EARTH aside, Mars is the most-studied planet in the solar system. Over the past half century a fleet of probes has photographed it, prodded it, sniffed it and blasted it with lasers. Most of the data those probes have gathered suggest Mars is biologically and geologically dead. Most, but not all. Over the past decade or more, several groups of scientists claim to have detected methane in the planet’s atmosphere, using observations both from orbiting spacecraft such as Mars Express, a European probe, and from Earth-bound telescopes. Some have reported that the methane comes and goes in vast plumes, which cover much of Mars’s surface.

For planetologists, methane is exciting. At the least, its presence suggests Mars is more geologically interesting than some had feared. At best it could be evidence that, somewhere on the orb, alien bacteria are eking a living—in other words, there really are Martians, albeit primitive ones. But not everyone is convinced the detections of methane are real. Sceptics got a boost in 2013 when measurements by Curiosity, NASA’s latest and most sophisticated Mars rover (pictured above), found only tiny, boring quantities of the gas.

On December 16th, though, Curiosity’s guardians stirred things up again by publishing some new results, garnered over the past few months, in Science. These give succour to both camps. Most of the time, the rover recorded only a faint whiff of methane on the air, with an average concentration of 0.69 parts per billion—about the same as it had seen in 2013. But for a period of about two months it found itself in the middle of a methane plume. Concentrations of the gas rose more than tenfold, to 7.2 parts per billion, before falling off as quickly as they had come.

It’s a gas! Or is it?

Everyone agrees that there should be some methane on Mars. Space dust and meteorites will bring organic compounds, which are then split apart into simpler molecules (including methane) by ultraviolet light from the sun.

When researchers crunched the numbers in 2012, they predicted this would mean about 2.2 parts per billion of the Martian atmosphere was methane. That is more than Curiosity’s background reading suggests, but the difference is not a serious discrepancy.

The idea of periodic plumes, though, seems to fly in the face of the rules of atmospheric chemistry. Occasional methane burps might be created by residual geological processes, or even by bacterial respiration. The problem is what happens once the methane is emitted. Some studies report levels as high as 60 parts per billion. All agree that, after a plume, methane levels fall rapidly within a few months. But Mars should not be able to scrub the gas from its atmosphere that quickly. Its destruction by ultraviolet radiation from the sun, the only scrubbing process known, should take about 300 years.

If the plumes are real, something else must be clearing the methane out. One possibility is oxidation. But that would consume oxygen as well as methane, depleting Mars’s atmosphere of the stuff in a few thousand years—something that, evidently, has not happened. (Martian air is about 0.15% oxygen.) Another option is that the gas gets adsorbed onto rocks. But adsorption should affect other gases, too. Xenon, in particular, should be even keener than methane to lock itself away like this, yet there is far more free xenon than methane in the Martian air.

Another possibility is that the putative extra methane is a chimera. Certainly, previously claimed detections of methane might have been mirages. Those from orbit were at the edge of what the instruments aboard Mars Express could manage. Those from Earth-bound telescopes had to distinguish between Martian methane and methane in Earth’s atmosphere, a tricky job that comes with necessary margins of error.

There are few doubts about the quality of Curiosity’s data, though. Its instruments are extremely sensitive and NASA’s scientists have checked and rechecked them for any signs of bias or excess noise and come up with nothing. “This is a real signal,” says Chris McKay, a longtime sceptic of the plume hypothesis who is nevertheless one of the authors of the paper in Science.

So what explains Curiosity’s plume? Dr McKay, who still does not believe in the giant plumes reported previously, says that there must have been a nearby meteorite strike, and that Curiosity was merely smelling the freshly delivered organic compounds decaying. Once that process had finished, and the methane had been carried away by the Martian winds, the readings would fall quickly back to background levels (as they did), with no need to invoke any new or exotic chemistry.

Perhaps. But there is a lot of Mars for meteorites to hit. That Curiosity should find itself nearby seems quite a stroke of luck. And, when Curiosity’s science team used orbiting spacecraft to look for fresh craters in the rover’s vicinity, they came up with nothing. Dr McKay admits his hypothesis sounds improbable. But he points out that the rover’s readings did not rise to anywhere near the levels that were supposedly detected before. Besides, he claims, the alternatives are even more unlikely. Quoting Sherlock Holmes, he says, “once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.”

The only way of solving the mystery is to gather yet more data. Another spacecraft, ExoMars Trace Gas Orbiter, run by the Europeans and the Russians, should arrive in 2016. As its name suggests, it is designed to probe the Martian atmosphere for rare gases, and its orbital perch will give it a view of the entire planet. Its data might allow researchers to decide once and for all whether Martian methane is a tantalising mirage or a real, but baffling, mystery.