Washington  - A team of NASA scientists has found proof of the existence of methane gas on Mars, again raising questions about the possibility of life on the red planet, the US space agency said Thursday.

In an article published in the journal Science, researchers said they found seasonal variations in methane in the planet's atmosphere that could be evidence of geological or biological processes. On Earth, 90 per cent of the gas is produced by living things such as the fermentation of organic material or gas released by livestock, but geochemical processes also produce levels of the gas similar to those found on Mars, the scientists said.

"Mars could be more active than we have long believed," astrophysicist Geronimo Villaneuva told Deutsche Presse-Agentur dpa in an interview before the findings were released.

It is unclear whether the methane was produced by bacteria or from so called methane ice deposits, he said. "The question of the possibility of life on Mars is now asked anew."

Scientists monitored the Martian atmosphere using Earth-bound infra-red telescopes to look at 90 per cent of the planet over three years, finding plumes of methane in
2003. More studies are needed to locate the source of the gas.

"Methane molecules have a short life span in the Martian atmosphere. The discovery of larger quantities in recent years points to a current release process," Villaneuva added.

Last year, a NASA rover confirmed the existence of water on Mars, in a key finding that leaves open the possibility of past or present life there. (dpa)