Вадим Дудченко
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In a new paper published in the Journal of the Geological Society, University of Edinburgh’s Dr. Sean McMahon and University of Oxford’s Dr. Julie Cosmidis attempt to summarize the physicochemical processes so far known to generate life-like morphologies, minerals, molecules and other phenomena, and consider how and where they may have taken place on early Mars, with a particular focus on the materials that NASA’s Perseverance rover and ESA’s planned Rosalind Franklin rover may encounter and sample on the Martian surface.

Organic and inorganic biomorphs (small abiotic structures that bear a morphological resemblance to living or fossil microbes): (a) asteroidal trichites radiating from central magnetite grains in a natural volcanic glass; (b) organic microspheres obtained from RNA and quartz mixtures under diagenetic conditions; (c) amphiphilic spherical and tube-like vesicles formed by mixed fatty acids and 1-alkanols (C10-C15) in 10 mM CaCl2; (d) coacervate microdroplets prepared in water by mixing PDDA with ATP; (e, f ) chain-forming and twinning proteinoid microspheres obtained by heating mixtures of amino acids; (g, h) apatite particles formed by double diffusion in gelatin mimicking dividing cells and raised polygonal features in embryos; (i) rod-shaped silica particles formed on the walls of basalt fractures injected with supercritical CO2; (j) cryogenic opal filaments and sheets formed in rapidly freezing silica-rich fluids; (k) cryogenic carbonate segmented filaments obtained by freezing of silica-rich alkaline brines; (l) rod-shaped apatite particles forming within organic films in sediments; (m) fluoroapatite particle precipitated in the presence of citrate, mimicking dividing coccoid cells. Image credit: McMahon & Cosmidis, doi: 10.1144/jgs2021-050.

“It is often acknowledged that the search for life on Mars might produce false positive results, particularly via the detection of objects, patterns or substances that resemble the products of life in some way but are not biogenic,” Dr. McMahon and Dr. Cosmidis said.

“The success of major current and forthcoming rover missions now calls for significant efforts to mitigate this risk.”

In the new research, the authors reviewed evidence of all known processes that could have created life-like deposits in Martian rocks.

They identified dozens of processes — with many more likely still undiscovered — that can produce structures that mimic those of microscopic, simple lifeforms that may once have existed on the Red Planet.

Among the life-like specimens these processes can create are deposits that look like bacterial cells and carbon-based molecules that closely resemble the building blocks of all known life.

“We have been fooled by life-mimicking processes in the past,” Dr. Cosmidis said.

“On many occasions, objects that looked like fossil microbes were described in ancient rocks on Earth and even in meteorites from Mars, but after deeper examination they turned out to have non-biological origins.”

“Our paper is a cautionary tale in which we call for further research on life-mimicking processes in the context of Mars, so that we avoid falling into the same traps over and over again.”

Because signs of life can be so closely mimicked by non-living processes, the origins of any fossil-like specimens found on Mars are likely to be very ambiguous.

Dr. McMahon and Dr. Cosmidis call for greater interdisciplinary research to shed more light on how life-like deposits could form on Mars, and thereby aid the search for evidence of ancient life there and elsewhere in the Solar System.

“At some stage a Mars rover will almost certainly find something that looks a lot like a fossil, so being able to confidently distinguish these from structures and substances made by chemical reactions is vital,” Dr. McMahon said.

“For every type of fossil out there, there is at least one non-biological process that creates very similar things, so there is a real need to improve our understanding of how these form.”

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S. McMahon & J. Cosmidis. False Biosignatures on Mars: Anticipating Ambiguity. Journal of the Geological Society, published online November 17, 2021; doi: 10.1144/jgs2021-050

 



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