Scientists have discovered methane hidden in Martian meteorites, which could hint that the elusive gas, which on Earth is often linked to life, might be lurking beneath the surface of Mars today.
The findings, published in the journal Nature Communications, could have implications for the biological potential of the Red Planet.
"The availability of methane and hydrogen is critical to the potential of the Martian crust as a habitat for microbial life," the study authors wrote. "The hostile Martian surface is probably less habitable than the subsurface, and several scenarios have been proposed for deep Martian life."
The mystery of methane on Mars has long dogged astrobiologists looking to assess whether life could have existed on the Red Planet. Methane can be made in nonbiological ways, but on Earth, the majority of it is made by living things. Methane can also be consumed by certain types of microbes, so the presence of methane could potentially be a food source for them.
But even as Mars increasingly looks like it had a life-friendly past, methane has proved remarkably difficult to pin down on the Red Planet. NASA's Curiosity rover searched for months only to turn up empty, but later picked up what appear to be intermittent plumes. (Some scientists think the methane readings might actually be contamination from the rover itself, so the jury is still out on Martian methane for the moment.)
"The putative occurrence of methane in the Martian atmosphere has had a major influence on the exploration of Mars, especially by the implication of active biology," the study authors wrote. "The occurrence has not been borne out by measurements of atmosphere by the MSL rover Curiosity but, as on Earth, methane on Mars is most likely in the subsurface of the crust."
For this paper, an international team of researchers turned to data they could hold in their hands: six Martian meteorites that have landed on Earth. These little chunks of the Red Planet might not be as pristine as a rock sitting on Mars right now, but they still offer a chance to answer some pressing scientific questions up close and personal.
The researchers crushed rock from the meteorites, thus forcing out the gases trapped inside. (Many experiments involve "cooking" the rock to reveal its contents, but that often ends up forming new molecules in the process that can muddle the readings.)
"It's an interesting, complementary way of getting information," said Paul Mahaffy, a planetary scientist at NASA's Goddard Space Flight Center who was not involved in the paper. Mahaffy serves as lead scientist for Curiosity's Sample Analysis at Mars instrument suite, which among other things uses the cooking method to study rocks on Mars.
Among the released gases (which included carbon dioxide, hydrogen, nitrogen and trace amounts of oxygen and argon), the researchers found significant amounts of methane and hydrogen. The relatively high levels of these two very important gases make sense, the authors said, given that the rocks were altered in the presence of water.
Methane is a pretty big deal in the Earth's "deep biosphere," where methane-eating microbes may use it as fodder. If Martian microbes ever existed, perhaps they played a similar role on the Red Planet, the scientists said.
"The evidence presented here indicates that a methane-bearing subsurface habitat is similarly available on Mars," the authors wrote. "Whether or not the habitat has been occupied remains to be determined."
But Mahaffy warned that there are plenty of ways that methane could have been brought to or produced on Mars that have nothing to do with living things. Very little is known about the amount of methane, its origins and its dynamics on the Red Planet, he said.
"I think that the whole methane story together, where it comes from and how often it appears in the atmosphere, is still not a solved problem," he said.