Methane, a potential source of energy for life, has been found trapped in meteorites from Mars.
The discovery by Canadian and Scottish scientists suggests that microbes could potentially live deep underground on Mars. It also points to a possible source of the methane that has been detected in Mars's atmosphere by the Curiosity rover and telescopes on Earth, baffling planetary scientists.
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The six meteorites studied by the researchers are volcanic rocks that formed near the surface of Mars or a few kilometres deep about a billion to 1.5 billion years ago, says Matthew Izawa, a co-author of the paper describing the discovery. The research was published in Nature Communications this week.
When a comet or meteor smashed into the surface of Mars between a few million and tens of millions years ago, the rocks were ejected into space, and eventually fell to Earth.
There, they were examined by Izawa and other researchers at Western University's department of Earth Sciences.
They discovered that while the rocks were still on Mars, they had been exposed to water – a liquid once plentiful on Mars.
The minerals the rocks were made of — olivine and pyroxene — would have reacted with water to release hydrogen gas.
In theory, that hydrogen could react with carbon dioxide in the surrounding environment to form methane – something that happens to those types of minerals on Earth. That methane, could, in turn, provide the energy needed to support life.
"On Earth, there's a famous sea floor locality called Lost City where methane produced like that is the basis of a whole ecosystem," Izawa said.
On Mars, there's quite a lot of carbon dioxide for hydrogen to react with, but methane doesn't last long above ground – Mars's thin atmosphere lets through lots of ultraviolet radiation that destroys methane both directly and indirectly, via other reactive chemicals that it generates from water.
Trapped in pores
Still, it's possible that methane formed when the rock was exposed to water could get trapped in the pockets or pores in the rock as they were closed up by minerals recrystallizing from the water.
Nigel Blamey, geologist and geochemist at Brock University in St. Catharines, Ont., along with researchers at the University of Aberdeen, the Scottish Universities Environmental Research Centre and the University of Glasgow, wanted to see if this was the case.
Blamey, lead author of the paper, and his colleagues ground up some of the Martian meteorites inside a chamber that contained no air or other gases. That would break open any pores or pockets in the rock, releasing the gases inside.
Using a chemical detector called a mass spectrometer, the researchers found that the mixture of gases released from all the meteorites contained mostly methane. The pores also contained smaller amounts of carbon dioxide, hydrogen, nitrogen, and traces of oxygen and argon.
Izawa said the researchers were "pretty excited" by the results. While the amounts of methane were extremely tiny, he added, "it's certainly enough that something might be able to live on it."
Consistent results from so many different meteorites suggest that the production of methane is likely to be widespread on the crust of Mars, the researchers say.
Izawa said methane that accumulated over time in underground caves might later be released by cracking in the rocks above – that might explain some of the sudden "plumes" of methane detected from time to time in the atmosphere.
This kind of chemical reaction could also provide a continuous source of methane that would be available to microbes living deep underground.
"We know from the experience of studying earth rocks, that you find life way down deep into the crust, almost as deep as anyone's ever drilled," Izawa said. "Often they live on the reaction products of the same sort of water-rock reaction that seems to be occurring on Mars."
Based on the new results, he said, "we think that the deep surface of Mars is one of the best places for life to continue to inhabit."