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.
The methane emissions, observed over three Mars years (seven Earth years), come from three distinct 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 known to be a fragile compound, and the observed variations in its concentration indicate that the methane in the Martian atmosphere lasts for less than one Earth year and is constantly being replenished, says Mumma. That suggests that even if the planet isn’t biologically active, some unknown geological process is very much alive, continually releasing methane into the atmosphere, he says.
To detect the methane, Mumma and colleagues monitored Mars from Earth, using three ground-based spectrometers to spread infrared light into its component wavelengths. The observations, which began in 2003, were unusually challenging because absorption from water vapor, ozone and methane in Earth’s own atmosphere make it difficult to isolate the Martian signals. Using a new algorithm that removed the extraneous terrestrial signals, 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. It’s fantastic.”
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, comments Jack Mustard of Brown University in Providence, R.I.
In 2003, one of the plumes released about 19,000 metric tons of methane, comparable to the methane naturally seeping from the ocean at the Coal Oil Point Reserve in Santa Barbara, Calif. The plumes were detected over locales that show either evidence of ancient ground ice or the flow of liquid water, including an area east of Arabia Terra, the Nili Fossae region and the southeast quadrant of an ancient volcano called Syrtis Major.
Another group of researchers recently reported that Nili Fossae contains carbonates, which can only form in the presence of liquid water.
Mumma and his collaborators emphasize that their measurements can’t discriminate between biological and nonbiological sources for the methane. But the team cites two possibilities for how the methane may be delivered into the atmosphere.
In one scenario, the warmer temperatures of Martian spring and summer vaporize ice that blocks cracks and fissures in rock, allowing a supply of 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 greater amount of 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 well 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 the hydrocarbon 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. The bacteria could have survived for eons if they lie below the permafrost layer, where water would be liquid, carbon dioxide provides a source of carbon and the planet’s core supplies heat, he says.
To test whether life could have produced the methane on Mars, Mumma and his collaborators are examining the isotopic ratios of carbon and hydrogen in the gas. Living things preferentially use the lighter isotopes of carbon and hydrogen. Mumma said that his team has already gathered the data on hydrogen but has not yet analyzed it.
