LOUISVILLE, Ky. – Methane gas detected on Mars could be a sign of extraterrestrial life, scientists announced Thursday.

But don’t get ready for E.T. just yet. There are many possible explanations for the methane, and tiny martian critters are just one.

Still, the detection of methane had scientists buzzing in Louisville at a meeting of the American Astronomical Society’s Division of Planetary Sciences.

“I stand before you and tell you, quite honestly, I’m shocked by these results,” said Michael Mumma, an astrobiologist at NASA’s Goddard Space Flight Center in Greenbelt, Md.

Mumma and colleagues discovered unusually high levels of methane at two places in Mars’ atmosphere: above the Hellas Basin, a giant impact scar in Mars’ southern hemisphere, and Valles Marineris, the great canyon system near the martian equator.

Methane is a gas that, on Earth, is produced naturally by plants and animals, such as in wetlands and in the stomachs of cows. On Mars, methane is much rarer. It isn’t produced in the atmosphere and would likely be destroyed there by chemical reactions within a few hundred years.

So finding methane in the atmosphere suggests that something on Mars’ surface is producing it, Mumma said. The question is whether that something is alive.

Last month, researchers from the European Space Agency announced that the Mars Express spacecraft found three regions of Mars with relatively high levels of atmospheric methane. Possible sources include bacteria, hot springs and comets, the scientists reported in the online edition of the journal Science.

At first glance, Mumma’s findings don’t seem to jibe. His two methane-rich regions aren’t the same as the Europeans’ three. But some of the European measurements were taken early this year, right after a dust storm smeared through Mars’ atmosphere and possibly affected the observations, Mumma said.

If martian bacteria exist, they probably live deep underground, munching on carbon dioxide and hydrogen and spitting out methane, said Vladimir Krasnopolsky, a researcher at the Catholic University of America.

Any bacteria would probably live in isolated spots on Mars, Krasnopolsky added. “Mars is generally sterile except for some small oases,” he said.

The possibility of life on Mars has fascinated, then disappointed, generations of scientists. A notorious 1996 announcement that a meteorite contained fossilized martian bacteria is now generally dismissed by the scientific community. The methane discovery may reopen many debates.

Mumma’s team used two large telescopes, in Hawaii and in Chile, to scrutinize the martian atmosphere. By studying the spectrum of light coming from various locations, the scientists detected methane unequivocally for the first time, he said.

Mumma is skeptical about the methane coming from bacteria. He favors the idea that methane-rich ices called clathrates break apart on Mars, releasing the gas into the atmosphere above.

“We have an exciting time ahead of us,” he said.