A trio of research teams independently probing the Martian atmosphere for signs of methane have found it, a combined discovery that opens the door for a host of theories as to how the smelly gas got there.
Among the most tantalizing, if not very likely, of scenarios, scientists say, is the possibility that the Mars methane could be the byproduct of some form of microbial life. But a safer bet, they say, centers on the geology of Mars, including anything from volcanic activity to long-ago impacts of methane-carrying comets.
"It's of course very exciting and quite a surprise," said Augustin Chicarro, project scientist for the European Mars Express mission, which detected Mars methane while orbiting the planet. "Mars seems to be a planet that is always surprising us, one week it's an ocean … now this."
The methane findings comes just weeks after NASA's Mars Exploration Rovers (MER) Spirit and Opportunity found conclusive evidence that water once flowed on the surface of the red planet, providing firm evidence for a location on Mars that could have supported life.
Water does not mean life, however, and neither does methane.
While not a rover itself, the Mars Express orbiter pieced together its methane picture after successive turns around the planet, detecting a small amount of the gas in the atmosphere. Two other projects, one led by NASA scientist Michael Mumma of Goddard Space Flight Center and the other by Vladimir Krasnopolosky, a researcher with the Catholic University of America in Washington, D.C., used ground-based telescopes to detect Mars methane as well.
"I would say that they confirm our results," Krasnopolosky told SPACE.com, adding that his study predicted almost the exact concentration of methane, about 10.5 parts per billion, seen by Mars Express.
Volcanoes may be a source
Since methane has a relatively short lifetime on Mars for atmospheric gases, about 300 years or so, scientists believe there must be some process at work to keep replenishing its concentration in the atmosphere.
On Earth, methane is belched into the air during volcanic eruptions. It seeps out from fissures in the crust. And it is expelled by methanogenic bacteria as a waste product. While the idea of subterranean microbes living just under the Martian surface is attractive, Mars researchers are hesitant to put the full weight of their belief behind it.
"I think the first possibility, volcanism, is probably best," Chicarro said. "Volcanism has not been ruled out as a modern phenomena on Mars."
Nothing so explosive as an eruption is needed to expel the gas. It could possibly seep out through gentle, consistent hydrothermal activity, Chicarro said.
Krasnopolosky, on the other hand, said while he believes that Martian microbes are the most likely methane culprits, he cannot definitely rule out other factors. It is just as possible, he said, that methane formed in Martian volcanoes and outgassed through primordial surface vents, or even crashed down onto the planet during comet and meteorite impacts.
Locating the source of Mars methane could pare down at least some of those scenarios if researchers are able to determine local concentrations. Both Mars Express and Krasnopolosky's study measured Mars methane on a global scale. However, it may be possible for Mars Express researchers to use their spacecraft's mineralogical mapping instrument to scan the surface for signs of volcanic activity, then compare the results with methane observations.
In the meantime, researchers plan to continue their Mars studies.
"It seems like with every set of missions to Mars, instead of a gradual increase in our understanding we have a quantum leap," Chicarro said. "It's really a complicated place."