There is methane in the Martian atmosphere, and it varies seasonally.
A NASA press release announced the discovery that methane – CH4 – in the atmosphere of Mars appears to vary temporally and spatially. Methane concentrations were measured spectroscopically using NASA’s Infrared Telescope Facility and the W.M. Keck telescope at Mauna Kea, Hawaii. The results over several Mars years show that more CH4 finds its way into the Martian atmosphere during summer there, and also there’s more CH4 in the northern hemisphere than in the southern. This could be a biological signature.
NASA is understandably and correctly cautious in its interpretation of this data.
“Right now, we do not have enough information to tell whether biology or geology — or both — is producing the methane on Mars,” Mumma said. “But it does tell us the planet is still alive, at least in a geologic sense. It is as if Mars is challenging us, saying, ‘hey, find out what this means.’ “
On Earth the majority of methane released into the atmosphere through natural processes is from methanogens, microbes that exhale methane gas as they respire. All methanogens on Earth belong to the ancient line of microbes called Archaea, which closely resemble the common ancestor of all terrestrial life. Some methane is also produced on Earth by volcanoes, and methane is also released by mining natural gas and other fossil fuels. The latter is biologically-derived, too, though volcanic CH4 is purely geological in origin.
Mars is thought to be volcanically dead, so geologic methane sources are unexpected there. An underground biosphere of microbes could account for the CH4, which might build up beneath the permafrost and seasonally diffuse into the atmosphere when the ground warms and partially melts the otherwise impermeable regolith ice. On Mars the photochemistry is wrong for making methane in the atmosphere, so the source has to be in the subsurface.
This isn’t definitive proof of life, but it is mightily odd. It would be plausible to perhaps see a one-off event where a little CH4 seeps from ancient subsurface methane ice. But a repeating temporal pattern of methane coming out of the ground every summer would be very difficult to explain as a purely geological process. If there were tons and tons of buried methane ice dating from 4 billion years ago that might be possible, if deposits were close to the surface… but so much buried methane ice would also be a dead giveaway that the planet once had an active biosphere, even if life is extinct there today.
A definitive answer is attainable, but it would require another mission. Isotopic data could be used to determine whether the CH4 in Mars’ atmosphere is biological or geological in origin. Methane produced biologically on Earth – and almost certainly anywhere else – has isotopically light carbon. Stable carbon normally comes in two isotopes, 12 and 13. Biological processes prefer the light isotope very slightly, which means that methane exhaled from microbes has a distinct carbon isotopic signature relative to volcanic methane. The Martian methane mystery could be solved with just a few measurements, but that would require a new probe with a built-in carbon mass spectrometer with which to sample the atmosphere and regolith.
Right now I’d put the odds of extant life on Mars at ~60%, based on this report and everything else we know about the planet. I’d put the odds of life having been there at least in the remote past at 80%, perhaps 90% after a few margaritas. But if the carbon isotopic data someday comes back with Martian CH4 having a δ13C of -20? Then I’d put the odds at 99% for Mars bacteria. Nobody’s getting 100% until they can capture, culture and sequence the damn things.