CAMBRIDGE, U.K.--Forget microbes or Martians. The trace amount of methane gas in the atmosphere of the red planet is not produced by living organisms. Instead, a geochemical process known as low-temperature serpentinization, which involves the alteration of basalt by water, is the source, according to a new theory presented here at the 37th annual meeting of the Division of Planetary Sciences of the American Astronomical Society.
Last year, a spectrometer on board the European Space Agency's Mars Express spacecraft detected methane in regions where there also appears to be sub-surface ice (ScienceNOW, 29 September 2004,). This led some researchers to suggest that bacteria living in the ice might be producing the gas. After all, almost all methane in Earth's atmosphere comes from living organisms. According to planetary scientist Sushil Atreya of the University of Michigan in Ann Arbor, many alternative explanations for the existence of Martian methane don't make sense. Volcanic activity would also release sulphur dioxide, which is not observed. In addition, a freak cometary impact in the past few thousand years could have delivered methane to the Martian surface, but then the gas wouldn't be concentrated in specific regions.
During a presentation at the meeting on Tuesday, Atreya launched a new idea. He described a phenomenon in which water reacts with rocks, producing minerals known as serpentines and releasing hydrogen molecules. The hydrogen then reacts with carbon dioxide to produce methane. Low-temperature serpentinization in presumed aquifers at a few kilometers beneath the Martian surface could produce up to 200,000 tons of methane per year, Atreya calculates--more than enough to explain the observed concentrations of some 10 parts per billion.
But Michael Mumma of NASA's Goddard Space Flight Center in Greenbelt, Maryland, says he's not sure that the process applies to Mars. Using ground-based telescopes, Mumma has observed much higher methane concentrations (around 250 parts per billion) in some equatorial regions of Mars. "I'd keep the biological option open," he says. Atreya counters that such large concentrations would almost blind Mars Express's sensitive spectrometer, which hasn't happened.
A definitive check on the origin of methane will likely have to await NASA's future Mars Science Laboratory, scheduled for launch in 2009. Says Mumma: "This is going to be a long tale."