WASHINGTON—For years, scientists searching for life on other worlds have concentrated on finding water. No creature on Earth, from whales to bacteria, can live without it.
But now they are also pursuing another line of evidence—swamp gas.
Formally known as methane or natural gas, this common compound of carbon and hydrogen is produced by decaying vegetation, in coalmines, rice paddies and cows' stomachs, and by myriad of tiny microbes called methanogens. Even the world's trillions of termites pump out little puffs of methane when they consume wood.
The recent discovery of trace amounts of methane on Mars stirred hopes that it might be another signal that microbial life exists—or once existed—on the Red Planet.
"We have clear evidence that there is a source of methane on Mars," said Michael Mumma, an astrobiologist at NASA's Goddard Space Flight Center in Greenbelt, Md. "We're going to test this as a search for other evidence (of life)."
The space agency and its European counterpart have scheduled an international "Methane-on-Mars" workshop next month. They want to figure out how to follow this tantalizing hint that the seemingly dead planet might still be biologically active.
Interest in methane is high because this gas, a fragile compound of one atom of carbon and four atoms of hydrogen, is easily destroyed by the intense ultra-violet radiation raking the Martian surface.
Because methane's lifetime is short, scientists figure it must have been created within the last 300 years, implying that life may still exist—or very recently existed—on Mars.
"The presence of significant methane on Mars requires recent release from sub-surface reservoirs," said Mumma, who discussed his findings at an astrobiology conference in Boulder, Colo., this month.
There is a major problem, however. Although most of the methane on Earth is generated biologically, a significant amount—perhaps as much as 20 percent—comes from non-biological chemical reactions. Volcanoes, the hot springs of Yellowstone National Park, and hot-water vents at the bottom of the oceans all produce quantities of methane.
Furthermore, huge amounts of non-biological methane are found on gaseous planets like Jupiter and Saturn. Frozen lakes of methane were detected this year on Saturn's moon, Titan. Comets are full of it.
Since living creatures are not the only way to generate swamp gas, scientists can't be sure how the Martian methane originated.
"We don't have a conclusion about its origin," Mumma admitted.
"Many different lines of evidence are going to be needed to convince people that it's biological," said Stephen Mojzsis, a geologist at the University of Colorado in Boulder.
Because of this uncertainty, Norman Pace, a University of Colorado molecular biologist, warned researchers at the conference not to take the traces of methane on Mars as proof of life.
"This is not the smoking gun," he said.
Jonathan Telling, a geologist at the University of Toronto, said scientists might be able to distinguish between biological and non-biological methane on Mars by testing the carbon and hydrogen atoms in the gas. Each of these elements comes in a lighter and heavier form. On Earth, the lighter forms are generated by living organisms, the heavier ones by inorganic chemistry. Presumably the same thing could happen on Mars.
Tullis Onstott, a geoscientist at Princeton University, Princeton, N.J., has developed a device to measure the heavier and lighter forms of carbon and hydrogen in Martian methane. He wants to put his instrument on a future Mars rover, hoping to determine whether the gas is biological or non-biological in origin.
The Martian methane was detected in 2003 and 2004 by three separate observatories—ground telescopes in Hawaii and Chile and a European space satellite. It was coming from at least five different locations on the planet.
The amounts were tiny—from 10 to 200 parts of methane per billion parts of atmosphere—compared to an average of 1,500 parts per billion on Earth. But that was enough to set off a new wave of excitement among those seeking extraterrestrial life.
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For more information, go to: http://www.wcsscience.com/methane/page.html
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(c) 2005, Knight Ridder/Tribune Information Services.
GRAPHIC (from KRT Graphics, 202-383-6064): 20050428 METHANE
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