The gas might have been emitted from microscopic organisms living under the Martian surface, the scientists said, although it could also have come from geochemical processes not connected with life. The research, funded by Nasa, the US space agency, was published in the online edition of the journal Science.

Michael Mumma and colleagues made the discovery by searching the Martian surface for the spectroscopic signature of methane, using three Earth-based observatories. They found that several “plumes” of methane were released from specific points on the planet during 2003; the gas had largely disappeared by 2006.

The most interesting possibility is that Mars harbours “deep bio-communities” that use hydrogen as an energy source and emit methane, similar to those found 2-3km below ground in the Witwatersrand basin of South Africa. They could have been isolated from millions of years from the inhospitable conditions on the planet's surface.