Tag: Mars Science Laboratory

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.

Are Martian clays magmatic in origin?

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Artist’s Concept of Curiosity’s touchdown(credit: Wikipedia)

The remote detection of spectral features in the infrared that suggest abundant clay minerals on the surface of Mars is the basis for a widely-held view that Mars may once have had moist climatic conditions that encouraged life to form (see The Martian ‘sexy beast’ in September 2012  EPN). The presence of clays, along with suggestive landforms, has also been used to speculate that Mars once harboured long-lived lakes and perhaps even a huge ocean on its northern hemisphere, between 3.7 to 4.1 Ga. It was the clays that pitched the recently arrived Curiosity (aka Mars Exploration)Rover at the Gale crater and its central Aeolis Mons. The latter, also known as Mount Sharp, preserves about 5 km of layered rocks, the lowest of which are clay-rich and hypothesised to be sediments laid down in a lake that filled the crater. Provided Curiosity operates according to plan, we will know soon enough whether or not the layered rocks of Mount Sharp are indeed sediments, but a soon-to-be-published article suggests another explanation than weathering for the production of abundant clay minerals on Mars (Meunier, A. et al. 2012. Magmatic precipitation as a possible origin of Noachian clays on Mars. Nature Geoscience, published online 9 September 2012; DOI: 10.1038/NGEO1572).

Focusing the 100-millimeter Mastcam [detail]
Layered rocks on the flanks of Mount Sharp in Gale crater from Curiosity’s Mastcam (NASA Goddard via Flickr)
The French-US team provides evidence from terrestrial lavas that abundant iron- and magnesium-rich clays, known as smectites, may form at a late stage during crystallization of magma. If magma contains water – and most magmas do – as more and more anhydrous silicates crystallise during cooling water builds up in the remaining liquid. Once silicate crystallisation is complete there remains a watery fluid capable of reacting with some of the silicates to form clay minerals; a process often referred to as pneumatolysis. How much clay is formed depends on the initial water content of the magma. Pneumatolysisoperates on hot lava, whereas weathering occurs at ambient temperature provided the climate is able to support liquid water at the surface. Mars is currently far too cold for that, and ideas of a wet surface environment earlier in the planet’s history demand an explanation for a much warmer climate. Clay minerals do not appear to be present in Mars’s younger rocks, so Meunier and colleagues suggest that as the planet’s mantle evolved early water-rich magmas were gradually replaced by ones with less water: interior Mars was gradually de-gassed and its magmas lost the ability to alter minerals that crystallised from them.

Now, clay minerals are extremely resistant to change except through high-temperature metamorphism. Once formed they can be blown around – Mars has probably always been a very windy place – to end up in aeolian sediments that are plentiful on Mars.  Also, if occasionally water flowed on the surface perhaps by subsurface water venting suddenly, fine-grained pneumatolytic clays would easily be picked up, concentrated as flow speed lessened and deposited in waterlain sedimentary layers.  A dilemma that faces the Curiosity science team is what significance to assign to clays in sediment layers, when they no longer provide unequivocal evidence of weathering.  But will the resistant layers on Mount Sharp turn out to be pneumatolytically altered lava flows?
Note added 28 September 2012: The first scientific triumph of the Curiosity Rover is imagery of sediments in what had been suggested to be an alluvial fan washed into Gale crater. They show gravels with rounded pebbles.

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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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