Mars may once had habitat suitable for subterranean life, say scientists

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Monday, September 24, 2018

In a new study available in the current volume of Earth and Planetary Science Letters, a team led by scientists from Brown University in the United States says that the planet Mars once had the right water and temperatures to host simple life forms—just not on its surface. Mars’ rocky, subterranean layer once had enough water and reductants to support some of the same kinds of microbial communities seen on Earth, and this habitat lasted for hundreds of millions of years.

“We showed, based on basic physics and chemistry calculations, that the ancient Martian subsurface likely had enough dissolved hydrogen to power a global subsurface biosphere,” reports lead author Jesse Tarnas, currently a graduate student at Brown. The paper does not prove that life on Mars did exist but rather that conditions suitable for life are very likely to have lasted for eons. This habitable zone, located beneath Mars’ then-frozen surface, would have reached several kilometers into Mars’ surface, protected from freezing by the ice above.

The the study shows that, during Mars’ Noachian period (3.7–4.1 billion years ago), radiolysis, the process by which radiation splits water molecules apart, produced enough hydrogen gas (H2) for microbial organisms to live on so long as they remained within the area just beneath the cryosphere, the SHZ. The concentration of hydrogen in the groundwater could have ranged from about 35 to about 55 millimoles depending on whether ancient Mars was warm or cold, respectively, and higher if the subsurface medium also contained enough salt. The researchers determined this by establishing three factors. First, they examined data from the gamma ray spectrometer aboard NASA’s Odyssey spacecraft, from which they inferred how much uranium would have been present in Mars’ crust during the Noachian, and therefore how much radiation would have been available to split water and so produce hydrogen. Then built on existing models of water flow on Mars to determine how much groundwater would have been present. Third, they used climate and geothermal modeling to determine how much of that water would have been in liquid form and at a suitable temperature for living things.

In subterranean environments on Earth called subsurface lithotrophic microbial ecosystems, or SLiMEs, ecosystems sustain themselves not on plants that harness sunlight through photosynthesis but on microbes that harvest electrons from nearby molecules. Molecular hydrogen is an especially good electron donor.

One of the study authors, Brown Professor John Mustard, is on the team designing the next Mars Rover mission, scheduled for 2020. He and his co-authors recommend that the Rover examine the sites of meteorite crashes, which may have excavated rocks from this possibly habitable depth that may hold traces of ancient life.

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