Chemical evolution of the early dense Martian atmosphere

We focus on the geological system of the Noachian. In the early part of the corresponding period (4.6 to 3.7 Gyrs before the present, BP) there must have been a reduced amount of sulphur compounds. Later on, due to volcanism, likely candidates for the dense atmospheric components (towards the later Noachian/Hesperian transition) were SO₂ and H₂, (Greely, Reference Greely2013). The resulting high atmospheric pressure may have inhibited SO₂ outgassing while allowing H₂O and CO₂ (Gaillard et al., Reference Gaillard, Michalski, Berger, McLennan and Scaillet2013). In addition, it is only until the Noachian/Hesperian transition that widespread sulphate compounds appear when the valley networks were formed (Bibring et al., Reference Bibring, Langevin, Mustard, Poulet, Arvidson, Gendrin, Gondet, Mangold, Pinet and Forget2006). For surficial sulphur minerals, biogeochemistry yields large negative values for isotopic fractionation. This should not be confused with the results of photolysis experiments involving the atmospheric components SO₂ and H₂S (Farquhar et al., Reference Farquhar, Savarino, Jackson and Thiemens2000) that can produce analogous fractionation.

The significant relevance of these experiments demonstrate exclusively that sulphur was indeed one of the chemical elements that were present in the early atmosphere. For that reason, Farquhar and coworkers suggest isotopic fractionation of sulphur isotopes in Martian meteorites. However, the period of Martian geologic evolution that concerns us in the present paper is the Early Noachian, rather than the period of formation of those SNC meteorites that were available to the experimenters. Unlike the SNC meteorites, which are of a younger age, the photolysis experiments did not include the oldest known sample of Mars’ crust – the approximately 4.1 Gyr-old meteorite Allan Hills 84001.

The emergence of abundant Martian surficial sulphates

Sulphates were abundant towards the Noachian/Hesperian. However, in the later Noachian, more precisely, towards the Noachian/Hesperian transition and later, volcanic activity argues in favour of sulphur compounds playing a significant role, either maintaining a warm and wet period (Halevy and Schrag, Reference Halevy and Schrag2009), or subsequently working as a factor contributing to general cooling that ended the wet warm period (Kite et al., Reference Kite, Williams and Aharonson2014; Kerber et al., Reference Kerber, Forget and Wordsworth2015).

At this stage in the evolution of the Martian atmosphere, there is ample evidence of large emissions of SO₂ due to volcanism. In the section ‘Can stable isotopes ratios yield biomarkers of sulphate-reducing microorganisms?’, we are mainly concerned with effects from the early atmosphere and the corresponding required sensitive instrumentation that could detect a sulphur biomarker in sulphur minerals. There are possible testing sites at Mawrth Vallis and Oxia Planum, which are probable landing sites for the forthcoming missions: ExoMars and Mars 2020.

Can stable isotopes ratios yield biomarkers of sulphate-reducing microorganisms?

We have seen that at a terrestrial time contemporary with the transition Hadean/Archean, there is ample evidence that Noachian Mars was in the presence of liquid water on its surface. There were also possible hydrothermal vents, environments where life emerged, as it did in the analogous terrestrial case. At least if life did originate elsewhere, we expect that it would likely prosper near vents (Baross and Hoffman, Reference Baross and Hoffman1985; Reysenbach and Cady, Reference Reysenbach and Cady2001), or at any rate, near the sedimentary layer between oceanic crust and seawater, given the convincing evidence from the terrestrial Hadean Eon. On the Martian surface, it is possible to test biosignature detection with miniaturized mass spectrometry that has already reached a significant degree of development, but we expect some feasible improvements in the foreseeable future (Tulej et al., Reference Tulej, Neubeck, Ivarsson, Riedo, Neuland, Meyer and Wurz2015; Wiesendanger et al., Reference Wiesendanger, Wacey, Tulej, Neubeck, Ivarsson, Grimaudo, Moreno-Garcia, Cedeno-Lopez, Riedo and Wurz2018).

To answer with confidence whether variations of stable isotopes ratios could be indicative of biogenic origin, certain questions have to be taken into account, especially the possible effects of diagenesis and thermochemical sulphate reduction on sedimentation. Terrestrial sulphate reducing bacteria are of Archean time (Kaplan, Reference Kaplan1975). Some reliable estimates for their first emergence are available. Indeed, some of these chemosynthesizers have been detected in sediments of approximately 3.47 Gyrs BP (Shen et al., Reference Shen, Buick and Canfield2001).

These early microbes have demonstrated that on Earth isotopic fractionation δ³⁴ exceeds by some 20‰ of the standard value. This data may be interpreted as the result of sulphate reduction, in spite of diagenesis and sedimentation. Such geologic measurements place sulphate reducers low in the phylogenetic tree, among the most ancient of our ancestors (Philippot et al., Reference Philippot, Van Zuilen, Lepot, Thomazo, Farquhar and Van Kranendonk2007). To constrain our search for the first appearance of life on Mars, we should consider the evidence so far in our own planet: It is possible that life had already established a terrestrial habitat near submarine-hydrothermal vents before 3.77 Gyrs BP and, possibly as early as 3.95 Gyrs BP, or even 4.28 Gyrs BP (Dodd et al., Reference Dodd, Papineau, Grenne, Slack, Rittner, Pirajno, O'Neil and Little2017; Tashiro et al., Reference Tashiro, Ishida, Hori, Igisu, Koike, Méjean, Takahata, Sano and Komiya2017). These events may have taken place not long after the earliest evidence for a continental crust and oceans (Wilde et al., Reference Wilde, Valley, Peck and Graham2001). Hadean zircons suggest that liquid water was present at the surface of the Earth, possibly already by 4.36 Gyrs BP, the age of the oldest zircons (Mojzsis et al., Reference Mojzsis, Harrison and Pidgeon2001; Wilde et al., Reference Wilde, Valley, Peck and Graham2001; Nemchin et al., Reference Nemchin, Pidgeon and Whitehouse2006; Bell et al., Reference Bell, Boehnike, Mark Harrison and Mao2015). In any case, the bounds for the emergence of terrestrial life are most likely to be constrained to the time range of 4.5 to 3.9 Gyrs BP, as it has been recently discussed (Pearce et al., Reference Pearce, Tupper, Pudritz and Higgs2018). Such early genesis of life on Earth strongly suggests constraining our search for an early emergence of life on Mars to the Noachian before the transition to the Hesperian.

Where are the best sites for testing isotopes as biomarkers?

The southern hemisphere may still retain analogous conditions to those that led to the origin of life on Earth, where they have totally disappeared, due to the harsh effects of metamorphism and plate tectonics. In any case, the early geological history of Mars has left a window for testing the emergence of life, whereas it has disappeared on Earth (cf., the section ‘Can stable isotopes ratios yield biomarkers of sulphate-reducing microorganisms?’). While there are numerous factors that militate in favour of present inhabitability of the Red Planet, or even simply contemporary habitability, there is no evidence yet for inhabitability, or ancient inhabitability that could be detectable with biosignatures of reliable microbial fossils.

But we have argued that focusing on testing for the more robust S isotopes as biosignatures will lead to useful information with a careful analysis of the changes that the first sulphur metabolizing bacteria may have made in isotopic ratios on Mars.

However, a possible eventual search is suggested due to the newly discovered lake of liquid water that is 20 km across buried 15 km beneath Mars surface close to the southern polar ice cap (Orosei et al., Reference Orosei, Lauro, Pettinelli, Cicchetti, Coradini, Cosciotti, Di Paolo, Flamini, Mattei, Pajola, Soldovieri, Cartacci, Cassenti, Frigeri, Giuppi, Martufi, Masdea, Mitri, Nenna, Noschese, Restano and Seu2018). It is not deeper than the analogous subterranean lake Vostok in Antarctica (Siegert et al., Reference Siegert, Ellis-Evans, Tranter, Mayer, Petit, Salamatin and Priscu2001). Fortunately, it is known that the ice above the terrestrial lake is a testimony of great diversity of single-celled organisms: yeast, actinomycetes, mycelian fungi, the alga Crucigenia tetrapodia and diatoms (Siegert et al., Reference Siegert, Carter, Tabacco, Popov and Blankenship2005). Besides, it appears that in Vostok, water temperatures do not drop too far below zero centigrade, with the possibility of geothermal heating raising the temperatures above this level. In the Martian lake, there are also conditions that keep the water liquid (Orosei et al., Reference Orosei, Lauro, Pettinelli, Cicchetti, Coradini, Cosciotti, Di Paolo, Flamini, Mattei, Pajola, Soldovieri, Cartacci, Cassenti, Frigeri, Giuppi, Martufi, Masdea, Mitri, Nenna, Noschese, Restano and Seu2018). This suggests a search for isotope biomarkers closely beneath the surface of the polar cap in Planum Australe (reachable by future rovers), in order to test whether biomarkers of fossilized microbes or even multicellular microorganisms may have emerged from the interior of the Martian lake.

The detection of biosignatures awaits the challenge of innovative use of approved instrumentation for forthcoming missions. We conclude that with so much valuable and appealing potential astrobiological information to be retrieved, rovers should also use their approved instrumentation for testing isotopic biomarkers (Goesmann et al., Reference Goesmann, Brinckerhoff, Raulin, Goetz, Danell, Getty, Siljestrom, Mißbach, Steininger, Arevalo, Buch, Freissinet, Grubisic, Meierhenrich, Pinnick, Stalport, Szopa, Vago, Lindner, Schulte, Brucato, Glavin, Grand, Xiang and van Amerom2017).