Issue Date: January 19, 2009
Methane On Mars
THE MOST DETAILED search yet for methane on Mars has yielded a dramatic result: three discrete, seasonal plumes of the gas emanating from the martian surface.
On Earth, methane is produced almost entirely by organisms, so the tantalizing possibility that the martian plumes might also be biogenic has set the planetary science community abuzz.
Michael J. Mumma, director of the Center for Astrobiology at NASA's Goddard Space Flight Center, in Greenbelt, Md., and colleagues reported the discovery in Science (DOI: 10.1126/science.1165243) and at a Jan. 15 press conference at NASA headquarters in Washington, D.C.
Scientists have known for several years that methane exists on Mars. Four years ago, three groups, including Mumma's, announced they had detected the gas, but those discoveries were based on observations of a weak, single spectral band.
The new announcement is based on seven Earth years of observations dating back to 2000. With data gathered from high-resolution infrared spectrometers at three different Earth-based telescopes, including the W. M. Keck Observatory telescopes on Mauna Kea, in Hawaii, Mumma's group spent years on painstaking analyses, removing interfering signals from Earth's atmosphere. "We think ours is the first truly reliable detection," Mumma says.
The three martian methane sources were located in various regions of Mars, separated from each other by hundreds of miles. The plumes were initially detected in 2003 and peaked during midsummer, but they had almost disappeared by the martian vernal equinox in 2006.
The variable concentrations of methane highlight another mystery: Why are methane plumes disappearing? The sunlight-driven photochemical process that would consume methane on Mars takes over 300 years, yet the plumes are vanishing within just a few years. One possible route of faster destruction is a reaction of methane with dust grains coated with strong oxidants, such as peroxide, which has been detected in the martian atmosphere.
The question of the methane's source also can't be answered right now, Mumma says. Biogenic and chemical methane have distinctly different isotopic ratios of both carbon and hydrogen. Additionally, the chemical reactions that produce methane also produce a host of other larger hydrocarbons. But telescopes don't have the sensitivity to detect isotopic ratios, and methane-associated compounds haven't yet been observed.
The upcoming Mars Science Laboratory, a rover scheduled for launch in 2011, will be able to detect such compounds directly in Mars's soil or atmosphere. Nili Fossae, one of the methane-producing regions, appears to have been wet in the past and is one promising potential landing site, Mars scientists say.
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