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.

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.

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.

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.