Tag: Gale Crater

Curiosity rover hints at the carbon cycle on Mars

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The Mars Science Laboratory carried by the Curiosity rover is still functioning 10 years after a jetpack lowered Curiosity onto the surface of Gale crater. It includes a system aimed at scooping and drilling samples of soil and rock from the sedimentary strata deposited in the lake that once filled the crater about 3.5 to 3.8 billion years ago. The system on the rover is also capable of analysing the samples in various ways. A central objective of the mission was to obtain data on oxygen and carbon isotopes in carbon dioxide and methane released by heating samples, which uses a miniature mass spectrometer. In early 2022 a paper on Martian carbon isotopes was published in the Proceedings of the National Academy of Sciences (PNAS) that I have only just found (House, C.H. et al. 2022. Depleted carbon isotope compositions observed at Gale crater, Mars. Proceedings of the National Academy of Sciences, v. 119, article e2115651119; DOI: 10.1073/pnas.2115651119). PNAS deemed it to be one of the 12 most important of its articles during 2022.

Oblique view of Curiosity’s landing site in Gale crater on Mars, from which the rover has traversed the lower slopes of Mount Sharp. Credit: NASA-Jet Propulsion Laboratory

Carbon isotopic analyses chart the type and degree of fractionation between carbon’s two stable isotopes 12C and 13C. This is expressed by their relative abundances to one another in a sample and in a reference standard, signified by δ13C. The measure is a natural tracer of both inorganic and biological chemical processes: hence the potential importance of the paper by Christopher House and colleagues from the University of California, San Diego. The thin atmosphere of Mars contains both CO2 and traces of CH4, so a carbon cycle is part and parcel of the planet’s geochemical functioning. The ‘big question’ is: Did that involve living processes at any stage in the distant past and even now? Carbon held in various forms within Mars’s ancient rocks and soils may provide at least a hint, one way of the other. At the very least it should say something about the Martian carbon cycle.

House et al. focus on methane released by heating 22 samples drilled from sandstones and mudstones traversed by Curiosity up a slope leading from the floor of Gale crater towards its central peak, Mount Sharp.  The sampled sedimentary rocks span a 0.5 km thick sequence. Carbon in the expelled methane has δ13C values that range from -137 to +22 ‰ (per mil). Samples from six possibly ancient exposed surfaces were below -70 ‰. This depletion in 13C is similar to the highly negative δ13C that characterises carbon-rich sediments on Earth that were deposited at the Palaeocene-Eocene boundary. That anomaly is suspected to have resulted from releases of methane from destabilised gas hydrate on the sea floor during the Palaeocene–Eocene Thermal Maximum. Organic photosynthesis takes up ‘light’ 12C in preference to 13C, thereby imparting low δ13C to organic matter. In the case of the Mars data that might seem to point to the lake that filled Gale crater 3.5 to 3.8 billion years ago has contained living organisms of some kind. Perhaps on exposed surfaces of wet sediment primitive organisms consumed methane and inherited its δ13C. Some Archaean sediments of about the same age on Earth show similar 13C depletion associated with evidence for microbial mats that are attributed to the activities of such methanotrophs.

Before exobiologists become too excited, no images of possible microbial mats in Gale crater sediments have been captured by Curiosity. Moreover, there are equally plausible scenarios with no recourse to once-living organisms that may account for the carbon-isotope data,. Extreme depletion in 13C is commonly found in the carbon within meteorites, almost certainly inherited from the interstellar dust from which they accreted. It is estimated that the solar system passes through giant molecular clouds every 100 Ma or so: the low δ13C may be inherited from interstellar dust. Alternatively, because Mars has an atmosphere almost entirely composed or CO2 – albeit thin at present – various non-biological chemical reactions driven by sunlight or electrically charged particles may have reduced that gas to form methane and other compounds based on C-H bonds. Carbon dioxide still in Mars’s atmosphere is highly enriched in 13C, suggesting that earlier abiotic reduction may have formed 13C-depleted methane that became locked in sediments. Yet such an abundant supply of inorganic methane may have encouraged the evolution of methanotrophs, had life emerged on early Mars. No one knows …

It’s becoming a cliché that, ‘We may have to await the return of samples from the currently active Perseverance rover, or a crewed mission at some unspecifiable time in the future. The Curiosity carbon-isotope data keep the lamp lit for those whose livelihoods have grown around humans going to the Red Planet.

A first for geochronology: ages from Mars

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Remote sensing, including mapping of topographic elevation, and the recent exploits of three surface vehicles – the Spirit, Opportunity and Curiosity Rovers – have provided lots of data for a host of geological interpreters. Producing a time frame for Martian geological and geomorphological events has, understandably, been limited mainly to the use of stratigraphic principles. Various rock units and surface features can be placed in relative time order through simple stratigraphic principles, such as what sits on top of what and which features cut through pre-existing rock units or are masked by them. The most important guide up to now has been interpretation of the relations between impact craters and both rock units and other geomorphological features. The Inner Planets are assumed to have recorded the same variation through time of the frequency and energies of bombardment, and that has been calibrated to some extent by radiometric dating of impact-related rocks returned from the Moon by the crewed Apollo missions. Some detail of relative timings also emerge from some craters cutting earlier ones. The only other source of Martian ages has been from rare meteorites (there are only 114 of them) whose stable isotope compositions are different from those of terrestrial rocks and more common meteorites. By a process of elimination it is surmised that they were flung from Mars as a result of large impacts in the past to land eventually on Earth. The oldest of them date back to 4.5 Ga, much the same as the estimated age of the earliest crystallisation of magmas on Earth.

MOLA colorized relief map of the western hemis...
Colorised relief map of the western hemisphere of Mars, showing Valles Marineris at centre and the four largest volcanoes on the planet (credit: Wikipedia)

But all Martian stratigraphy is still pretty vague by comparison with that here, with only 4 time divisions based on reference to the lunar crater chronology and 3 based on evidence from detailed orbital spectroscopy and Rover data about the alteration of minerals on the Martian surface. Apart from meteorite dates there is very little knowledge of the earliest events, other than Mars must have had a solid, probably crystalline crust made of mainly anhydrous igneous minerals. This was the ‘target’ on which much of the impact record was impressed: by analogy with the Moon it probably spanned the period of the Late Heavy Bombardment from about 4.1 to 3.7 Ga, equivalent to the Eoarchaean on Earth. That period takes its name – Noachian – from Noachis Terra (‘land of Noah’), an intensely cratered, topographically high region of Mars’s southern hemisphere, whose name was given to this large area of high albedo by classical astronomers. Perhaps coincidentally, the Noachian provides the clearest evidence for the former presence of huge amounts of water on the surface of Mars and its erosional power that formed the gigantic Valles Marineris canyon system. The rocky surface that the craters punctured is imaginatively referred to as the pre-Noachian. A major episode of volcanic activity that formed Olympus Mons and other lava domes is named the Hesperian (another legacy of early astronomical nomenclature). It is vaguely ascribed to the period between 3.7 and 3.0 Ga, and followed by three billion years during which erosion and deposition under hyper-arid conditions formed smooth  surfaces with very few craters and rare evidence for the influence of surface water and ice. It is named, inappropriately as it turns out, the Amazonian.

Remote sensing has provided evidence of  episodes of mineral alteration. Clay minerals have been mapped on the pre-Noachian surface, suggesting that aqueous weathering occurred during the earliest times. Sulfates occur in exposed rocks of early Hesperian age, suggesting abundant atmospheric SO2 during this period of massive volcanicity. The last 3.5 billion years saw only the development of the surface iron oxides whose dominance led to Mars being nickname the ‘Red Planet’.

Curiosity Rover's Self Portrait at 'John Klein...
A ‘selfie’ of Curiosity Rover drilling in Gale Crater (credit: Euclid vanderKroew)

A recent paper (Farley, K.A. and 33 others plus the entire Mars Science Laboratory 2014. In Situ Radiometric and Exposure Age Dating of the Martian Surface. Science, v. 343, online publication DOI: 10.1126/science.1247166) suggests that radiometric ages can be measured ‘in the field’, as it were, by instruments carried by the Curiosity rover. How is that done? Curiosity carries a miniature mass spectrometer and other analytical devices. Drilling a rock surface produces a powder which is then heated to almost 900°C for half an hour to drive off all the gases present in the sample. The mass spectrometer can measure isotopes of noble gases, notably 40Ar, 36Ar, 21Ne and 3He. Together with potassium measured by an instrument akin to and XRF, the 40Ar yields a K-Ar age for the rock. A sample drilled from a fine-grained sedimentary in Gale Crater gave an age of 4.2 Ga, most likely that of the detrital feldspars derived from the ancient rocks that form the crater’s wall, rather than an age of sedimentation. The values for 36Ar, 21Ne and 3He provide a means for establishing how long the rock has been exposed at the surface: all three isotopes can be generated by cosmic-ray bombardment. The sample from Gale Crater gave an age of about 78 Ma that probably dates the eventual exposure of the rock by protracted wind erosion.

By themselves, these ages do not tell geologists a great deal about the history of Mars, but if Curiosity makes it through the higher levels of the sediments that once filled Gale Crater – and there is enough power to repeat the mass spectrometry at other levels – it could provide a benchmark for Noachian events. The exposure age, interesting in its own right, also suggests that sediments in the crater have not been exposed to cosmic-ray bombardment for long enough to have destroyed any organic materials that the science community longs for.

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The Martian ‘sexy beast’

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Artist’s concept of NASA’s Mars Science Laboratory (Curiosity) near a canyon on Mars. (Credit: NASA-JPL via Wikipedia)

Why is ’Curiosity’ the latest Mars rover aimed to land at Gale Crater? It seems to have been filled with stratified sediments deposited in the crater over perhaps as long as two billion years after it formed by a meteorite impact. The sediments now occur as a relic of later aeolian erosion at the centre of the crater in the form of a large mound that Curiosity is designed to climb and sample. The big attraction is the detection of clays and sulfate minerals in the sediments using multispectral remote sensing. They clearly suggest the influence of water in the formation of the sediments, hence the suggestion that they are lake sediments. On that assumption, Gale Crater is hoped to be a fruitful site for seeking signs of former biological processes: given the technical circumstances of the mission it is deemed the best site there is on Mars for NASA’s Mars Science Laboratory.

Sulfates on Mars have excited geologists enormously, along with their companion clays, because they signify the influence of abundant acid water in the breakdown of Martian primary igneous rocks from which the sediments have undoubtedly been derived. Their formation is undoubtedly the geoscientific ‘sexy beast’ of the last four or five years. Given multi-channel remotely sensed data – and Mars labs are awash with them from several previous missions – sulfates are easy to detect from their distinctive reflectance spectra so there has been abundant pay-back for geologists involved with the Red Planet. But there is water and there is…water. It is hoped to be proved that the depositional medium was standing water or at least abundant subsurface aqueous fluids, which may have lingered for long enough for living organisms to have formed. But there is a possibility that sulfates can form, and so too clays, by superficial weathering processes beneath a humid atmosphere.

English: This oblique, southward-looking view ...
An oblique view of Gale crater showing the landing site and the mound of layered rocks that NASA’s Curiosity rover will investigate. The landing site is outlined in yellow. (Credit: NASA-JPL via Wikipedia)

Erwin Dehouck and  team of French geochemists set out experimentally to recreate conceivable atmospheric and climatic conditions from Mars’s early history to mimic weathering processes (Dehouck, E. et al. 2012. Evaluating the role of sulfide-weathering in the formation of sulfates or carbonates on Mars. Geochimica et Cosmochimica Acta, v. 90, p. 47-63). The experiment involved liquid water and hydrogen peroxide (detected in Mars’s present atmosphere and probably produced photochemically from water vapour) in contact with a CO2 atmosphere.  Martian surface conditions were simulated by evaporation of H2O and H2O2 to mix with dominant CO2, which allowed ‘dew’ to form on the experimental samples. The samples consisted of ground up olivine and pyroxene, important mineral constituents of basalt – feldspar was not used. – mixed with the iron sulfide pyrrhotite, commonly found in terrestrial basalts and meteorites judged to have come from Mars. Samples of each pure mineral and mixtures with the sulfide were left in the apparatus for four years and then analysed in detail.

Even in such a short exposure the silicate-sulfide mixtures reacted to produce sulfate minerals –hexahydrite (MgSO4_6H2O), gypsum (CaSO4_2H2O) and jarosite( KFe3 (OH)6(SO4)2), together with goethite (FeOOH) and hematite (Fe2O3). Without the presence of sulfides, the silicate minerals barely broke down under the simulated Martian conditions but did produce traces of magnesium carbonate. The sulfate bearing assemblages look very like those reported from many locations on Mars. The acid conditions produced by weathering of sulfides to yield sulfate ions are incompatible with preservation of carbonates, as the experiment indicates. However, there are reports of Martian sediments that do contain abundant carbonate minerals.

The researchers’ conclusions are interesting: “These results raise doubts on the need for a global acidic event to produce the sulfate-bearing assemblages, suggest that regional sequestration of sulfate deposits is due to regional differences in sulfide content of the bedrock, and pave the way for reevaluating the likelihood that early sediments preserved biosignatures from the earliest times”. Weathering by dew formation seems quite adequate to match existing observations.

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