Large quantities of methane gas have been detected on Mars, Nasa scientists have announced in Science journal.
The gas could be produced either by geological activity or by life.
Methane was detected in the Martian atmosphere five years ago; scientists have found it is more abundant over particular parts of the planet.
It should last for only a short time in the atmosphere until it is destroyed by sunlight, and so its continued presence means it is being replenished.
This suggests the methane is made by an ongoing process.
We've demonstrated there are regions of active (methane) release
Michael Mumma, Nasa
But the ultimate origin of the methane could either be an ancient or a modern one, say the researchers.
"The fact that we have found three discrete regions where Mars is releasing methane at this time means we have a window into processes occurring under the surface of the planet," said co-author Michael Mumma, a senior planetary scientist at Nasa's Goddard Spaceflight Center in Maryland, US.
"The production (of methane) is likely due to only one of two possibilities. The first is geochemistry, the second is biology. That raises much interest on which one is the dominant production mechanism."
If the methane is produced by geological activity, it could either originate from active Martian volcanoes or from a process called serpentinisation.
The latter process occurs at low temperatures and occurs when rocks rich in the minerals olivine and pyroxene react chemically with water, releasing methane.
"It could be occurring on Mars. In that case, we might expect the methane to be released only under regions that are volcanic districts," said Dr Mumma.
"On the other hand, there could be biology that's going on either very near the surface or deep below the permafrost layer.
NASA's Sushil Atreya describes the latest research on atmospheric activity on Mars
"Then in order to release captured gases to the atmosphere one needs to access those sub-permafrost regions."
Some scientists consider it possible that microbes could have survived for aeons below the Martian permafrost layer, where water changes from ice into liquid.
In deep canyons, or the walls of yawning craters, ice might plug fissures or pores connecting these sub-permafrost regions to the atmosphere.
But the ice could disappear during spring and summer, Dr Mumma speculated.
The team used a technique called infrared spectroscopy at three different Earth-based telescopes to monitor about 90% of the planet's surface for seven years (equivalent to three Martian years).
They identified "plumes" of methane on Mars during 2003; at one point in their study, the primary plume of methane contained an estimated 19,000 tonnes of the gas.
This implies the gas was released from discrete regions on the planet. However, these regions do not move the researchers significantly towards one theory of methane production more than another.
"We've demonstrated there are regions of active (methane) release," said Dr Mumma.
"What we'd really like to do is map the entire planet identifying all such regions of release and validating that some are reproducible from year to year.
"In addition, we'd like to map the gases being released to establish which of those are more likely biological in origin versus others that may be geochemical in origin.
"On this basis we could target several interesting sites for lander missions."
Professor Colin Pillinger on what he makes of the methane discovery
Andrew Coates, from the Mullard Space Science Laboratory in Surrey, UK, said: "The observation of short-lived, seasonally varying clouds of methane from specific regions on Mars is a tantalising result. It shows there must be an underground source, past or present."
Dr Coates, who is not involved with the latest research, added: "Seasonal effects may open up fissures to allow increased escape into the atmosphere. But this could be a sign of either geology or biology.
"Both are exciting; we will not be sure which it is until we can analyse the methane in-situ at the surface."
Nasa's Mars Science Laboratory (MSL) rover, due to launch to the Red Planet in 2011, will carry instruments that have the potential to distinguish between carbon in gases produced by biological activity and those with a geochemical origin.
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