Nasa’s Curiosity rover has detected periodic “burps” of methane at the surface of Mars.
Detecting this gas is intriguing, since it could point towards there being some kind of life on the planet - almost all the methane on Earth comes from life forms.
But this discovery, by itself, is not evidence of life.
Methane can naturally form in several ways on a planet. It might get bound into the soil and, once in a while, get released because of geological processes.
Alternately, methane could be released when light hits the surface of a planet and breaks down naturally-occurring organic (carbon-based) compounds present there - these might be naturally present or have been brought in from meteorites.
Methane can also be released when underground volcanic deposits, which have been trapped in water ice, start to melt.
The Nasa team think that these natural processes do not account for the levels of methane they have measured, so they added that it might have come from methane-producing microbes.
But the scientists have also been careful to argue that this is only a hypothesis at this stage.
“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,” said John Grotzinger, the lead scientist on the Curiosity team, at a meeting of the American Geophysical Union meeting in San Francisco.
The methane discovery is a strange and surprising one because, for most of its first year of operations, the Curiosity rover seems to have found none of this gas at all in the atmosphere.
Telescopes and orbiting probes first detected signs of methane on Mars in around 2004, but those discoveries had not been verified with experiments on the ground.
Curiosity arrived on the Martian surface in August 2012 and, after almost a year, had not seen any signs of methane, which led scientists to believe that those previous measurements might have been wrong.
This announcement by Nasa turns that idea around again. The team showed that, when one of Curiosity’s instruments gulped some Martian air last year, it detected an average of seven molecules of methane per billion molecules of Martian atmosphere (a measure known as parts per billion by volume or ppbv) over the course of a few months.
Not only that but, the level of methane spiked to 9ppbv in the measurement window of 60 Martian days and then fell back to around 0.7ppbv. The wind measurements suggested a source north of the rover.
The sudden disappearance is strange because methane usually stays in the atmosphere for around 300 years once it appears. If there is a persistent source, the levels should not fluctuate anything like those measurements made by Curiosity.
The results suggest, therefore, that methane is being produced or vented somewhere near the rover and that the gas seems to disperse into the atmosphere soon after it appears.
"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 University of Michigan, Ann Arbor, and Curiosity rover science team.
"There are many possible sources, biological or non-biological, such as interaction of water and rock."
The results are published in this week’s edition of the journal Science.
Curiosity is not equipped to definitively answer whether or not the source of the Martian methane is a life form. To do that we’ll have to wait for future Nasa rovers and the European Space Agency’s ExoMars mission.
The latter will land on the surface in 2019 and drill several metres under the surface to look for more unequivocal biomarkers for life and, perhaps, even existing or fossilised life itself.