Curiosity has been exploring the planet's Gale Crater, a 96-mile-(155km)-wide depression caused by an asteroid strike, since 2012. The rover has previously found ice in the fine soil of the crater, suggesting it was once filled with water, which is believed to be crucial to sustaining life.
In the latest discovery, reported in the journal Science, spikes of methane were recorded as Curiosity scanned a 279sq m area of the crater over 60 Martian days, which are slightly longer than Earth days.
"What is interesting is that these spikes of methane are coming and going. They are transient," said Dr Paul Mahaffy at Nasa. "At the moment we can't really tell anything, but these burps are intriguing. We have to keep an open mind.
"We don't want to eliminate anything, and potentially it could indicate life or evidence of ancient methane trapped which could show ancient life. But it's interesting to think about why it comes and goes. It seems to be suggestive of a localised source."
Nasa claimed that with more readings it would be possible to test isotope levels which would prove if the emissions came from a biological source. Previous satellite observations have detected unusual plumes of methane on Mars, which has a thin atmosphere composed largely of carbon dioxide, but none as extraordinary as the sudden "venting" measured at the crater.
A low background level of methane can be explained by the Sun's rays degrading organic material deposited by meteors. But the readings in a 279sq m area spiked 10-fold over the 60-day period. By the time Curiosity had travelled another two-thirds of a mile, the higher methane levels had disappeared.
In their paper, the US scientists led by Dr Chris Webster, from Nasa's Jet Propulsion Laboratory in Pasadena, California, wrote: "The persistence of the high methane values over 60 sols [Martian days] and their sudden drop 47 sols later is not consistent with a well-mixed event, but rather with a local production or venting that, once terminated, disperses quickly."
The short timescale of the methane spikes also means it is unlikely that the gas was released from volcanic deposits trapped in ice. Nor did it appear to come from the release of gaseous methane that had become bound to the soil.
The question of whether there is, or was, life on Mars may finally be answered by the European Space Agency's ExoMars mission, which aims to land a 272kg rover on the Red Planet in 2019.
Bacteria search
• Curiosity Rover has recorded "burps" of methane on Mars.
• The gas may have been produced by primitive microorganisms living beneath the surface of the Red Planet.
• Alternatively, it could have been left over from bacteria that existed billions of years ago.
• If the existence of living, breathing microbes is confirmed, it would be the first hard evidence of life outside Earth.
