Methane is making a comeback on Mars.

NASA’s Curiosity rover quashed the hopes of scientists and dreamers alike in 2013 when it found scant signs of methane on the Red Planet. On Earth, most of this gas is produced by living things, and a healthy dose of this organic molecule on our rust-hued neighbor would have given researchers hope of finding signs of life.

Now scientists say the rover, sniffing the air using its sensitive laboratory equipment, picked up a dramatic tenfold spike in methane gas that lasted several weeks. The find, described Tuesday at the American Geophysical Union and published online by the journal Science, reignites an already flammable issue: the chances for methane-linked life on Mars.

“It opens up a whole debate of methane on Mars again and life on Mars,” said lead author Christopher Webster at the Jet Propulsion Laboratory.

In 2003, scientists using ground-based telescopes picked up signs of an enormous amount of methane – as much as 45 parts per billion – coming from the Red Planet in plumes.

At the time those results were hotly debated, particularly since the methane readings eventually disappeared and did not return. Some argued that the readings were off; the scientists stuck by their measurements. Curiosity, with a suite of high-tech instruments loaded in its belly, was expected to help settle the debate.

In 2013, after testing the air for roughly eight months using its Tunable Laser Spectrometer, the rover gave the scientific community an answer – though perhaps not the one that many researchers had hoped for. Methane levels were extremely low – no more than 1.3 parts per billion, a tiny fraction of what had been detected 10 years earlier.

“There was basically no methane on Mars, and that was a big disappointment to the planetary community,” Webster said. “So we thought we had closed the book on the Mars methane story at the time.”

But then, around Thanksgiving last year, the rover picked up a strange spike – methane levels in the air jumped from an average of about 0.7 parts per billion to about 5 parts per billion.

“We were very surprised – we said, ‘What the heck is going on?’ ” Webster said.

When they checked a week later, the methane had risen further, to 7 parts per billion. They waited a month and still saw 7 parts per billion. Three weeks after that, it was 9 parts per billion. During this strange spike, the methane levels averaged 7.2 parts per billion – a tenfold increase from the baseline average. Six weeks later, it had completely disappeared.

“The methane measurements are saying something about modern Mars,” said study co-author Paul Mahaffy of NASA’s Goddard Space Flight Center, lead scientist for the Sample Analysis at Mars instrument package (which includes the Tunable Laser Spectrometer). “It’s alive at some level; it’s living and breathing and giving off little spurts of methane somehow.”

The scientists don’t know much about the origins of this methane. It seems to be coming in from the north, although it’s not clear whether the plume is big and far away, or small and close by. (They think it’s small and nearby because it disappeared fairly quickly, by methane’s standards.)

This does not at all mean that there must be microbes on Mars, because the gas could be the product of geophysical processes. In fact, it’s unclear whether the methane is old or new – it could have been stored in ice traps called clathrates under the ground for eons and recently released upon the ice’s melting, Webster said.

These are questions that Curiosity may help answer as it continues exploring Mount Sharp, a three-mile-high mountain in the middle of Gale Crater.

“We thought we closed that chapter,” Webster said of methane on Mars, “but now we’re on to the next chapter.”

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