NASA and Science magazine announced Thursday that large amounts of methane have been found on the Red Planet, which could be a sign of biological activity.

Nearly 21,000 tons (19,000 metric tons) of methane were released all at once during the late summer of 2003, according to a study published Thursday in the online edition of Science.

"This raises the probability substantially that life was there or still survives at the present," study author Michael Mumma of NASA's Goddard Space Flight Center told The Associated Press. "We think the probability is much higher now based on this evidence."

By 2006, most of the methane had disappeared from the Martian atmosphere, adding to the mystery of the gas, Mumma wrote.

On Earth, methane comes mainly from belching animals such as cows and rotting organic matter such as dead leaves. But it's also pumped out by volcanoes.

"The most obvious source of methane is organisms," planetary scientist Colin Pillinger told London's Sun tabloid. "So if you find methane in an atmosphere, you can suspect there is life. It's not proof, but it makes it worth a much closer look."

The catch is that it breaks down quickly in the atmosphere due to reactions with sunlight, and there haven't been any active volcanoes on Mars for millions of years.

So it could be that the large amounts of methane spotted floating over Mars were created by microbes buried under the soil.

Or it could just be the result of some little-understood geological process.

It's "the most important discovery of all time," former British civil servant and fervent UFO hunter Nick Pope told the Sun. "We've really only scratched the surface — it's an absolute certainty that there is life out there and we are not alone."

London's Daily Mail added that "three giant telescopes on Hawaii" — presumably among the 12 or so observatories clustered atop the two-mile-high summit of Mauna Kea on the Big Island — had detected the seasonal methane emissions.

The European Mars Express Orbiter first detected small amounts of methane in the Martian atmosphere in 2004.

Other spacecraft have found significant amounts of water on the Red Planet, as well as geologic formations that appear to have carved by flowing liquids.

Mars is much smaller, colder and drier than Earth, with a thin atmosphere and high surface levels of radiation, but there may be large buried glaciers at the poles.

It's thought primitive bacteria may exist underground, subsisting on chemicals and water — and possibly emitting methane.