Methane is making a comeback on Mars.
NASA's Curiosity rover has detected a tenfold spike in the gas that lasted for several weeks, scientists announced Tuesday. On Earth, most methane is produced by living things, and a healthy dose of this organic molecule on Mars may give scientists new hope of finding signs of life there.
The discovery represents a stark reversal from last year, when the rover found scant signs of methane on the Red Planet.
"It opens up a whole debate of methane on Mars again, and life on Mars," said Christopher Webster, lead scientist for the rover's tunable laser spectrometer instrument at NASA's Jet Propulsion Laboratory in La Cañada Flintridge. "We thought we closed that chapter, but now we're on to the next chapter."
The finding was described at a meeting of the American Geophysical Union in San Francisco and published online by the journal Science.
In 2003, telescopes on Earth picked up signs of an enormous amount of methane emanating from the Red Planet. As much as 45 parts per billion of the gas seemed to be rising from the surface in plumes.
At the time, those results were hotly debated, particularly since the methane readings eventually dropped to zero and did not return. Some skeptics argued that the readings were off, but the scientists stuck by their measurements.
Curiosity, which landed on Mars in 2012 with a suite of high-tech instruments loaded in its belly, was expected to help settle the debate.
In 2013, after sniffing the air for about eight months with its laser spectrometer, the rover gave the scientific community an answer. Methane levels were no higher than 1.3 parts per billion, a tiny fraction of what had been detected a decade earlier.
"There was basically no methane on Mars, and that was a big disappointment to the planetary community," Webster said. "We thought we had closed the book on the Mars methane story at the time."
But then, around Thanksgiving last year, the rover noticed something surprising: Methane levels in the air jumped from an average of about 0.7 parts per billion to about 5 parts per billion.
"We said, 'What the heck is going on?'" said Webster, who led the Science study.
When the scientific team checked again a week later, the methane level had risen further, to 7 parts per billion. One month after that, it was still holding steady. Three weeks later, it was up to 9 parts per billion.
And then, six weeks later, the methane had completely disappeared.
Though puzzling, the measurements reveal something important about modern Mars, said study coauthor Paul Mahaffy of NASA's Goddard Space Flight Center in Greenbelt, Md.
"It's alive at some level," said Mahaffy, lead scientist for Curiosity's Sample Analysis at Mars instrument package, which includes the tunable laser spectrometer. "It's living and breathing and giving off little spurts of methane somehow."
The scientists don't know much about the origins of this Martian methane. It seems to be coming from the north, though it's not clear whether the plume is big and far away from the rover or small and close by. (They said they were leaning toward the latter scenario because the gas disappeared fairly quickly.)
"It does indeed mean that we have an active planet," said Michael Mumma, a planetary scientist at Goddard Space Flight Center who led the team that picked up the methane signal in 2003. "We don't know where the methane is coming from yet, nor do we know whether it's generated by biology or generated by geochemistry. But we do know how to go about testing that."
The findings are not necessarily a sign that there are or were microbes on Mars, since the gas could be the product of geophysical processes. It's also unclear whether the methane is old or new — it could have been stored for eons in underground ice traps called clathrates and recently released upon the ice's melting, Webster said.
"One has to treat each possibility as equal to the other," said Mumma, who was not involved in the latest analysis.
"If it's geochemistry, then that tells us we have a window into the interior processes of the planet. That's very important," he said. "If it is biology, then that's important too, for other reasons. It would mean that there would be likely an independent origin of life."
Some researchers expressed doubt about the new measurements and suggested the newly detected methane might have come from the rover itself.
"There is considerable methane on the rover," said Kevin Zahnle, a scientist at NASA's Ames Research Center in Silicon Valley who was not involved in the work. "They are looking for Martian methane through a cloud of their own methane."
Zahnle added that while the team had been diligent, "they can't rule out the possibility that something funny is going on."