Inhibition of carbonate synthesis in acidic oceans on early Mars

Several lines of evidence have recently reinforced the hypothesis that an ocean existed on early Mars. Carbonates are accordingly expected to have formed from oceanic sedimentation of carbon dioxide from the ancient martian atmosphere. But spectral imaging of the martian surface has revealed the presence of only a small amount of carbonate, widely distributed in the martian dust. Here we examine the feasibility of carbonate synthesis in ancient martian oceans using aqueous equilibrium calculations. We show that partial pressures of atmospheric carbon dioxide in the range 0.8-4 bar, in the presence of up to 13.5 mM sulphate and 0.8 mM iron in sea water, result in an acidic oceanic environment with a pH of less than 6.2. This precludes the formation of siderite, usually expected to be the first major carbonate mineral to precipitate. We conclude that extensive interaction between an atmosphere dominated by carbon dioxide and a lasting sulphate- and iron-enriched acidic ocean on early Mars is a plausible explanation for the observed absence of carbonates.     

Phyllosilicates on Mars and implications for early martian climate

The recent identification of large deposits of sulphates by remote sensing and in situ observations has been considered evidence of the past presence of liquid water on Mars. Here we report the unambiguous detection of diverse phyllosilicates, a family of aqueous alteration products, on the basis of observations by the OMEGA imaging spectrometer on board the Mars Express spacecraft. These minerals are mainly associated with Noachian outcrops, which is consistent with an early active hydrological system, sustaining the long-term contact of igneous minerals with liquid water. We infer that the two main families of hydrated alteration products detected-phyllosilicates and sulphates--result from different formation processes. These occurred during two distinct climatic episodes: an early Noachian Mars, resulting in the formation of hydrated silicates, followed by a more acidic environment, in which sulphates formed.     

Heterogeneous chemistry in the atmosphere of Mars

Hydrogen radicals are produced in the martian atmosphere by the photolysis of water vapour and subsequently initiate catalytic cycles that recycle carbon dioxide from its photolysis product carbon monoxide. These processes provide a qualitative explanation for the stability of the atmosphere of Mars, which contains 95 per cent carbon dioxide. Balancing carbon dioxide production and loss based on our current understanding of the gas-phase chemistry in the martian atmosphere has, however, proven to be difficult. Interactions between gaseous chemical species and ice cloud particles have been shown to be key factors in the loss of polar ozone observed in the Earth's stratosphere, and may significantly perturb the chemistry of the Earth's upper troposphere. Water-ice clouds are also commonly observed in the atmosphere of Mars and it has been suggested previously that heterogeneous chemistry could have an important impact on the composition of the martian atmosphere. Here we use a state-of-the-art general circulation model together with new observations of the martian ozone layer to show that model simulations that include chemical reactions occurring on ice clouds lead to much improved quantitative agreement with observed martian ozone levels in comparison with model simulations based on gas-phase chemistry alone. Ozone is readily destroyed by hydrogen radicals and is therefore a sensitive tracer of the chemistry that regulates the atmosphere of Mars. Our results suggest that heterogeneous chemistry on ice clouds plays an important role in controlling the stability and composition of the martian atmosphere.     

A lower limit for atmospheric carbon dioxide levels 3.2 billion years ago

The quantification of greenhouse gases present in the Archaean atmosphere is critical for understanding the evolution of atmospheric oxygen, surface temperatures and the conditions for life on early Earth. For instance, it has been argued that small changes in the balance between two potential greenhouse gases, carbon dioxide and methane, may have dictated the feedback cycle involving organic haze production and global cooling. Climate models have focused on carbon dioxide as the greenhouse gas responsible for maintaining above-freezing surface temperatures during a time of low solar luminosity. However, the analysis of 2.75-billion-year (Gyr)-old palaeosols--soil samples preserved in the geologic record--have recently provided an upper constraint on atmospheric carbon dioxide levels well below that required in most climate models to prevent the Earth's surface from freezing. This finding prompted many to look towards methane as an additional greenhouse gas to satisfy climate models. Here we use model equilibrium reactions for weathering rinds on 3.2-Gyr-old river gravels to show that the presence of iron-rich carbonate relative to common clay minerals requires a minimum partial pressure of carbon dioxide several times higher than present-day values. Unless actual carbon dioxide levels were considerably greater than this, climate models predict that additional greenhouse gases would still need to have a role in maintaining above-freezing surface temperatures.     

Perennial water ice identified in the south polar cap of Mars

The inventory of water and carbon dioxide reservoirs on Mars are important clues for understanding the geological, climatic and potentially exobiological evolution of the planet. From the early mapping observation of the permanent ice caps on the martian poles, the northern cap was believed to be mainly composed of water ice, whereas the southern cap was thought to be constituted of carbon dioxide ice. However, recent missions (NASA missions Mars Global Surveyor and Odyssey) have revealed surface structures, altimetry profiles, underlying buried hydrogen, and temperatures of the south polar regions that are thermodynamically consistent with a mixture of surface water ice and carbon dioxide. Here we present the first direct identification and mapping of both carbon dioxide and water ice in the martian high southern latitudes, at a resolution of 2 km, during the local summer, when the extent of the polar ice is at its minimum. We observe that this south polar cap contains perennial water ice in extended areas: as a small admixture to carbon dioxide in the bright regions; associated with dust, without carbon dioxide, at the edges of this bright cap; and, unexpectedly, in large areas tens of kilometres away from the bright cap.     

Hydrated silicate minerals on Mars observed by the Mars Reconnaissance Orbiter CRISM instrument

Phyllosilicates, a class of hydrous mineral first definitively identified on Mars by the OMEGA (Observatoire pour la Mineralogie, L'Eau, les Glaces et l'Activitié) instrument, preserve a record of the interaction of water with rocks on Mars. Global mapping showed that phyllosilicates are widespread but are apparently restricted to ancient terrains and a relatively narrow range of mineralogy (Fe/Mg and Al smectite clays). This was interpreted to indicate that phyllosilicate formation occurred during the Noachian (the earliest geological era of Mars), and that the conditions necessary for phyllosilicate formation (moderate to high pH and high water activity) were specific to surface environments during the earliest era of Mars's history. Here we report results from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) of phyllosilicate-rich regions. We expand the diversity of phyllosilicate mineralogy with the identification of kaolinite, chlorite and illite or muscovite, and a new class of hydrated silicate (hydrated silica). We observe diverse Fe/Mg-OH phyllosilicates and find that smectites such as nontronite and saponite are the most common, but chlorites are also present in some locations. Stratigraphic relationships in the Nili Fossae region show olivine-rich materials overlying phyllosilicate-bearing units, indicating the cessation of aqueous alteration before emplacement of the olivine-bearing unit. Hundreds of detections of Fe/Mg phyllosilicate in rims, ejecta and central peaks of craters in the southern highland Noachian cratered terrain indicate excavation of altered crust from depth. We also find phyllosilicate in sedimentary deposits clearly laid by water. These results point to a rich diversity of Noachian environments conducive to habitability.     

Impact erosion of the primordial atmosphere of Mars

Abundant geomorphic evidence for fluvial processes on the surface of Mars suggests that during the era of heavy bombardment, Mars's atmospheric pressure was high enough for liquid water to flow on the surface. Many authors have proposed mechanisms by which Mars could have lost (or sequestered) an earlier, thicker atmosphere but none of these proposals has gained general acceptance. Here we examine the process of atmospheric erosion by impacts and show that it may account for an early episode of atmosphere loss from Mars. On the basis of this model, the primordial atmospheric pressure on Mars must have been in the vicinity of 1 bar, barring other sources or sinks of CO2. Current impact fluxes are too small to erode significantly the present martian atmosphere.     

The hydrologic cycle in deep-time climate problems

Hydrology refers to the whole panoply of effects the water molecule has on climate and on the land surface during its journey there and back again between ocean and atmosphere. On its way, it is cycled through vapour, cloud water, snow, sea ice and glacier ice, as well as acting as a catalyst for silicate-carbonate weathering reactions governing atmospheric carbon dioxide. Because carbon dioxide affects the hydrologic cycle through temperature, climate is a pas des deux between carbon dioxide and water, with important guest appearances by surface ice cover.     

Subsurface water and clay mineral formation during the early history of Mars

Clay minerals, recently discovered to be widespread in Mars's Noachian terrains, indicate long-duration interaction between water and rock over 3.7 billion years ago. Analysis of how they formed should indicate what environmental conditions prevailed on early Mars. If clays formed near the surface by weathering, as is common on Earth, their presence would indicate past surface conditions warmer and wetter than at present. However, available data instead indicate substantial Martian clay formation by hydrothermal groundwater circulation and a Noachian rock record dominated by evidence of subsurface waters. Cold, arid conditions with only transient surface water may have characterized Mars's surface for over 4 billion years, since the early-Noachian period, and the longest-duration aqueous, potentially habitable environments may have been in the subsurface.     

An integrated view of the chemistry and mineralogy of martian soils

The mineralogical and elemental compositions of the martian soil are indicators of chemical and physical weathering processes. Using data from the Mars Exploration Rovers, we show that bright dust deposits on opposite sides of the planet are part of a global unit and not dominated by the composition of local rocks. Dark soil deposits at both sites have similar basaltic mineralogies, and could reflect either a global component or the general similarity in the compositions of the rocks from which they were derived. Increased levels of bromine are consistent with mobilization of soluble salts by thin films of liquid water, but the presence of olivine in analysed soil samples indicates that the extent of aqueous alteration of soils has been limited. Nickel abundances are enhanced at the immediate surface and indicate that the upper few millimetres of soil could contain up to one per cent meteoritic material.     

Jarosite as an indicator of water-limited chemical weathering on Mars

The Mars Exploration Rover Opportunity identified the ferric sulphate mineral jarosite and possible relicts of gypsum at the Meridiani Planum landing site. On Earth, jarosite has been found to form in acid mine drainage environments, during the oxidation of sulphide minerals, and during alteration of volcanic rocks by acidic, sulphur-rich fluids near volcanic vents. Jarosite formation is thus thought to require a wet, oxidizing and acidic environment. But jarosite on Earth only persists over geologically relevant time periods in arid environments because it rapidly decomposes to produce ferric oxyhydroxides in more humid climates. Here we present equilibrium thermodynamic reaction-path simulations that constrain the range of possible conditions under which such aqueous alteration phases are likely to have formed on Mars. These calculations simulate the chemical weathering of basalt at relevant martian conditions. We conclude that the presence of jarosite combined with residual basalt at Meridiani Planum indicates that the alteration process did not proceed to completion, and that following jarosite formation, arid conditions must have prevailed.     

Albedo of the south pole on Mars determined by topographic forcing of atmosphere dynamics

The nature of the martian south polar cap has remained enigmatic since the first spacecraft observations. In particular, the presence of a perennial carbon dioxide ice cap, the formation of a vast area of black 'slab ice' known as the Cryptic region and the asymmetric springtime retreat of the cap have eluded explanation. Here we present observations and climate modelling that indicate the south pole of Mars is characterized by two distinct regional climates that are the result of dynamical forcing by the largest southern impact basins, Argyre and Hellas. The style of surface frost deposition is controlled by these regional climates. In the cold and stormy conditions that exist poleward of 60 degrees S and extend 180 degrees in longitude west from the Mountains of Mitchel (approximately 30 degrees W), surface frost accumulation is dominated by precipitation. In the opposite hemisphere, the polar atmosphere is relatively warm and clear and frost accumulation is dominated by direct vapour deposition. It is the differences in these deposition styles that determine the cap albedo.     

Negligible glacial-interglacial variation in continental chemical weathering rates

Chemical weathering of the continents is central to the regulation of atmospheric carbon dioxide concentrations, and hence global climate. On million-year timescales silicate weathering leads to the draw-down of carbon dioxide, and on millennial timescales chemical weathering affects the calcium carbonate saturation state of the oceans and hence their uptake of carbon dioxide. However, variations in chemical weathering rates over glacial-interglacial cycles remain uncertain. During glacial periods, cold and dry conditions reduce the rate of chemical weathering, but intense physical weathering and the exposure of carbonates on continental shelves due to low sea levels may increase this rate. Here we present high-resolution records of the lead isotope composition of ferromanganese crusts from the North Atlantic Ocean that cover the past 550,000 years. Combining these records with a simple quantitative model of changes in the lead isotope composition of the deep North Atlantic Ocean in response to chemical weathering, we find that chemical weathering rates were two to three times lower in the glaciated interior of the North Atlantic Region during glacial periods than during the intervening interglacial periods. This decrease roughly balances the increase in chemical weathering caused by the exposure of continental shelves, indicating that chemical weathering rates remained relatively constant on glacial-interglacial timescales. On timescales of more than a million years, however, we suggest that enhanced weathering of silicate glacial sediments during interglacial periods results in a net draw-down of atmospheric carbon dioxide, creating a positive feedback on global climate that, once initiated, promotes cooling and further glaciation.     

Geochemical evidence for magmatic water within Mars from pyroxenes in the Shergotty meteorite

Observations of martian surface morphology have been used to argue that an ancient ocean once existed on Mars. It has been thought that significant quantities of such water could have been supplied to the martian surface through volcanic outgassing, but this suggestion is contradicted by the low magmatic water content that is generally inferred from chemical analyses of igneous martian meteorites. Here, however, we report the distributions of trace elements within pyroxenes of the Shergotty meteorite--a basalt body ejected 175 million years ago from Mars--as well as hydrous and anhydrous crystallization experiments that, together, imply that water contents of pre-eruptive magma on Mars could have been up to 1.8%. We found that in the Shergotty meteorite, the inner cores of pyroxene minerals (which formed at depth in the martian crust) are enriched in soluble trace elements when compared to the outer rims (which crystallized on or near to the martian surface). This implies that water was present in pyroxenes at depth but was largely lost as pyroxenes were carried to the surface during magma ascent. We conclude that ascending magmas possibly delivered significant quantities of water to the martian surface in recent times, reconciling geologic and petrologic constraints on the outgassing history of Mars.     

Martian stepped-delta formation by rapid water release

Deltas and alluvial fans preserved on the surface of Mars provide an important record of surface water flow. Understanding how surface water flow could have produced the observed morphology is fundamental to understanding the history of water on Mars. To date, morphological studies have provided only minimum time estimates for the longevity of martian hydrologic events, which range from decades to millions of years. Here we use sand flume studies to show that the distinct morphology of martian stepped (terraced) deltas could only have originated from a single basin-filling event on a timescale of tens of years. Stepped deltas therefore provide a minimum and maximum constraint on the duration and magnitude of some surface flows on Mars. We estimate that the amount of water required to fill the basin and deposit the delta is comparable to the amount of water discharged by large terrestrial rivers, such as the Mississippi. The massive discharge, short timescale, and the associated short canyon lengths favour the hypothesis that stepped fans are terraced delta deposits draped over an alluvial fan and formed by water released suddenly from subsurface storage.     

Activation of old carbon by erosion of coastal and subsea permafrost in Arctic Siberia

The future trajectory of greenhouse gas concentrations depends on interactions between climate and the biogeosphere. Thawing of Arctic permafrost could release significant amounts of carbon into the atmosphere in this century. Ancient Ice Complex deposits outcropping along the ~7,000-kilometre-long coastline of the East Siberian Arctic Shelf (ESAS), and associated shallow subsea permafrost, are two large pools of permafrost carbon, yet their vulnerabilities towards thawing and decomposition are largely unknown. Recent Arctic warming is stronger than has been predicted by several degrees, and is particularly pronounced over the coastal ESAS region. There is thus a pressing need to improve our understanding of the links between permafrost carbon and climate in this relatively inaccessible region. Here we show that extensive release of carbon from these Ice Complex deposits dominates (57?±?2 per cent) the sedimentary carbon budget of the ESAS, the world’s largest continental shelf, overwhelming the marine and topsoil terrestrial components. Inverse modelling of the dual-carbon isotope composition of organic carbon accumulating in ESAS surface sediments, using Monte Carlo simulations to account for uncertainties, suggests that 44?±?10 teragrams of old carbon is activated annually from Ice Complex permafrost, an order of magnitude more than has been suggested by previous studies. We estimate that about two-thirds (66?±?16 per cent) of this old carbon escapes to the atmosphere as carbon dioxide, with the remainder being re-buried in shelf sediments. Thermal collapse and erosion of these carbon-rich Pleistocene coastline and seafloor deposits may accelerate with Arctic amplification of climate warming.     

Detection of carbonates in dust shells around evolved stars.

Carbonates on large Solar System bodies like Earth and Mars (the latter represented by the meteorite ALH84001) form through the weathering of silicates in a watery (CO3)2- solution. The presence of carbonates in interplanetary dust particles and asteroids (again, represented by meteorites) is not completely understood, but has been attributed to aqueous alteration on a large parent body, which was subsequently shattered into smaller pieces. Despite efforts, the presence of carbonates outside the Solar System has hitherto not been established. Here we report the discovery of the carbonates calcite and dolomite in the dust shells of evolved stars, where the conditions are too primitive for the formation of large parent bodies with liquid water. These carbonates, therefore, are not formed by aqueous alteration, but perhaps through processes on the surfaces of dust or ice grains or gas phase condensation. The presence of carbonates which did not form by aqueous alteration suggests that some of the carbonates found in Solar System bodies no longer provide direct evidence that liquid water was present on large parent bodies early in the history of the Solar System.     

Atmospheric carbon dioxide concentrations before 2.2 billion years ago

The composition of the Earth's early atmosphere is a subject of continuing debate. In particular, it has been suggested that elevated concentrations of atmospheric carbon dioxide would have been necessary to maintain normal surface temperatures in the face of lower solar luminosity in early Earth history. Fossil weathering profiles, known as palaeosols, have provided semi-quantitative constraints on atmospheric oxygen partial pressure (pO2) before 2.2 Gyr ago. Here we use the same well studied palaeosols to constrain atmospheric pCO2 between 2.75 and 2.2 Gyr ago. The observation that iron lost from the tops of these profiles was reprecipitated lower down as iron silicate minerals, rather than as iron carbonate, indicates that atmospheric pCO2 must have been less than 10(-1.4) atm--about 100 times today's level of 360 p.p.m., and at least five times lower than that required in one-dimensional climate models to compensate for lower solar luminosity at 2.75 Gyr. Our results suggest that either the Earth's early climate was much more sensitive to increases in pCO2 than has been thought, or that one or more greenhouse gases other than CO2 contributed significantly to the atmosphere's radiative balance during the late Archaean and early Proterozoic eons.     

大气有效发射率与水汽和CO2的纬带关系研究

根据水汽和ＣＯ２的辐射传输和它们的纬带分布观测值，建立了一个大气有效发射率εａ，与大气中的水汽含量Ｗａ和ＣＯ２含量ｃ的半经验公式。该公式对于把水汽和ＣＯ２对气候的温室作用纳入气候模式或地球表层动力学模式十分有用。     

MARSIS radar sounder evidence of buried basins in the northern lowlands of Mars

A hemispheric dichotomy on Mars is marked by the sharp contrast between the sparsely cratered northern lowland plains and the heavily cratered southern highlands. Mechanisms proposed to remove ancient crust or form younger lowland crust include one or more giant impacts, subcrustal transport by mantle convection, the generation of thinner crust by plate tectonics, and mantle overturn following solidification of an early magma ocean. The age of the northern lowland crust is a significant constraint on these models. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on the European Space Agency's Mars Express spacecraft is providing new constraints on the martian subsurface. Here we show evidence of buried impact basins ranging in diameter from about 130 km to 470 km found over approximately 14 per cent of the northern lowlands. The number of detected buried basins >200 km in diameter indicates that the lowland crust is ancient, dating back to the Early Noachian epoch. This crater density is a lower limit because of the likelihood that not all buried basins in the area surveyed by MARSIS have been detected. An Early Noachian age for the lowland crust has been previously suggested on the basis of a large number of quasi-circular topographic depressions interpreted to be evidence of buried basins. Only a few of these depressions in the area surveyed by MARSIS, however, correlate with the detected subsurface echoes. On the basis of the MARSIS data, we conclude that the northern lowland crust is at least as old as the oldest exposed highland crust. This suggests that the crustal dichotomy formed early in the geologic evolution of Mars.