Investigating the potential for pseudofossil formation in Earth's ancient sediments.

The earliest evidence for life on Earth comes in two forms; chemical evidence, i.e. compounds believed to have been derived from the activities of living organisms and isotopic signals thought to be indicative of biological activity, and through the presence of structures believed to be microfossils in ancient sedimentary deposits. In both cases, identifying these with confidence requires considerable skill, as there remains a danger that abiotic processes might have achieved the same results. One of these dangers is the potential formation of pseudofossils (structures which resemble fossils but are of non-biological origin). The most ancient structures interpreted as being microfossils take two forms, microbial filamentous and spherical cells, but structures morphologically similar to both of these have been synthesised on the lab by non-biological means, raising the possibility that none of these ancient 'fossils' are actually of biological origin. The researchers were able to recreate these strucutures, also known as organic biomorphs, by oxidising sulphides in the presence of organic materials, under which circumstances they formed spontaneously. This has been shown to be possible under a wide range of conditions likely to have been present on the ancient Earth, and with a wide range of organic compounds serving as precursors.

However, the fact that such biomorphs can be formed in the laboratory does not automatically prove that all, or indeed any, of the various Archaean and Palaeoproterozoic fossil deposits are in fact pseudofossils; indeed some of these have been studied for many years and their status as being of biological origin is not really in doubt. Nevertheless, some caution is clearly needed when establishing the nature of any apparent fossils in these ancient deposits, particularly if their host rocks are sulphide-rich.

In the oceans of the Proterozoic, euxinic (low oxygen, high sulphur) conditions are thought to have been fairly common, and this may have also have sometimes been the case in the Archaean, at least at a local level, although ocean sulphide levels appear to have been low for the most part. Many of the microfossils known from these deposits are associated with pyrite, which indicates the deposits which produced them did indeed have raised sulphur levels. However, there is not currently any clear data on the likelihood of biomorphs being preserved in these deposits.

In a paper published in the journal Geology on 28 January 2021, Christine Nims and Julia LaFond of the Department of Geosciences at Pennsylvania State University, Julien Alleon of the Institut des Sciences de la Terre at the Université de Lausanne, Alexis Templeton of the Department of Geological Sciences at the University of Colorado, Boulder, and Julie Cosmidis, also of the Department of Geosciences at Pennsylvania State University, describe the results of an experiment in which they performed experimental silicification of organic biomorphs along with the Sulphur Bacterium Thiothrix, in order to assess the likelihood of their becoming preserved in the fossil record as pseudofossils.

 

Side-by-side comparison of Precambrian putative organic microfossils and organic biomorphs synthesized in the laboratory. (A) Organic strand from the 3.5 billion-year-old Dresser Formation (Western Australia). OM—organic material; Py—pyrite. (C), (G) Cluster of spheres (C) and 'straw-like' filaments (G) from the 2.4–2.2 billion-year-old Turee Creek Group (Western Australia). Spheres in panel (C) inset are from the 3.4 billion-year-old Strelley Pool Formation (Western Australia). (E), (I) Rosette (E) and cluster of filaments (I) from the 1.9 billion-year-old Gunflint Formation (northeastern North America). (K), (M), (O) Rigid branching filaments (K), 'river' of flexible filaments (M), and cobweb-like network of filaments (O) from the 2.4–2.2 billion-year-old Turee Creek Group. (B), (D), (F), (H), (J), (L), (N), & (P) Organic biomorphs synthesized in the laboratory. Nims et al. (2021).

As the majority of putative Precambrian microfossils are preserved in chert (fine-grained sedimentary rock composed of microcrystalline crystals of quartz), Nims et al. decided to investigate the preservational potential of organic biomorphs through silicification. They also carried out the same experimental procedures on mats of the sulphur-oxidising Bacterium Thiothrix as a control measure; experiments on the silicification of colonial micro-organisms have been undertaken before, but the majority of these have been performed on Cyanobacteria, despite many putative Precambrian microfossils being interpreted as most likely being sulphur-cycling organisms. Nims et al. felt that Thiothrix would be a good analogue for these organisms, as it forms intracellular sulphur globules.

Nims et al. obtained organic biomorphs by reacting dissolved sulphides with yeast extract (which contains a variety of complex organic compounds) in a sterile environment. Both the biomorphs and the Thiothrix mats were then placed into a supersaturated sodium-metasilicate solution, then stored for up to five months at room temperature. Samples were taken from these experiments at regular intervals, and examined using scanning electron microscopy and transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, Raman spectromicroscopy, attenuated total reflectance–Fourier transform infrared spectroscopy, X-ray absorption near-edge structure at the sulphur K-edge, and scanning transmission X-ray microscopy at the carbon K-edge, nitrogen K-edge, and sulphur L-edge.

The biomorphs produced were a mixture of spheres (0.5–3 μm in diameter) and filaments (0.1–1 μm thick). These generally retained their shape during the silicification process; whereas the spherical Thiothrix cells rapidly lost their shape, first becoming elongate, then deforming into flattened ribbons. As non-silicified Thiothrix cells retained their shape. Nims et al. presume this flattening to be a result of the silicification process, rather than the microscopy preparation. In contrast, the silicification process appeared to have little impact upon the shape of the organic biomorphs, other than a small degree of fragmentation in some of the filamentous specimens. In fact, the biomorphs were preserved very rapidly, by the precipitation of nano-colloidal silica on their surfaces, which formed a thin crust around each specimen, preserving it from any further degredation. The Thiothrix cells, in contrast, quickly became covered in a thick silica-gel, possibly due to the presence of extracellular polymeric substances around the Bacterial cells, which may have caused the silica to behave in a different way, or possibly due to the metabolic activity of the cells, which initially reduced the pH of the surrounding media, enabling an increase in the silica saturation, whereas the pH around the biomorphs remained constant at about 7.

 

Scanning electron microscopy images of organic biomorphs (A)–(D) and Sulphur Bacterium Thiothrix cells (E)–(H) prior to and at different times throughout silicification. Note the silica nano-colloids at the surfaces of spherical biomorph in (B) and of Thiothrix filaments in (F). Nims et al. (2021).

During the silification process, the discrete globules of elemental sulphur present within the cells of Thiothrix broke up, with the sulphur becoming diffused out of the cells into the surrounding medium. Sulphur also diffused out of the spherical biomorphs, leaving empty organic vesicles, but here it subsequently re-precipitated along the envelope of the silicified biomorphs, as both sulphur and oxidised sulphur forms (such as sulphate, thiosulphate, and/or sulphones and ester sulphates).

Nims et al. suggest it is likely that the diffusion of sulphur from both the Thiothrix cells and the organic biomorphs was caused by solubilisation as polysulphides, which are highly reactive toward organics, causing rapid organic-matter sulphurisation. This may be the cause of the formation of the sulphur-rich organic envelopes around the organic biomorphs. Attenuated total reflectance–Fourier transform infrared spectroscopy showed that sulphur-bearing groups such as sulphates and sulphones were forming during silification, although it was not possible to confirm the incorporation of intramolecular sulphur into the biomorphs. Early digenetic sulphurisation would favour the preservation of microstructures in the rock record, so establishing whether this actually happens with the organic biomorphs would be a major step in establishing their preservation potential.

 

High-resolution imaging and chemical mapping of organic biomorphs and Sulphur Bacterium Thiothrix cells throughout silicification. (A)–(F) High-angle annular dark field–scanning transmission electron microscopy images and corresponding energy-dispersive X-ray spectroscopy maps of biomorphs prior to silicification (A), (B) and two weeks into silicification (C)–(F). (G)–(L) High-angle annular dark field–scanning transmission electron microscopy images and corresponding energy-dispersive X-ray spectroscopy maps of Thiothrix cells prior to (G), (H) and two months into (I)–(L) silicification. Energy-dispersive X-ray spectroscopy maps show distribution of sulphur in yellow and silica in cyan, except in (H), where carbon is in blue and sulphur in red. Nims et el. (2021).

If sulphur was lost from the organic biomorphs during silicification, then the result would be organic microstructures in the chert which did not contain sulphur-bearing minerals. However, if they were being preserved in an iron- and sulphur-rich environment, we would expect to see the formation of pyrites close to the preserved biomorph structures, something which is commonly observed in cherts which host ancient organic microfossils.

Prior to silicification, the composition of the biomorphs was dominated by carboxylic groups and unsaturated carbon, along with aliphatics, alcohols, and carbon-oxygen groups. During the silicification process, the proportions of aliphatic, aromatic, and unsaturated carbon compounds rose, while those of other organic compounds fell. In Thiothrix cells the composition was dominated by amide groups (the major component of proteins) both prior to and during silicification. It has previously been established that if peptides are present in the synthesis medium, then amides can also be incorporated into organic biomorphs. Nims et al. did not detect any amide groups in the biomorphs, despite these having been formed using yeast extract, which contains peptides. However, nitrogen was found in the envelopes of the biomorphs, in an unidentified inorganic or organic form. Thus, the carbon and nitrogen species present in the biomorphs and Thiothrix were quite different.

The initial carbon/nitrogen ratio of the biomorphs was 0.27, which rose to 0.40 during the silicification process, whereas that of the Thiothrix cells started at 0.16 and rose to 0.42, i.e. a similar value to that of the biomorphs. It is unclear how this proportion would be affected by subsequent high-temperature and pressure diagenesis, although it is possible that this might result in some detectable difference in the carbon/nitrogen ratio of organic biomorphs and Bacterial cells developing.

 

Scanning transmission X-ray microscopy analyses of organic biomorphs and Sulphur Bacterium Thiothrix cells prior to and one week into silicification. (A)–(H) Scanning transmission X-ray microscopyimages and corresponding scanning transmission X-ray microscopy chemical maps of biomorphs (A)–(D) and Thiothrix (E)–(H). Pre-silicification images (A) and (E) show dense sulphur spheres inside spherical biomorphs and Thiothrix cells. Scanning transmission X-ray microscopy maps (B), (D), (F), (H) show distribution of carbon (red), nitrogen (green), and sulphur (blue; in (F) only). (I) X-ray absorption near-edge structure spectra covering carbon (C) and nitrogen (N) K-edges, and calculated N/C ratios. Black rectangle shows the carbon K-edge spectral range (closeup in (J)). Black curves show the fitting functions for N/C ratio calculations. Spectrum of silicified Thiothrix includes a feature at ∼350 eV, corresponding to calcium. (J) Closeup of carbon K-edge X-ray absorption near-edge structure spectra. Energies of the main absorbance features are indicated. Nims et al. (2021).

Nims et al. conclude that organic biomorphs can form via the reaction of sulphides with organic compounds, and are highly likely to be preserved as pseudofossils in chert by the process of rapid silica encrustation, possibly in combination with the sulphurisation of organic matter. Such pseudofossils would not only be extremely similar to fossils produced by Bacteria, or similar Prokaryotes, they would also have similar chemical characteristics. Indeed, such organic biomorphs might actually have better preservational potential than actual microbes.

None of this proves that any Precambrian microfossil assemblage is in fact made up of pseudofossils, but it certainly suggests that there is a possibility for pseudofossil assemblages to exist in these ancient rocks, and indicates that a degree of caution must be used before such assemblages are accepted as being of biological origin. In particular, specimens with an apparently Bacterial morphology and chemical composition will need to be viewed with some caution, particularly if they originate from deposits interpreted as having been laid down in high-sulphur environments. Nims et al. strongly feel that more work must be done to identify possible geochemical signatures which might indicate specimens are of non-biological origin. 

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Ingredients for life found in 3.5 billion-year-old fluid inclusions from Pilbara Craton, Western Australia.

Primeval Microbes likely required small organic molecules to act as building blocks for biomass and as catabolic substrates for heterotrophic metabolism. A potential source of such compounds includes recycled and redistributed organic matter from pre-existing biomass. In addition, ample exogenous organic matter probably had been delivered to the early Earth by interplanetary dust particles and meteorites. Experiments have also shown that organic molecules relevant for primordial life can be formed by synthesising organic compounds from inorganic atmospheric gases. As important, endogenous synthesis and processing of organic molecules could have occurred in marine and terrestrial (i.e. hot spring) hydrothermal environments. In such settings, organic molecules may form, or react, at elevated temperatures and pressures within the steady flow of inorganic hydrothermal chemistry (e.g. hydrogen sulphide, carbon dioxide, molecular hydrogen). One hypothesis on organic synthesis at hydrothermal sites suggests that the reaction of iron(II) sulphide to pyrite with hydrogen sulphide drives the reduction of carbon dioxide to organic molecules. Moreover, a primordial carbon fixation mechanism involving the reaction of carbon monoxide with methanethiol on catalytic metal (nickel or iron) sulphide surfaces could be demonstrated in the laboratory under hydrothermal conditions. This experiment produced an activated form of acetic acid that represents a plausible building block for further organic synthesis, for example, into acyl lipids. As yet, however, such distinctive organic molecules have not been found in rocks that directly testify to the emergence of life on our planet.

The roughly 3.5 billion-year-old Dresser Formation (Pilbara Craton, Western Australia) is one of the most important windows into hydrothermal habitats on early Earth. The rocks are only mildly metamorphosed (prehnite-pumpellyite to lower greenschist facies) and still preserve numerous putative biosignatures, including Stromatolites, microfossils, and isotopic anomalies. Further, cherts and barites of the Dresser Formation contain kerogenous organic material of supposedly biological origin. Detailed field mapping, petrographic observations, and mineralogical analyses revealed that the Dresser Formation was formed in a hydrothermal setting, most likely a volcanic caldera. Thus, it appears plausible that organisms in the Dresser environments grew chemotrophically, fuelled by hydrothermal fluids that delivered inorganic and organic substrates. Indeed, stable carbon and sulphur isotopic anomalies indicate methanogenic and sulphur-disproportionating Microbes as key players in these early Microbial communities, although the exact metabolisms still await further evidence and testing.

Cherts and barites of the Dresser Formation contain abundant primary fluid inclusions, that is, fluids and/or gases entrapped in minerals. These fluid inclusions represent a valuable archive, as their chemistry can potentially be preserved for billions of years. Barite appears to be a particularly robust host mineral because of its low solubility and high stability under a wide range of pressure, temperature and redox conditions. Therefore, fluid inclusions in the Dresser barites are excellent candidates in the search for organic molecules that once supported Microbial life. Previous work identified water, carbon dioxide, hydrogen sulphide, and minor methane as the main inorganic constituents of the fluid inclusions in Dresser barites. However, the content of organic molecules, potential key ingredients for early life, is as yet unknown.

In a paper published in the journal Nature Communications on 17 February 2021, Helge Mißbach of Geobiology at the University of Göttingen, and Geobiology at the University of Cologne, Jan-Peter Duda, also of Geobiology at the University of Göttingen, the 'Origin of Life' Group at the Göttingen Academy of Sciences and Humanities, and Sedimentology & Organic Geochemistry at the University of Tübingen, Alfons van den Kerkhof of Applied Geology at the University of Göttingen, Volker Lüders of the GFZ German Research Centre for Geosciences, Andreas Pack of the Isotope Geology Divison at the University of Göttingen, Joachim Reitner, also of Geobiology at the University of Göttingen, and the 'Origin of Life' Group at the Göttingen Academy of Sciences and Humanities, and Volker Thiel, once again of Geobiology at the University of Göttingen, report on the presence of biologically-relevant primordial organic molecules in primary fluid inclusions trapped in barites of the roughly 3.5 billion-year-old Dresser Formation. To explore the full range of volatiles, Mißbach et al. combined gas chromatography–mass spectrometry, microthermometry, fluid inclusion petrography, and stable isotope analysis. Their findings reveal an intriguing diversity of organic molecules with known or inferred metabolic relevance and provide a strong clue as to how ancient hydrothermal fluids sustained Microbial life about 3.5 billion years ago.

The Dresser Formation contains thick barite units with colours ranging from white and grey to black. Black barites exhibit coarse crystalline textures and yield a strong hydrogen sulphide odour when freshly crushed. The sedimentary black barite studied here was sampled in the Dresser mine, where it was interbedded with originally sulphidic Stromatolites. Field and petrographic evidence clearly suggest a primary origin of the barite (e.g. no progressive replacement of stromatolite interbeds, no relicts of potential precursor materials within the barite). Thin section analysis revealed the presence of abundant primary and rare secondary inclusions. Most primary fluid inclusions are small (about 10 μm), translucent, and often oriented parallel to planes of barite crystals, thereby tracing succeeding growth phases. Morphologies of some fluid inclusions indicated necking down, which is a typical modification under stress conditions after crystallisation. These inclusions are typically stretched and may split up in segments that then usually show different composition and density.

 

Study area and field evidence. Location of the Dresser mine in Western Australia near Marble Bar (a) and black barite associated with originally sulphidic Stromatolites at the sampling site (b) and in the working area (c). The close association between the inclusion-bearing black barites and Stromatolites suggests that hydrothermal fluids might have influenced ancient microbial communities. Mißbach et al. (2021).

The fluid inclusions were analysed optically on a heating-freezing stage and by Raman spectroscopy. The black barites contain aqueous carbonic-sulfuric and non-aqueous carbonic-sulfuric fluid inclusions (hereafter, aqueous, and non-aqueous, respectively). Aqueous inclusions show highly variable water volume fractions of 0.1–1. At room temperature, they typically exhibit a double meniscus, indicating the presence of three phases: water + another (carbon dioxide–hydrogen sulphide-rich) liquid + vapour. In some cases, the other liquid is only visible during cooling runs. In comparison, non-aqueous fluid inclusions usually contain a carbon dioxie–hydrogen-sulphide-rich liquid and a vapour phase, although the liquid phase is sometimes absent at room temperature.

 

Fluid inclusions in representative black barites from the Dresser mine. (a), (b) Thin section images (reflected light) showing primary fluid inclusion trails parallel to barite crystal growth bands (marked by black arrows). (c) Thin section image (transmitted light) showing primary fluid inclusions which are dispersed or oriented parallel to barite crystal growth bands (exemplified by dashed line). The image also shows a minor secondary inclusion trail (marked by black arrow). (d) Thick section image (transmitted light) of an aqueous carbonic-sulfuric fluid inclusion containing three volatile phases (including hydrogen sulphide), plus pyrite, native sulfur, and strontianite as solid phases. (e) Thick section image (transmitted light) of a non-aqueous fluid inclusion bearing a vapour phase, native sulphur, and kerogen. These fluid inclusions are usually rich in hydrogen sulphide. V vapour/gas, Lw liquid water, L other liquid (e.g. CO2carbon dioxide). Organic compounds and gases preserved in these primary fluid inclusions could have provided a substrate to primordial microbial life in the Dresser Formation. Mißbach et al. (2021).

Both types of fluid inclusions typically contain solid daughter phases. Aqueous inclusions usually contain strontianite and sulphur as daughter crystals. Varieties with pure carbon dioxide in the vapour phase (volume fractions of about 0.9) may additionally include anatase, pyrite, and possibly also halite. In non-aqueous inclusions, typical daughter phases are sulphur, kerogen and, in few cases, halite.

The main gas components in both fluid inclusion types are carbon dioxide and hydrogen sulphide, accompanied by minor amounts of methane, nitrogen, and carbonyl sulphide. Aqueous fluid inclusions contain less hydrogen sulphide than non-aqueous fluid inclusions (0–24% by molarity and 21–36% by molarity, respectively). Furthermore, aqueous fluid inclusions typically enclose up to 1% by molarity nitrogen, which is not present in non-aqueous fluid inclusions. Instead, non-aqueous fluid inclusions additionally contain small amounts of methane (less than 2% by molarity).

 

Gas compositions of fluid inclusions in black barites as measured by Raman analysis. 0 % hydrogen sulphide (H₂S)  implies that the fluid inclusion largely contains carbon dioxide (CO₂). FI fluid inclusions. Mißbach et al. (2021).

Aqueous fluid inclusions typically reveal liquid compositions ranging from pure water to more saline solutions with 14% by weight sodium chloride-equivalents. Higher salinities of up to 25% by weight sodium chloride-equivalents are rare. The corresponding ice melting temperatures vary between 0°C and −26°C (peak at −7 °C). Aqueous fluid inclusions form clathrates upon freezing and subsequent melting between 7°C (pure carbon dioxide) and 20°C (hydrogen sulphide-rich). Total homogenisation temperatures, describing the minimum temperature of fluid entrapment, range from 100 to 195°C, with a maximum between 110 and 150°C. Most fluid inclusions decrepitate at temperatures of over 230°C.

 

Bar plots showing phase transition temperatures. From top to bottom: (i) homogenisation of the non-aqueous phase (total homogenisation temperature carbon dioxide-hydrogen sulphide), (ii) melting of carbon dioxide and hydrogen sulphide (melt temperature carbon dioxide-hydrogen sulphide), (iii) ice and clathrate melting (melt temperature ice/(melt temperature ice, metastable, and melt temperature clathrates, respectively), (iv) total homogenisation and decrepitation (total homogenisation and total decrepitation, respectively) temperatures. C, critical; V, vapour, L, liquid. Mißbach et al. (2021).

Non-aqueous fluid inclusions show total homogenisation temperatures (carbon dioxide-hydrogen sulphide) between 16 and 38°C. Those containing higher concentrations of hydrogen sulphide typically homogenise at the higher end of this range, that is, above the critical temperature of carbon dioxide (31.1°C). Phases usually homogenise to liquid, and only rarely to the gas or critical phase. During cooling runs, the subsequent melting of solid carbon dioxide and hydrogen sulphide can be observed at lower temperatures compared to the pure compounds (−56.6°C and −83.6°C, respectively).

Mißbach et al.'s data demonstrate that the majority of aqueous and nonaqueous inclusions formed during crystal growth (i.e. primary inclusions). Thus, fluids must have been immiscible at the time of encapsulation, and experienced identical trapping and homogenisation temperatures (i.e. heterogeneous trapping). Therefore, no pressure correction is necessary.

Online analyses of black barite fragments using thermal decrepitation-gas chromatography–mass spectrometry yielded high amounts of carbon dioxide, hydrogen sulphide, and water, thus confirming results from Raman analysis on fluid inclusions. The diversity and intensity of compounds was considerably higher in the 250°C than in the 150°C experiment. This finding is consistent with the microthermometry data revealing that most fluid inclusions remain intact up to about 230°C.

 

Total ion current chromatogram of volatile compounds from black barite fluid inclusions as detected by thermal decrepitation/desorption thermal decrepitation-gas chromatography–mass spectrometry analysis at 250 C. Inserts (a), (b) represent enlargements of respective areas in the chromatogram marked by dashed lines. Triangles denote oxygen-bearing compounds, circles denote aromatic hydrocarbons and stars denote sulfur-bearing compounds. n-Hexane (Hex) was used as a retention time standard (RT std.). COS carbonyl sulphide, Ea ethanal, MT methanethiol, Bu but-1-ene, Pa prop-2-enal, Pa’ propanal, ET ethanethiol, MSM (methylsulfanyl)methane, Po propan-2-one, Ba but-2-enal, Ox oxolane, TP thiophene, B benzene, Ac acetic acid, TL thiolane. Note the presence of methanethiol and acetic acid, the stable building blocks of activated acetic acid. Mißbach et al. (2021).

Offline analysis using solid phase micro extraction-gas chromatography–mass spectrometry revealed numerous organic molecules containing oxygen (aldehydes, ketones, acetic acid, oxolane) and/or sulphur (thiophene, thiols, organic polysulphanes), along with some aromatic hydrocarbons (e.g. benzene, alkylbenzenes). Compounds detected with both analytical techniques showed a lower abundance in olid phase micro extraction-gas chromatography–mass spectrometry as compared to thermal decrepitation-gas chromatography–mass spectrometry at 250°C. On the other hand, solid phase micro extraction-gas chromatography–mass spectrometry yielded a considerably greater diversity of compounds, especially in the higher molecular weight range. The absence of carbon doxide and hydrogen sulphide in the solid phase micro extraction-gas chromatography–mass spectrometry runs is due to an analytical bias, as these compounds do not adsorb onto the solid phase micro extraction fibre.

 

Total ion current chromatogram of volatile compounds from black barite fluid inclusions as obtained by solid phase micro extraction solid phase micro extraction-gas chromatography–mass spectrometry. Inserts (a)–(c) represent enlargements of respective areas in the chromatogram marked by dashed lines. Triangles denote oxygen-bearing compounds, circles denote aromatic hydrocarbons and stars denote sulphur-bearing compounds. n-Hexane (Hex) was used as a retention time standard (RT std.). COS carbonyl sulfide, Ea ethanal, MT methanethiol, Pa prop-2-enal, Pa’ propanal, ET ethanethiol, MSM (methylsulfanyl)methane, Po propan-2-one, Ba but-2-enal, Ox oxolane, Bo butan-2-one, TP thiophene, B benzene, Ac acetic acid, MB 3-methylbutan-2-one, Mxp 1-methoxypropan-2-ol, Pe pentanal, MDSM (methyldisulfanyl)methane, To toluene, MP 4-methylpentan-2-one, Ha hexanal, MEDS (methyldisulfanyl)ethane, Xy I p-xylene, Xy II m-xylene, Pac 1-methoxyprop-2-yl acetate, Xy III o-xylene, Sty styrene, CH cyclohexanone, Hp heptanal, BA benzaldehyde, MTSM (methyltrisulfanyl)methane, TMB I 1,3,5-trimethyl benzene, TMB II 1,2,4-trimethyl benzene, MH 6-methylheptan-3-one, TMB III 1,2,3-trimethyl benzene. Note the higher diversity of compounds as compared to thermal decrepitation/desorption analysis. Oxygen- and sulfur-bearing organic compounds may have provided substrates for microbial life in the Dresser Formation. Mißbach et al. (2021).

The mean total organic carbon content of the black barite is 0.31% by weight. Stable carbon isotope analysis revealed a mean proportional value of −27.6±0.6‰ carbon¹³ in total organic carbon, compared to the Vienna Pee Dee Belemnite standard. Offline analysis revealed porportions of carbon¹³ and oxygen¹⁶ in carbon dioxide values of −10.0±0.3‰ and 34.1±0.6‰, respectively, compared to the Vienna Pee Dee Belemnite and Vienna Standard Mean Ocean Water standards. Online analyses yielded porportions of carbon¹³ in carbon dioxide values ranging from −14.3 to −8.9±0.3 ‰ for black barites (mean = –10.3 ‰) and from −8.6 to −4.0±0.3‰ for grey barites (mean = −6.3 ‰). Thus, black barites are consistently more depleted in carbon¹³ than their grey counterparts. In all cases, methan and nitrogen contents were too low for stable isotope analyses (less than 2% by molarity).

 

Molecular structures of oxygen-bearing compounds, aromatic hydrocarbons, and sulphur-bearing compounds found in black barite fluid inclusions. Mißbach et al. (2021).

The Black barites studied by Mißbach et al. classify as primary hydrothermal sediments that precipitated from discharging fluids. This interpretation is additionally supported by the facts that (i) the originally sulphidic Stromatolite interbeds are still largely intact and show no indications for a progressive replacement by barite and (ii) that the barite does not contain relicts of potential precursor materials. Mißbach et al.'s  observations are therefore consistent with earlier studies that argued for a primary, synsedimentary origin of the Dresser barites analysed herein (i.e. precipitation in surface environments linked to hydrothermal activity).

 

Distribution of stable carbon isotope signatures of carbon dioxide from black and grey barite fluid inclusions. Reproducibility of the stable isotope measurements is 0.3‰. A total of 11 black barite samples and 11 grey barite samples was analysed. The relatively low proportional carbon¹³ values in the black barites possibly reflect the addition of a biomass-derived carbon component to the fluids. Mißbach et al. (2021).

Barite is highly chemically stable under a wide range of geological conditions. Hence, barite-hosted fluid inclusions can preserve information on the original composition of hydrothermal fluids. The black and grey barites from the Dresser Formation primarily grew as coarse crystals and contain abundant primary fluid inclusions. Most fluid inclusions show no indication of post-entrapment modification. The results are reproducible and total homogenisation temperature values (100–195°C) are internally consistent for different coevolutionary fluid inclusions. The measured total homogenisation temperature is in line with (i) formation temperatures estimated for coexisting cherts (100–200°C), and (iii) maximum formation temperatures of barite-hosted fluid inclusions in a modern hydrothermal system (the Jade hydrothermal field in the Izena Hole, mid-Okinawa Trough, 150–200°C).

The aqueous and non-aqueous fluid inclusions distinguished herein appear to include those described in earlier studies. Particularly key-characteristics such as sizes (5–30 μm), ice and clathrate melting temperatures (−7.5 to −0.6°C and −0.9 to 19.2°C, respectively), and the fundamental volatile inventories (carbon dioxide, water, hydrogen sulphide, methane) are all remarkably similar. A notable exception is the presence of trace amounts of nitrogen in some of the aqueous fluid inclusions, which has not been reported previously.

The presence of aqueous and non-aqueous fluid inclusions can be explained by the presence of two coexisting fluids at the time of trapping as a result of phase separation from boiling fluids during cooling (effervescence). Hence, the major fluid composition of the black barites can be considered primary. However, there are indications that a few fluid inclusions were locally modified immediately after emplacement (e.g. necking down after crystallisation), explaining the wide variations observed in total homogenisation temperatures. This information is not relevant to the interpretation of the fluids as being primary, because they would be trapped again instantly with their overall composition remaining unchanged.

Organic molecules detected by gas chromatography–mass spectrometry are derived from the fluid inclusions as evidenced by (i) clean pre-analysis blanks, (ii) retrieval of products exclusively after grinding of barite, (iii) reproducibility of the results from five thermal decrepitation-gas chromatography–mass spectrometry and seven solid phase micro extraction-gas chromatography–mass spectrometry experiments, (iv) presence of highly volatile compounds in gas chromatography–mass spectrometry analyses, (v) consistency of data obtained by independent analytical techniques (Raman spectroscopy vs. gas chromatography–mass spectrometry), (vi) temperature dependency of product yields, meaning that higher temperature analyses above the decrepitation temperature of fluid inclusions result in higher abundances (thermal decrepitation-gas chromatography–mass spectrometry 150°C vs. thermal decrepitation-gas chromatography–mass spectrometry 250°C), and (vii) absence of molecular contamination indications. Together, these multiple lines of evidence strongly suggest that the analysed compounds derived from the fluid inclusions, while a minor contribution of organic compounds from the rock matrix cannot entirely be ruled out. This result adds to earlier studies, which demonstrated that fluid inclusions form closed systems that can preserve molecules even in billion-year-old metamorphic rocks.

Organic molecules detected by thermal decrepitation-gas chromatography–mass spectrometry and solid phase micro extraction-gas chromatography–mass spectrometry display major differences in diversity and abundance. Solid phase micro extraction probably provides a more authentic picture of the compounds contained in the fluid inclusions, because no heating to more than 50°C is applied before gas chromatography–mass spectrometry analysis. In contrast, thermal decrepitation resulted in abundant sulphur dioxide formation during heating to higher temperatures (250°C experiment), reflecting thermally driven artefact formation by reaction of the components in the interior of the fluid inclusions. Additionally, and even more important, the mild solid phase micro extraction offline approach can be applied on much greater sample amounts (gram vs. milligrams), resulting in detectable yields of trace compounds that are indiscernible with the thermal decrepitation approach.

The Dresser Formation formed in a hydrothermal environment. Hence, compounds entrapped in barite-hosted fluid inclusions may have been derived from abiotic sources. Indeed, gaseous compounds such as sulphur dioxide, carbon dioxide, hydrogen sulphide, carbonyl sulphide, carbon disulphide, and (methylsulphanyl)methane are known to be delivered to surface environments via volcanic outgassing. Functionalised lipid-like organic molecules such as ketones, aldehydes, carboxylic acids, and alcohols can be formed by Fischer–Tropsch-type processes under hydrothermal conditions. Further compounds of possibly abiotic origin are acetic acid and organic sulfur molecules (e.g. thiols, organic polysulphanes). These molecules may be synthesized in the presence of sulphide catalysts and with carbon disulphide or carbon dioxide as a carbon source. Extraterrestrial delivery by meteorites could have provided an additional source for many of the observed compounds (e.g. carbonyl sulphide, carbon disulphide, hydrogen sulphide, methanethiol, benzaldehyde, acetic acid, benzene, toluene, various aldehydes, and ketones).

While many compounds observed in the barite-hosted fluid inclusions from the Dresser Formation are consistent with an abiotic origin, the Dresser Formation also contains a variety of evidence for life. Thus, biology is another potential source for the observed compounds. In fact, organisms synthesise most lipids on modern Earth, and proportional carbon¹³ signatures of kerogen in the black barite (roughly –28 ‰) are in good accordance with biological carbon fixation. Furthermore, compounds such as hydrogen sulphide, carbonyl sulphide, carbon disulphide, (methylsulphanyl)methane, (methyldisulphanyl) methane, and thiols are typically formed during microbial sulphur cycling in modern environments, and there is isotopic evidence for the presence of sulfur-processing metabolisms during Dresser times. 

Taken together, it is likely that the barite-hosted fluid inclusions contain mixtures of various abiotic and biotic compounds. Such contributions from different sources would plausibly explain the contrasting proportional carbon¹³ signatures of carbon dioxide in grey and black barites. Carbon dioxide released from grey barites exhibits a mean proportional carbon¹³ value of –6.3‰, which might be indicative of a magmatic source (typically between −2 and −8‰). In contrast, lower proportional carbon¹³ values of −10.3‰ in carbon dioxide from black barites might fingerprint a biomass-derived carbon component that had been converted to carbon dioxide via Bacterial and/or thermochemical sulphate reduction before it was absorbed and transported by fluids. The processing, re-distribution, and mixing of fluids from different sources is well known from modern and ancient hydrothermal systems (hydrothermal pump).

It is widely assumed that hydrothermal processes fuelled primeval life on Earth, but it is difficult to pinpoint the exact nature of such relationships in the Archaean rock record. The fluid inclusion-bearing black barites are interbedded with Stromatolites, suggesting that hydrothermal fluids may have influenced the ancient Microbial communities. Indeed, many compounds discovered in the barite-hosted fluid inclusions (e.g. carbonyl sulphide, carbon disulphide, acetic acid, (methylsulphanyl)methane, (methyldisulphanyl)methane, thiols, methane) would have provided ideal substrates for the sulfur-based and methanogenic microbes previously proposed as players in the Dresser environment. For instance, acetic acid may have fuelled acetoclastic methanogenesis, while organic sulphides such as methanethiol and (methylsulphanyl)methane might have served as substrates for fermenting methanogenic Bacteria. This hypothesis is in full agreement with isotopic evidence indicating the existence of methanogenic and sulphur-cycling Microbes in Dresser environments. The activity of sulphate reducing or sulphur disproportioning Bacteria could also account for the presence of abundant pyrite in the originally sulphidised Dresser Stromatolites. Thus, Mißbach et al.'s findings provide a strong clue that Microbial life associated with the black barites of the Dresser Formation was (partly at least) fuelled by hydrothermal fluid flow.

In addition to potential nutrients and/or substrates, hydrothermal fluids captured in the Dresser fluid inclusions contain molecules closely related to putative key agents in the emergence of life. It has been proposed that carbon monoxide and methanethiol can react in the presence of catalytic metallic sulphides to methyl thioacetate. This compound, also known as activated acetic acid, was proposed as being important for the formation of lipids under primordial conditions and as an energy source for early Microbial metabolisms. Whereas this highly energetic molecule is readily hydrolysed and cannot be preserved over geological time, our data evidence the presence of its stable building blocks, methanethiol and acetic acid, in the Dresser fluids. In other words, essential ingredients of methyl thioacetate, a proposed critical agent in the emergence of life, were available in the Dresser environments.

Mißbach et al.'s data provide the first detailed picture of the organic composition of primordial fluids that had evidently been available for the ancient Microbes roughly 3.5 billion years ago. These fluids delivered ample catabolic substrates for chemoheterotrophic metabolisms. In addition, they might have conveyed the building blocks for chemoautotrophic carbon fixation and, thus, anabolic uptake of carbon into biomass. Taken together, Mißbach et al.'s data strongly support the idea that hydrothermal fluids supplied a fertile substrate for early Microbial life on Earth.

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Looking for evidence of a Martian origin of life.

An origin of Earth life on Mars would resolve significant inconsistencies between the inferred history of life and Earth’s geologic history. Life as we know it utilizes amino acids, nucleic acids, and lipids for the metabolic, informational, and compartment-forming subsystems of a cell. Such building blocks may have formed simultaneously from cyanosulfidic chemical precursors in a planetary surface scenario involving ultraviolet light, wet-dry cycling, and volcanism. However, early Earth was a water world, and the timing of the rise of oxygen on Earth is inconsistent with final fixation of the genetic code in response to oxidative stress. A cyanosulfidic origin of life could have taken place on Mars via photoredox chemistry, facilitated by orders of magnitude more sub-aerial crust than early Earth, and an earlier transition to oxidative conditions. Meteoritic bombardment may have generated transient habitable environments and ejected and transferred life to Earth. The Mars 2020 Perseverance Rover offers an unprecedented opportunity to confirm or refute evidence consistent with a cyanosulfidic origin of life on Mars, search for evidence of ancient life, and constrain the evolution of Mars’ oxidation state over time.

In a paper published on the arXiv database at Cornell University on 4 February 2021, Christopher Carr of the Daniel Guggenheim School of Aerospace Engineering and School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, examines the current evidence for a Martian origin of life on Earth, and ways in which we can look for evidence to support or disprove this hypothesis.

Life as we know it utilises amino acids, nucleic acids, and lipids for the metabolic, informational, and compartment-forming subsystems of a cell. Such building blocks may have formed simultaneously from cyanosulfidic chemical precursors in a planetary surface scenario involving ultraviolet light, wet-dry cycling, and volcanism. This process can be driven by photoredox chemistry with sulphite mediating cycling of ferrocyanide and ferricyanide in combination with ultraviolet irradiation. While this scenario does not rule out other models such as an origin of life at seafloor vents, it plausibly and simultaneously addresses key challenges including formation and concentration of organic building blocks, their polymerisation to yield functional molecules, and compartmentalisation to yield proto-cellular entities.

All life as we know it shares a common ancestor. The most conserved genome regions occur within genes encoding the translation machinery (16S and 23S ribosomal subunits, transfer RNAs), which are themselves RNA machines involved in translating RNAs to polypeptides via the genetic code. These regions have changed little over 4 billion years. The deep evolutionary conservation of these molecular fossils is one piece of evidence for an RNA-Protein world preceding the DNA world. Furthermore, RNA molecules are capable of both storing hereditary information as well as catalyzing reactions, a dual role that may have been critical before the emergence of translation and the fixation of the genetic code. Protocell-like growth and division, for example, mediated by feedstock supply and/or photochemical processes could facilitate compartmentalization, selection, and evolution in the context of an RNA-Protein world.

Genetic evidence suggests that the Last Universal Common Ancestor, which shares many features with modern life and was evolutionarily distant from its origin, inhabited an anoxic, 'geochemically active environment rich in hydrogen, carbon dioxide and iron. However, this setting on its own does not distinguish between sea floor vents and shallow-water hydrothermal habitats, nor between Earth and Mars.

Life as we know it utilises dehydration synthesis to form the metabolic (protein, carbohydrate), informational (nucleic acid), and compartment-forming (lipid) polymers. Driving forces for dehydration include evaporation, sublimation, crystallisation, or formation of hydrated minerals. Surface conditions thus offer plausible mechanisms to concentrate pre-biotic molecules and produce polymers. Dehydration could possibly occur due to nanoconfinement in metal sulphides at alkaline vents, yet, at present, high water activity does not seem consistent with an origin of life.

The cyanosulphidic origin theory is compelling, yet current data suggests that the early Earth was a water world with little to no sub-aerial continental crust before 3.5 billion years ago, reaching 1-2% land by 3.0 billion years ago, and 5-8% by 2.5 billion years ago. Consistent with these findings, recent analysis suggests initiation of continental weathering between 3 and 2.5 billion years ago.This would have limited the land area suitable for a cyanosulphidic origin of life to regions such as volcanic island hot spots.

 

Planetary context for a hypothesised cyanosulphidic origin of life on Mars and its transport to Earth via lithopanspermia. The horizonal axis for all panels is billions of years ago. (A) Early Mars had orders of magnitude more land area available to support a cyanosulphidic origin of life. (B) Life arising on early Mars would have been protected by a magnetic field. The lack of a rock record prevents similar knowledge of the early Earth, although it is likely Earth’s dynamo had started by 3.4 to 3.45 billion years ago. (C) Impact cratering on early Mars, especially from about 4.2 to about 3.8 billion years ago, would have provided both habitable environments for life to arise as well as facilitate its transport to Earth via meteoritic ejecta. Volcanism would have played a role in facilitating a cyanosulphidic origin of life in combination with hydrothermal subsurface activity and ephemeral surface waters. The aqueous record of this time and the geochemical transformation of Mars is recorded by the presence of hydrated silicates and salts, with sulphates as a particularly important record of early oxidation on Mars in comparison to Earth, where widespread oxidation occurred over a billion years later. (D) If a cyanosulphidic origin of life occurred on early Mars, such life could have been transferred to Earth around the time of, but not necessarily coincident with, the Last Universal Common Ancestor. For example, the Last Universal Common Ancestor may have existed on Mars and life on Earth could have arrived via multiple transfer events. Carr (2021).

Estimates of the water inventory on Mars at the Noachian/Hesperian boundary (about 3.7 billion years ago) range from tens of meters to about1 km global equivalent layer. The upper bound assumes a putative northern ocean after formation of the Martian dichotomy, e.g. the Borealis basin resulting from a massive impact event at or before 4.2 billion years ago and possibly as old as 4.5 billion years ago. Even in the most extreme scenarios, Mars would have had orders of magnitude more land area than early Earth.

Additional support for a Mars origin of life on Earth comes from amino acids and the evolution of the genetic code itself. In 2017 Matthias Granold, Parvana Hajieva, Monica Ioana Toşa, Florin-Dan Irimie, and Bernd Moosmann, proposed that the genetic code used a simpler set of amino acids and that the final diversification of amino acids happened in response to oxygen, suggesting that the diversification was late, e.g. coincident with the appearance of early local oxygen on Earth. It this hypothesis is correct, it would imply exposure to oxidizing conditions on the early Earth. However, the geologic record has revealed that Earth was largely devoid of oxygen for its first 2+ billion years, and appreciable quantities only accumulated after Cyanobacteria invented oxygenic photosynthesis, resulting in the Great Oxidation Event.

While the Great Oxidation Event occurred around 2.33 billion years ago, evidence for early local oxidative weathering suggests there could have been transient local oxygen pulses at or before 3 billion years ago. Nevertheless, the late diversification is problematic because the root of the Archaeal phylum Euryarchaeota, which arose after the Last Universal Common Ancestor, has been dated to more than 4 billion years ago, and the genetic code must have been fully established before this time including use of selenocysteine by the Last Universal Common Ancestor. Time-calibrated phylogenomics extending this deep into life’s history comes with caveats and wide error bars, yet it highlights a more than 1.5 billion year inconsistency. Thus, the timing of the fixation of the genetic code does not align with the inferred oxidation state of the early Earth. However, on Mars, surface conditions became oxidising much earlier.

Today Mars is dry and cold, yet early Mars was habitable for life as we know it, with significantly more water; availability of the main elements used by life (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulphur) and energy sources including variable redox states of iron and sulphur minerals are recorded in the stratigraphy of Gale Crater. Widespread clay minerals confirm extensive periods of subsurface water-rock interactions before 3.7 billion years ago, yet also suggest even early Mars had mostly cold and relatively arid surface conditions, which could have aided accumulation of organic molecules through concentration and low hydrolysis rates.

In 2020 Dimitar Sasselov, John Grotzinger, and  John Sutherland delineated a plausible pathway for a cyanosulphidic origin of life on early Mars. They suggest that igneous intrusions, volcanism, or impacts interacting with cyanide salt deposits could have generated the relevant feedstocks to produce nucleotide, amino acid, and lipid precursors. Carr also suggests a Mars cyanosulphidic origin of life could have seeded life on Earth, resolving the inconsistencies previously noted for an Earth origin of life on Earth.

Lightning would have provided hydrogen cyanide, representing a total fixed nitrogen budget on par with that of the early Earth. Meteorite impacts may also have generated hydrogen cyanide and provided phosphate. Sulphite would likely have been available on early Mars as a consequence of volcanic sulphur dioxide, a carbon dioxide atmosphere, and low temperatures; later oxidation would have led to the formation of sulphate minerals, consistent with remote sensing and in situ measurements. The presence of sulphites would also help to explain the relative dearth of carbonates on Mars.

A cyanosulphidic origin of life would produce all the building blocks required for an RNA-protein world. What is still lacking is knowledge about whether ancient Mars, especially before the Noachian-Hesperian boundary, was conducive to the formation, stabilisation, and evolution of an RNA-Protein world and ultimately cellular life.

Angel Mojarro, Lin Jin,  Jack Szostak, James Head, and Maria Zuber evaluated plausible divalent cations (iron, magnesium, and manganese) at different pH conditions and found the lowest rate of metal-catalysed hydrolysis of RNA for magnesium at pH 5.4, rather more acidic than the circumneutral conditions suggested by the cyanosulphidic model. In 2018 team led by Marcus Bray found that iron and manganese can replace the modern role of magnesium for ribosomal RNA folding and translation in anoxic conditions, relevant to early Earth or early Mars. An improved understanding of Mars redox history is necessary to understand the potential origin and evolution of an RNA world on Mars.

For all known life, DNA ultimately replaced RNA as the carrier of hereditary information. However, if an RNA-protein world evolved on Mars, it may still be possible to sample fossilised or extant traces of it in the subsurface, or may find, like all known Earth life, that its role in heredity has been displaced by DNA.

While an oxidising atmosphere on Earth arose due to biological production of oxygen, oxidation on Mars is thought to have resulted from crustal water sequestration as well as photolysis of water and subsequent loss of hydrogen. The present-day surface of Mars is highly oxidising, and this state has likely persisted for billions of years as inferred from manganese hydroxides identified at Gale and Endeavor craters.

On Mars, the transition to more oxidising conditions is recorded by the deposition of sulphates from approximately the Noachian-Hesperian transition (3.7 billion years ago) onward. This is consistent with orbital observations of Noachianaged iron-bearing clay minerals (e.g. associated with surface oxidation) and in situ detection of their purported ferrous smectite precursors. As the full range of sulphate deposits on Mars has yet to be identified, oxidation may also have occurred earlier.

This timing is consistent with a potential role of oxidation in shaping fixation of the genetic code, followed by meteoritic transfer of life to Earth. This resolves the problematic interpretation that the genetic code was not fixed until 1.5 billion years after the timing predicted by genetic evidence and is consistent with the antiquity of antioxidant enzymes and recent work suggesting a potential role for reactive sulphur species in their evolution.

Early nucleic-acid based life would have required phosphate to build informational polymers and store energy. Release rates for phosphate-bearing minerals are estimated to be 45 times higher for Mars compared to Earth, with roughly two times higher equilibrium phosphate concentrations. In contrast, phosphorus availability was limited on the early Earth due to a dearth of oxidising power that limited recycling until the Great Oxidation Event. Cyanide can promote release of phosphate from iron phosphates, enabling generation of organophosphates under desiccating conditions; dry conditions were widespread on early Mars but lacking on early Earth.

The plausibility of lithopanspermia has been theoretically and experimentally tested, including nonsterilising ejection from planetary surfaces as measured in Mars meteorite ALH84001 (below 40°C based on residual magnetisation) and through experimental studies of microbial survival to hypervelocity shock pressures. Modeling suggests that the mass of Mars to Earth viable transfers is greater than 1 000 000 000 tonnes. Recent data supporting a Martian dynamo at 4.5 and 3.7 billion years ago suggests any life being ejected before 3.7 billion years ago would have been afforded protection from space radiation while on Mars, although perhaps not for a few hundred million years (3.4 to 3.45 billion years ago) after its arrival on Earth.

Organisms arriving on Earth, putative chemolithoautotrophs, would have adapted to available redox gradients, including hydrothermal systems. While thermophily near the base of the tree of life was originally interpreted to provide evidence of the vent origin model, it can also be explained by parallel adaptation to high temperatures, such as selection for heat resistant species during Mars-Earth meteoritic transfer, or via subsequent heating of Earth’s oceans during impact events. Reconstructed optimal growth temperature across the tree of life suggest hyperthermophily at the base of the Archaea and Bacteria domains, but a colder optimal growth temperature for the Last Universal Common Ancestor of about 45°C, consistent with non-sterilising temperatures during meteoritic ejection.

What we know about early Mars is consistent with the cyanosulphidic origin of life, and conditions there would have provided orders of magnitude more land area for prebiotic chemistry to cross the threshold to life. The Noachian-Hesperian transition and commensurate changes in surface redox balance on Mars are also consistent with the timing of fixation of the genetic code and the antiquity of antioxidant enzymes. Theoretical and experimental studies of lithopanspermia imply that Earth, Mars, and Venus would have shared viable microbes, if present in ejecta, and subsequent genetic evidence is consistent with this timeline.

Other options are possible: If cyanosulphidic chemistry and the transition to life is a planetary phenomenon, relatively fast and easy given the right conditions, then we may find that such life evolved independently on both early Mars and early Earth, despite the latter’s dearth of sub-aerial crust. If booting up life is harder or rare, this would raise the prior that we may share common ancestry with any life on Mars. Finding no life on Mars would be incredibly revealing, because it would suggest life is a rare event requiring very particular circumstances.

Given the inconsistencies between the inferred history of life as we know it and Earth’s geologic history, it is plausible that life on Earth could have originated on Mars. Such a history, while incredible, is a story of our past that explains the available genetic and geological evidence. It is also testable.

While the Perseverance Rover will focus on habitability and evidence of ancient, and not extant, life, it can seek to confirm or refute conditions consistent with a cyanosulphidic origin of life, as well as seek chemical precursors to life and identify high abundance organics.

Jezero Crater is a 45 km diameter impact crater that once harboured an open-basin lake system recorded by sedimentary deposits, deltas, and other features associated with fluvial activity that may have ended by around the Noachian-Hesperian boundary or earlier. Analysis of the western fan deposit at the crater inlet has identified clay minerals (iron/magnesium smectite) and stratigraphy that likely records an extended history of early Mars, drawn from a watershed much larger than the crater, which includes smectites, olivine, and magnesium-rich carbonate-bearing terrains, among others. Beyond likely having been habitable itself, the Jezero Lake and associated watershed should provide access to samples originating in a diverse set of other potentially habitable environments, and the lacustrine setting implies access to a well-ordered stratigraphic record of the Noachian period on Mars.

Because the fluvial activity associated with lacustrine sedimentary deposits at Jezero Crater likely ended by 3.8 billion years ago, evidence within those deposits of increasing oxidation over time would be consistent with the ancestral relationship scenario. Alternatively, a relatively reducing surface environment up to 3.8 billion years ago would not rule out life as we know it on Mars, but would weaken the case for any ancestral relationship, at least one linked to the Jezero Crater source region. For comparison, sedimentary rocks within Gale Crater, which formed between 3.6 and 3.8 billion years ago, record a mix of oxidation states even within the base of the stratigraphic section, as well as evidence of redox stratification in younger deposits, consistent with an oxidative surface environment by the Noachian-Hesperian boundary or early Hesperian.

If putative ancient life on Mars was exposed to an oxidising surface environment before the earliest evidence of life on Earth, a Mars-Earth transfer is not ruled out. If no part of Mars became oxidizing until significantly after the earliest signs of life on Earth (less than 3.7 billion years ago), or certainly after the earliest unambiguous evidence of fossil life on Earth (3.5 billion years ago), then the window of opportunity to complete fixation of the genetic code on an oxidised Mars prior to a hypothesized Mars-Earth transit would close, and lead to rejection of the hypothesis that oxidising conditions on Mars could explain fixation of the genetic code.

Silica has high preservation potential and hydrated silica deposits have been identified within Jezero crater in association with delta deposits. Organic materials can be concentrated locally and stabilised by adsorbing to silica and the aforementioned clays (iron/magnesium smectite). Lacustrine carbonates in Jezero Crater also offer high preservation potential. A study of amino acid preservation in simulated Mars conditions found the highest preservation in smectites and sulphates.

Extrapolation from modeling of hydrolysis and ancient DNA sequencing projects suggests that 100 base pairs DNA at –60°C could have a half-life over 2 billion years. However, other factors such as naturally-occurring radioisotope decay may limit the decay half-life to perhaps 10 million years. RNA generally has higher hydrolysis rates, although the pH dependence is different. Thus, direct detection of nucleic acid polymers in Noachian-aged terrain is extremely improbable. However, nucleobases, the informational component of nucleic acids, have been identified in meteorites, and thus, at least under cold conditions, nucleobases can survive more than 4 billion years.

Intact nucleic acids and nucleobases can adsorb to silica, and in the relative absence of later aqueous and thermal alterations, nucleobases can be retained. Silica is, in fact, used to isolate cross-linked RNA and proteins to study their interactions. Cross-linking would be expected as part of the diagenesis process, as would be deamination of cytosine to yield uracil. Mars 2020’s instruments are not designed to specifically interrogate uracil, but it could be targeted in returned samples along with other nucleobases.

Lipids are also one of the most geologically stable biomolecules, and are used as a biosignature in studies of Earth life in rocks older than 2 billion years. Studies of Mars-relevant diagenesis reveal caveats. Jonathan Tan and Mark Sephton found that hydrous pyrolysis from 200-280°C for 72 hours significantly degraded lipids, especially with low carbon content, high water-to-rock ratios, and iron-rich minerals, while a team led by Daniel Carrizo demonstrated the potential to detect lipid biomarkers using a tunable Raman laser spectrometer and found that of silica rich vs. iron rich samples tested, the silica rich material enhanced preservation. Thus, silica-rich deposits in Jezero Crater could be targeted in the search for nucleobases and lipids.

Many of the mineral deposits mentioned are expected to be accessible to the Mars 2020 Rover, for example, within the stratigraphy of the western delta deposits. Because Mars lacks a substantial atmosphere, organics within deposits exposed at the surface would be damaged over time by space radiation. To avoid this damage, low exposure ages are desirable. Mudstone on the floor of Gale Crater was found to have a exposure age of 78 ± 30 million years highlighting the potential to utilise wind-driven geomorphological change to obtain samples with potential for complex organic molecules as undegraded by cosmic rays as possible. Thus, it may be feasible to use a similar approach in Jezero Crater, leveraging prior wind analysis and in situ measurements to assess access to low exposure age stratigraphy. Because accessible exposure ages may still be large, drilling, including, in future missions, below the 1-2 meter penetration depth of space radiation, remains critical for accessing undegraded samples.

While the capabilities of the Mars 2020 Rover are unprecedented, and several instruments (SuperCam, SHERLOC, PIXL) will be used to detect or infer the presence of organics and specific chemical moieties, they may lack the ability to uniquely identify specific molecules. Instruments in development could one day target poly-peptides and nucleic acids. 

Mars 2020 is uniquely positioned to seek evidence consistent with a cyanosulphidic origin of life on Mars, search for ancient evidence of life, and constrain the evolution of Mars’ oxidation state over time. Unlike Earth, where ancient rocks are rare, on Mars we can access rocks across nearly the full 4.5 billion year history of that planet. Despite the modern thin (1% that of Earth) atmosphere and cold average surface temperature (–60°C), the subsurface of Mars likely remains habitable. Future missions, with access to special regions, including deep drilling to seek habitable environments in the sub-surface, may be required to target any extant life and unambiguously determine whether life as we know it exists on Mars today and if so, whether it is related to us.

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