Since methane was first detected on Mars' atmosphere in 2013, scientists have speculated that it could be created by life on the Martian planet.
Suggestions that methane may not be evenly distributed in the atmosphere, and that there are methane 'spikes' on Mars have done little to solve the mystery.
Recently, a team of researchers from Newcastle University ruled out one source for the gas in Mars' atmosphere.
Wind erosion ruled out
Researchers at Newcastle University, UK, have published a paper in Scientific Reports that rules out the theory that methane in Mars' atmosphere is released by wind erosion of rocks, leading to the release of trapped methane.
In a press release, study lead investigator Dr. Jon Telling, a geochemist based in the School of Natural and Environmental Sciences at Newcastle University, said:
"We realised one potential source of the methane that people hadn't really looked at in any detail before was wind erosion, releasing gases trapped within rocks. High resolution imagery from orbit over the last decade have shown that winds on Mars can drive much higher local rates of sand movement, and hence potential rates of sand erosion, than previously recognised.
"Using the data available, we estimated rates of erosion on the surface of Mars and how important it could be in releasing methane. And taking all that into account we found it was very unlikely to be the source."
Funded by the UK Space Agency, the Newcastle University researchers compared new and existing data on Mars' atmosphere to study the contents of different rock types on the planet - and whether they contain significant amounts of methane.
In order for wind erosion of rocks to be the main source of methane on Mars, the methane content of rocks would have to be similar to those of some of the richest hydrocarbon containing shales on Earth; an extremely unlikely scenario, the researchers say.
They haven't ruled out life on Mars
All of this means that life - existing or in fossilized form - has not been ruled out on Mars.
"The questions are—where is this methane coming from, and is the source biological? That's a massive question and to get to the answer we need to rule out lots of other factors first," Telling said.
However, as he points out, the new study is a small step towards answering the big question:
"What's important about this is that it strengthens the argument that the methane must be coming from a different source. Whether or not that's biological, we still don't know."
And the aim of the research is precisely that - to find out if life can exist on other planets.
As Lead author Dr. Emmal Safi, a postdoctoral researcher at Newcastle University, says, "ultimately, what we're trying to discover is if there's the possibility of life existing on planets other than our own, either living now or maybe life in the past that is now preserved as fossils or chemical signatures."