It may only be a very small part of Mars' atmosphere but methane waxes and wanes with the seasons, scientists say.
The discovery made by the Curiosity rover is important because it helps narrow the likely sources of the gas.
On Earth, those sources largely involve biological emissions - from wetlands, paddy fields, livestock and the like.
No-one can yet tie a life signature to Mars' methane, but the nature of its seasonal behaviour probably rules out some geological explanations for it.
"For the first time in the history of Mars methane measurements, we have something that's repeatable," said Dr Chris Webster, a US space agency (Nasa) scientist working on Curiosity.