Sunday, February 1, 2015

Methanogens in Mars?

Excitement over the Curiosity rover’s recently reported detection of a ‘spike’ in localized atmospheric methane – persisting over a couple of months – is well founded. It’s possible that this represents a very real clue to past or present life on Mars. Or rather, life in Mars. The great majority of methane that we find here on Earth (whether in the air or in subsurface deposits) has a biological origin. This methane is generated by methanogenesis, a metabolic process that seems to be confined to members of the domain Archaea.

There’s more than one chemical pathway for making methane, but the most obvious is the combination of carbon dioxide with molecular hydrogen, and that’s precisely the reaction that a slew of methanogenic archaea latch onto. Molecular hydrogen is a potent source of chemical energy, and other organisms such as sulfate-reducing bacteria also gobble it up. So where do they find that hydrogen?

Methanogens. Credit: Maryland Astrobiology Consortium, NASA, and STScI
One source is where rock and water sit together. Radioactivity from rocks laced with elements like uranium can break up water molecules (the process of radiolysis), and the geochemical process of serpentization also spits out molecular hydrogen in abundance. Active hydrothermal vent systems on ocean beds are one environment where hydrogen is readily made, and methanogenic organisms thrive there. Places like these are, in relative terms, packed with chemical energy that life can exploit, and does.

Now a new study by Lollar, Onstott, Lacrampe-Couloume & Ballentine reported in Nature (2014) suggests that the deep (5km) continental zones could indeed be a major producer of hydrogen. Specifically, the most ancient Precambrian continental subsurface (rock older than about 540 million years), could generate molecular hydrogen at a rate 40 to 250 times higher than previously thought – a production on par with that associated with the much younger marine lithosphere. This Precambrian material exists in about 70% of the Earth’s continental area, and could contain as much water as all surface rivers, swamps, and lakes.

The conclusion is that the global production of molecular hydrogen needs an upward revision and, most critically, at least half of that is coming from the deep, ancient continental subsurface – which is not dry, not inert, and seems to be filled with life.

Connecting the discoveries of methane on Mars could give us an idea of how the Earth was in its earliest stages. It could also give us a clue as to where to look to find microorganisms based on the known methanogens in Earth.

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