Plumes of Martian methane hint at possible underground microbial life: but emissions could just be signs of geochemical processes.

No one is suggesting that Mars has flatulent cows, but a new study shows that the Red Planet, like Earth, spews methane. Researchers say it's possible that the gas could be generated by bacteria living beneath the Martian surface.

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The methane emissions, observed over three Mars years (seven Earth years), come from three locations and vary with the seasons--strongest in Martian summer and weakest in winter, Mike Mumma of NASA's Goddard Space Flight Center in Greenbelt, Md., and his colleagues report online January 15 in Science.

Methane is a fragile compound, and the variations in its concentration indicate that methane in the Martian atmosphere lasts for less than one Earth year and is constantly being replenished, Mumma says. That suggests that even if the planet isn't biologically active, some unknown geological process is very much alive, continually releasing methane into the air.

To detect the methane, Mumma and colleagues monitored Mars from Earth, using three ground-based spectrometers to spread infrared light into its component wavelengths. Using a new algorithm that removed extraneous signals from Earth's atmosphere, the team detected three absorption features that conclusively prove the presence of methane plumes on Mars.

"Mumma and his team have been painstakingly careful," comments astrobiologist Christopher Chyba of Princeton University. "The reward is that we have observations of methane that show variations over season and by location."

The European Space Agency's orbiting Mars Express had previously found hints of methane, but the craft's spectrometer isn't sensitive enough to make a definitive measurement, says Jack Mustard of Brown University in Providence, R.I.

In 2003, when the observations began, one of the plumes released about 19,000 metric tons of methane. The plumes were detected over locales that show either evidence of ancient ground ice or the flow of liquid water, including the Nili Fossae region, an area east of Arabia Terra and the southeast quadrant of an ancient volcano called Syrtis Major.

Another team recently reported that Nili Fossae contains carbonates, which form only when liquid water is present.

Mumma and colleagues say that their measurements can't discriminate between biological and nonbiological sources of methane. But the team cites two possibilities for how the methane might be delivered into the atmosphere.

In one scenario, the warmer temperatures of Martian spring and summer vaporize ice that in colder months blocks cracks and fissures in rock, allowing methane that had accumulated underground to seep into the Martian atmosphere. During winter, the ice redeposits, once again plugging up the cracks.

Another possibility is that reserves of methane, rather than being sealed inside ice-covered rocks, are trapped inside molecular cages called clathrates. During Martian summer, the increased sunlight striking the icy clathrates directly liberates the trapped methane.

In either case, the methane might have been produced as a by-product of a purely geochemical process in which iron oxide is converted into another group of minerals called serpentines. That process occurs on Earth and might also happen on Mars.

"It's entirely possible that the signature is due to methanogenic microorganisms, but nonbiological mechanisms have also been proposed and a biological explanation can't be embraced unless we have stronger evidence," says Chyba. "Nor should it be dismissed."

Livestock and decomposing plants account for more than 90 percent of the methane in Earth's atmosphere, but terrestrial methane-producing bacteria are also known. If some kind of microscopic life is producing the Martian methane, it could reside underground, where temperatures may be warm enough for water to be liquid, says Mumma.

The team says it can't determine whether the methane now being released into the atmosphere was produced recently or billions of years ago. But an old reservoir of methane is problematic, Mumma says, because it would be hard to explain how it could be steadily released over billions of years. That would suggest that if bacteria are indeed the source of the methane, the organisms are active now.