Volcaniclastic habitats for early life on Earth and Mars: A case study from ∼3.5Ga-old rocks from the Pilbara, Australia

Abstract

Within the context of present and future in situ missions to Mars to investigate its habitability and to search for traces of life, we studied the habitability and traces of past life in $ 3.5 Ga-old volcanic sands deposited in littoral environments an analogue to Noachian environments on Mars. The environmental conditions on Noachian Mars (4.1-3.7 Ga) and the Early Archaean (4.0-3.3 Ga) Earth were, in many respects, similar: presence of liquid water, dense CO 2 atmosphere, availability of carbon and bioessential elements, and availability of energy. For this reason, information contained in Early Archaean terrestrial rocks concerning habitable conditions (on a microbial scale) and traces of past life are of relevance in defining strategies to be used to identify past habitats and past life on Mars. One such example is the 3.446 Ga-old Kitty's Gap Chert in the Pilbara Craton, NW. Australia. This formation consists of volcanic sediments deposited in a coastal mudflat environment and is thus a relevant analogue for sediments deposited in shallow water environments on Noachian Mars. Two main types of habitat are represented, a volcanic (lithic) habitat and planar stabilized sediment surfaces in sunlit shallow waters. The sediments hosted small (o 1 mm in size) microorganisms that formed colonies on volcanic particle surfaces and in pore waters within the volcanic sediments, as well as biofilms on stabilised sediment surfaces. The microorganisms included coccoids, filaments and rare rodshaped organisms associated with microbial polymer (EPS). The preserved microbial community was apparently dominated by chemotrophic organisms but some locally transported filaments and filamentous mat fragments indicate that possibly photosynthetic mats formed nearby. Both microorganisms and sediments were silicified during very early diagenesis. There are no macroscopic traces of fossilised life in these volcanic sediments and sophisticated instrumentation and specialized sample preparation techniques are required to establish the biogenicity and syngenicity of the traces of past life. The fact that the traces of life are cryptic, and the necessity of using sophisticated instrumentation, reinforces the challenges and difficulties of in situ robotic missions to identify past life on Mars. We therefore recommend the return of samples from Mars to Earth for a definitive search for traces of life.

Astrobiology and the Possibility of Life on Earth and Elsewhere…

“…Apart from the afore-mentioned corrosion tunnels, multi-species colonies of small (generally <1 µm diameter), mainly coccoidal fossilized microorganisms coat the surfaces of volcanic grains (Fig. 4) and are present in the interstitial pore spaces between the volcanic grains (Westall et al 2006(Westall et al , 2011a(Westall et al , 2011b. The carbonaceous cells were preserved by silica that was precipitated at a very early stage and the carbon isotope ratios of the carbon in the rock are consistent with microbial fractionation.…”

“…Their energy sources would have included molecular hydrogen produced by redox alteration of volcanic rocks and hydrothermal activity while their carbon source may have been CO 2 dissolved in seawater. Dissolution features in the surfaces of the volcanic rocks attest to microbial attack on these substrates in search of nutrients (Furnes et al 2004;Westall et al 2011aWestall et al , 2011b. Physical traces of the existence of these chemotrophs occur in volcanic sediments deposited in the shallow water littoral environment (Westall et al 2006(Westall et al , 2011a(Westall et al , 2011b and on the vitreous surfaces of pillow lavas erupted under water (Furnes et al 2004).…”

“…Dissolution features in the surfaces of the volcanic rocks attest to microbial attack on these substrates in search of nutrients (Furnes et al 2004;Westall et al 2011aWestall et al , 2011b. Physical traces of the existence of these chemotrophs occur in volcanic sediments deposited in the shallow water littoral environment (Westall et al 2006(Westall et al , 2011a(Westall et al , 2011b and on the vitreous surfaces of pillow lavas erupted under water (Furnes et al 2004). Apart from the afore-mentioned corrosion tunnels, multi-species colonies of small (generally <1 µm diameter), mainly coccoidal fossilized microorganisms coat the surfaces of volcanic grains (Fig.…”

Testing the ability of the ExoMars 2018 payload to document geological context and potential habitability on Mars

“…Structures observable at outcrop scale (layering) and textures observable at the microscopic scale indicate that the rock represents volcanic sediments that were deposited in a very shallow marine environment; such as an infilling tidal channel (de Vries, 2004). The traces of carbonaceous matter identified by Raman are related to the presence of fossilised (silicified) microbial colonies Westall et al, 2011). Concentrated on the surfaces of volcanic grains and in the pore spaces between the volcanic grains, these colonies most likely represent relatively simple microorganisms, such as chemolithotrophs that obtain their energy, nutrients and carbon from inorganic sources.…”

“…This sample is, thus, particularly relevant in terms of the search for life on Mars since these volcanoclastic sediments were deposited in a shallow water aqueous environmental setting that would have been relatively common in the Noachian period on Mars. The simple, chemotrophic life forms that they contain could therefore hypothetically reflect the kinds of simple life that may have occurred on Noachian Mars (Westall et al, 2011;Westall et al, 2013). Moreover, during the Noachian, hydrothermal processes associated with impacts and volcanic activity were likely to have been important on Mars (e.g.…”