Strange spikes in methane which could be bacteria-like organisms were identified by the Curiosity Mars Rover.
An instrument on the Nasa robot identified spikes in the gas which cannot be easily explained by geology or by organic material from comets or asteroids.
But scientists admit they are puzzled as to what has caused the methane spike.
The mysterious discovery has been reported in a paper published in the journal Science.
Dr Chris Webster, from Nasa's Jet Propulsion Laboratory, said: "Strictly speaking, our observations are evidence for methane production on Mars, and in themselves cannot directly provide evidence of microbial life.
"However, from our positive detection of methane on Mars, we cannot rule out the possibility that both the low background level and the high methane values originate in part from microbial activity (methanogenesis).
"Our observations indicate that either the source shut off or the wind field at the source location changed to direct the emission away from us.
"At this time, we have no idea what we will see in the future, or if we will ever see high values again."
Other possible explanations include the Sun's rays degrading organic material that was left behind by meteors, Nasa scientists said at the press conference announcing the news.
— Curiosity Rover (@MarsCuriosity) December 16, 2014
The Nasa authors are cautious about jumping to conclusions, but suggest that "methanogenesis" - the formation of methane by microbial bugs known as methanogens - may be one answer to the puzzle.
They wrote: "Our measurements spanning a full Mars year indicate that trace quantities of methane are being generated on Mars by more than one mechanism or a combination of proposed mechanisms - including methanogenesis either today or released from past reservoirs, or both."
"At the moment we can't really tell anything, but these burps are intriguing. We have to keep an open mind." Nasa scientists told the Telegraph.
"We don't want to eliminate anything, and potentially it could indicate life or evidence of ancient methane trapped which could show ancient life."