Nasa’s robotic rover, Curiosity, has detected fluctuating wafts of methane on Mars, fuelling speculation that the gas may be coming from a form of life on the red planet.

An instrument on the six-wheeled robot measured mysterious spikes of methane that cannot easily be explained by geology or organic material transported to the planet by comets or asteroids.

“That we detect methane in the atmosphere on Mars is not an argument that we have found evidence of life on Mars, but it’s one of the few hypotheses that we can propose that we must consider,” John Grotzinger, a scientist on the Curiosity team, told the American Geophysical Union meeting in San Francisco. “Large organic molecules present in ancient rocks on Mars is also not an argument that there was once life on ancient Mars, but it is the kind of material you’d look for if life had ever originated on Mars.”

The instrument recorded a 10-fold increase in methane in the atmosphere around it and detected other organic molecules in powdered rock collected by the rover’s drill, the first definitive detection of organics in surface materials of Mars. These organics could either have formed on Mars or landed on Mars via meteorites.

“This temporary increase in methane, sharply up and then back down, tells us there must be some relatively localised source,” said Sushil Atreya, of the Curiosity science team at the University of Michigan in Ann Arbor. “There are many possible sources, biological or non-biological, such as interaction of water and rock.”

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