Now NASA has confirmed at ground level by the Curiosity rover that there is methane on Mars but it does seem to appear and then disappear, or being created and destroyed, or formed and then transformed.
What is the source of the Mars methane? How did it get there? Why is the methane being released now and/or in bursts?
The presence of methane, however brief, is perplexing, because it does not last long. Calculations indicate that sunlight and chemical reactions in the Martian air would break up the molecules within a few hundred years, so anything there now must have been created recently.
The two mostly likely candidates are a geological process known as serpentinization (which requires both heat and liquid water) and life — a class of microbes that produce methane.
A New Clue in the Search for Life on Mars: Methane Gas
Methane on Mars - where does it come from? How did it get there?
The Curiosity team cannot identify the source of its methane, but the leading candidate is underground stores that are periodically disturbed.
Curiosity scientist Sushil Atreya said it was possible that so-called clathrates were involved. "These are molecular cages of water-ice in which methane gas is trapped. From time to time, these could be destabilised, perhaps by some mechanical or thermal stress, and the methane gas would be released to find its way up through cracks or fissures in the rock to enter the atmosphere," the University of Michigan professor told BBC News.
This, of course, still leaves open the question of how the methane (CH4) got into the clathrate stores in the first place. It could have come from Martian bugs; it could also have come from a natural process, such as serpentinisation, which sees methane produced when water interacts with certain rock types.
Methane 'belches' detected on Mars
Are there other methods or theories of how it could have got there or being produced? Science did not seem to predict this finding or the appearance and disappearance of it - so to use those same theories that didn't predict it to suggest the source and process of the Mars methane gas seems a bit of a way to guarantee failure.
Previous Methane gas detected in Mars atmosphere
"Methane is quickly destroyed in the Martian atmosphere in a variety of ways, so our discovery of substantial plumes of methane in the northern hemisphere of Mars in 2003 indicates some ongoing process is releasing the gas," said Dr. Michael Mumma of NASA's Goddard Space Flight Center in Greenbelt, Md. "At northern mid-summer, methane is released at a rate comparable to that of the massive hydrocarbon seep at Coal Oil Point in Santa Barbara, Calif."
Martian Methane Reveals the Red Planet is not a Dead Planet