The surface of Mars is an inhospitable desert bombarded by cosmic rays – but could microbes be lurking beneath the dust? Earth’s subsurface is teeming with unconventional life, including methane-munching microbes. The discovery of methane inside meteorites from Mars, reported in Nature Communications, confirms similar microbial life could exist on Mars.
Methane is a sign of an active planet; either geologically (from chemical reactions inside rocks) or biologically (most of Earth's methane is produced by living organisms). Scientists have been searching for signs of methane on Mars for nearly 50 years. Back in 1969, Mariner 7 scientists thought the spacecraft had detected methane bubbling up near Mars' south pole, but later researchers realised that it was carbon dioxide ice.
Then in 2003 and 2004 telescopes and spacecraft appeared to detect large methane clouds in Mars' atmosphere – they vanished after a few months, sparking a debate about whether the clouds were seasonal, or the measurements flawed. Most recently, NASA's Curiosity rover detected a burst of methane in 2013, but scientists continue to debate whether the measurement is real, or if the methane came from inside the rover.
The big question of course, is whether the methane on Mars has ever supported life.
Now scientists have tackled the Mars methane mystery from a different angle. John Parnell from the University of Aberdeen and his colleagues extracted small samples of rock from six rare Martian meteorites (only a few dozen have ever been found on Earth) and analysed what gases were released when the samples were crushed. They found that methane, trapped in pores inside the meteorites, was one of the dominant gases.
“This doesn't prove that there is life on Mars either now or in the past, but it does tell us that there is a potential habitat available to life,” says Parnell.
All the Martian meteorites sampled were volcanic rocks, thought to be similar to the rock that covers much of Mars today. “This is a very interesting new way of looking at Martian meteorites,” says Conel Alexander, a geochemist from the Carnegie Institution of Washington. That methane would be present is not a great shock, he adds. “Similar rocks on Earth also contain methane and I suspect that as long as a planet or moon contains carbon and water (or other hydrogen-bearing material), some methane will be present.”
Parnell agrees: “The chemical reaction between water and rock to produce methane is straightforward and is likely to take place on any planet with subsurface water.” But the researchers were “relieved and excited” to confirm experimentally that Martian rock really does contain the gas, adds Nigel Blamey, a member of the team.
The big question of course, is whether the methane on Mars has ever supported life. Mars isn’t an active planet, which limits opportunities for methane to escape from rocks and make itself available for microbes as a source of energy and carbon. But in the past Mars is thought to have had a more active crust, making it more likely that methane could have pervaded the subsurface, diffusing along underground cracks. “If there ever has been life on Mars it would have been more likely in the past," says Parnell.
And any life on Mars would likely have left a chemical fingerprint. Life on Earth prefers the lighter, slightly more reactive carbon isotope: carbon-12. As a result gases on Earth tends to be depleted in this isotope, because living things have absorbed it in preference to the heavier carbon-13. It is likely that any life on Mars was also selective about the carbon isotopes it consumed. Later this year Parnell and his colleagues will attempt to extract a little more gas from the Martian meteorites to analyse their carbon isotope composition. “If we can see any evidence of this kind of isotopic fractionation it is likely to be a sign that life was once there,” says Parnell.
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