars was substantially depleted in ¹³C compared with the modern atmosphere.

Mars may have once had a CO 2 -rich atmosphere, but carbonate rocks that could provide evidence for such conditions are sparse. Spectral analyses of rocks exposed from deep within an impact crater reveal that carbonate deposits may be extensive on Mars, but are buried under layers of younger volcanic rocks. The surface of Mars is cold, dry, oxidizing, acidic and inhospitable to life. Similar conditions may have persisted for billions of years, suggesting that the best place to search for habitable environments is the subsurface 1 . One hint of habitable conditions at depth is the presence of atmospheric methane, which may have formed through hydrothermal processes in the crust 2 , 3 in the presence of CO 2 . The observation of hydrated minerals excavated by some impact craters suggests that ancient hydrothermal systems may have existed in the subsurface 4 , 5 , 6 , 7 , 8 , 9 , but until now, none of those deposits has been linked to carbonate minerals and CO 2 -rich environments. Previous detections of carbonate minerals 10 that could be linked to an ancient CO 2 -rich surface environment 11 have been sparse. Here we show spectral evidence for carbonate- and phyllosilicate-bearing, layered and foliated bedrock exhumed from deep (about 6 km) within the martian crust by a meteor impact. The mineral assemblage, textural properties and geologic context of the deposits indicate that these rocks are probably ancient sediments that were metamorphosed during burial by younger volcanic materials from the nearby Syrtis Major volcano. We suggest that these buried layered carbonates might be only a small part of a much more extensive ancient carbonate sedimentary record that has been buried by volcanic resurfacing and impact ejecta. Our discovery may help explain the origin of other carbonates on Mars and indicates a high-priority site for future exobiological exploration.

Few traces of Earth’s geologic record are preserved from the time of life’s emergence, over 3,800 million years ago. Consequently, what little we understand about abiogenesis — the origin of life on Earth — is based primarily on laboratory experiments and theory. The best geological lens for understanding early Earth might actually come from Mars, a planet with a crust that’s overall far more ancient than our own. On Earth, surface sedimentary environments are thought to best preserve evidence of ancient life, but this is mostly because our planet has been dominated by high photosynthetic biomass production at the surface for the last ~2,500 million years or more. By the time oxygenic photosynthesis evolved on Earth, Mars had been a hyperarid, frozen desert with a surface bombarded by high-energy solar and cosmic radiation for more than a billion years, and as a result, photosynthetic surface life may never have occurred on Mars. Therefore, one must question whether searching for evidence of life in Martian surface sediments is the best strategy. This Perspective explores the possibility that the abundant hydrothermal environments on Mars might provide more valuable insights into life’s origins.

Clays form on Earth by near-surface weathering, precipitation in water bodies within basins, hydrothermal alteration (volcanic- or impact-induced), diagenesis, metamorphism, and magmatic precipitation. Diverse clay minerals have been detected from orbital investigation of terrains on Mars and are globally distributed, indicating geographically widespread aqueous alteration. Clay assemblages within deep stratigraphic units in the Martian crust include Fe/Mg smectites, chlorites and higher temperature hydrated silicates. Sedimentary clay mineral assemblages include Fe/Mg smectites, kaolinite, and sulfate, carbonate, and chloride salts. Stratigraphic sequences with multiple clay-bearing units have an upper unit with Al-clays and a lower unit with Fe/Mg-clays. The typical restriction of clay minerals to the oldest, Noachian terrains indicates a distinctive set of processes involving water-rock interaction that was prevalent early in Mars history and may have profoundly influenced the evolution of Martian geochemical systems. Current analyses of orbital data have led to the proposition of multiple clay-formation mechanisms, varying in space and time in their relative importance. These include near-surface weathering, formation in ice-dominated near-surface groundwaters, and formation by subsurface hydrothermal fluids. Near-surface, open system formation of clays would lead to fractionation of Mars’ crustal reservoir into an altered crustal reservoir and a sedimentary reservoir, potentially involving changes in the composition of Mars’ atmosphere. In contrast, formation of clays in the subsurface by either aqueous alteration or magmatic cooling would result in comparatively little geochemical fractionation or interaction of Mars’ atmospheric, crustal, and magmatic reservoirs, with the exception of long-term sequestration of water. Formation of clays within ice would have geochemical consequences intermediate between these endmembers. We outline the future analyses of orbital data, in situ measurements acquired within clay-bearing terrains, and analyses of Mars samples that are needed to more fully elucidate the mechanisms of martian clay formation and to determine the consequences for the geochemical evolution of the planet.