The planet Mars appears to be venting methane gas into its atmosphere from at least three places underground, say scientists who have been watching it with Earth-based instruments. And, while they want to be cautious, they say they are very intrigued.
Why does methane matter? Because on Earth, at least, 90 percent of it is created by living things.
"It's easier to explain methane as a product of biology than geological processes," said Michael Meyer, the lead scientist for NASA's Mars program. "We're only holding back because it's such an important result."
The observations were made over a seven-year period using instruments mounted on three large telescopes in Hawaii. The scientists were looking for methane, water vapor and other gases. The gases were detected by spectrometers, instruments that can analyze the chemical composition of distant objects by the light they give off or absorb.
The plumes of methane were spotted hundreds of miles apart and only during the warmer months of the Martian year, said Michael J. Mumma of NASA's Goddard Space Flight Center, the lead researcher.
"That rules out something like a rock slide exposing a pocket of gas underground," Mumma told ABC News. "It's easy to see why you might have that if there were just one site, but not three."
Mumma and his colleagues published their results online in this week's edition of the journal Science.
Methane is a colorless, odorless gas. It is the main component of natural gas, used as heating fuel here on Earth, and is largely formed from the decay of plant life. Most of the natural methane in the earth's atmosphere comes from swamps and bogs. It is also produced as animals, from cattle to the smallest microbes, digest their food.
Methane is abundant in the atmospheres of the large outer planets in the solar system -- Jupiter, Uranus and Neptune -- but those worlds do not have the earthlike characteristics that make Mars interesting to many biologists.
Scientists generally believe there is nothing alive on the surface of Mars today. The atmosphere there, mostly carbon dioxide, is thin, and the temperatures are sometimes hundreds of degrees below zero.
But NASA's Mars rovers found evidence that the Martian surface was once warm and wet, possibly with pools of brackish water, and scientists are curious to know if microbes could have lived there in the past, or could even survive underground today.
"The methane we detected is of unknown age; its origin could be ancient, or perhaps recent," Mumma and his colleagues write. "Both geochemical and biological origins have been explored, but no consensus has emerged."
But something mysterious is going on there, NASA's Meyer said. "Either Mars is a lot more geologically active than we thought, or there's biology there."
He urged caution, citing a famous quote from the late astronomer Carl Sagan: "Extraordinary claims require extraordinary evidence."
Mumma agreed. "We cannot confirm biological activity on Mars," he said. "To do that, we need to land there and check it out."