The mystery over methane in the Martian atmosphere — and whether it's being created by life on Mars — just got more mysterious: Scientists say the methane levels recorded by NASA's Curiosity rover are generally lower than expected, but they also saw a huge spike in the gas that lasted two months, and then disappeared.

What caused the spike? The Curiosity team doesn't know, but it could be methane-belching microbes.

"We can't rule it out," Chris Webster, a researcher at NASA's Jet Propulsion Laboratory who's the lead author of a paper reporting the mystery in the journal Science, told NBC News. "It's equally likely to be geophysical or biogenic. ... The fact that we've seen it, in a sense, argues that the stock in a possible biogenic source went up."

The findings are particularly exciting in light of the fact that, just a year earlier, Webster and his colleagues reported they weren't seeing nearly as much Martian methane as previous probes had. Those results came as a disappointment to astrobiologists, and these results are likely to boost their spirits again.

"They completely blow open the whole debate on Mars methane. ... I think it's going to create a lot of enthusiasm within the planetary community," Webster said.

Additional findings from other instruments on Curiosity, suggesting that organic chemistry is at work on Mars, added to the excitement.

Years-long debate

The methane debate goes back more than a decade, to atmospheric readings taken by the European Space Agency's Mars Express orbiter. Those readings showed the subtle signature of methane, a gas that typically lasts only a few hundred years before it breaks down.

Methane can be created through geological processes, such as volcanism or hydrothermal activity. But there's little evidence of those processes on Mars — and on Earth, methane is more typically created through biological digestion, most notably by microbes. (Although cows count, too.)

Since the Mars Express findings, measurements of Martian methane have varied from less than 5 parts per billion to 60 parts per billion. The background level recorded by Curiosity's Tunable Laser Spectrometer and reported in the newly published paper is very low: somewhere around 0.7 parts per billion. That's far lower than what was predicted by the models for Mars' atmospheric chemistry.

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Those low results suggest that Mars isn't getting as much organic material from cosmic impacts as expected, Webster said. "This is extremely important for scientists trying to understand how organics arrived at planets, and understand cross-fertilization [of organics] in the solar system," he said.

Even bigger questions surround a spike in methane levels that was seen in readings taken between late November 2013 and late January of this year. "What happened was, just after Thanksgiving last year we suddenly saw this very high level," Webster said. "We were completely surprised."

During that period, the average concentration of methane was about 7 parts per billion, or 10 times the background level. But after January, the level returned to just a trace and didn't rise again.

What's going on?

Since then, Curiosity's researchers have been working through the possible causes for the spike. To the best of their ability, they excluded the possibility of earthly contamination inside the rover itself. They've seen no evidence of fresh impact craters that might have caused a blast of gases, and there's no indication of a temperature rise associated with the spike.

Based on wind direction and the pattern of the rise and fall, scientists concluded that the spike is most consistent with a low-level, episodic release of the gas nearby, from somewhere to the north of the rover in Gale Crater.

That sort of phenomenon could conceivably be associated with an active colony of methane-producing microbes, perhaps living beneath the Martian surface. But Webster emphasized that other scenarios would fit the pattern as well.

"There's biology today, and there's biology in the past, and we can't rule out either of those as contributors."

For example, unknown circumstances might have sparked the release of stored-up methane from molecular cages known as clathrates. Our planet has a huge reservoir of methane clathrates deep in the oceans and in Arctic ice. Perhaps Mars has something similar.

But even then, questions would remain about the source of those methane clathrates. Perhaps that gas was stored up in ancient times — when Mars was warmer and wetter, and microbes conceivably held sway in Gale Crater's giant lake.

"There's biology today, and there's biology in the past, and we can't rule out either of those as contributors," Webster said.

In the months and years ahead, Curiosity's instruments will continue sniffing for signs of methane. If another spike occurs, the Tunable Laser Spectrometer will be primed to enrich the gas and try to determine how much carbon-13 and carbon-12 is contained in the methane molecules.

"This would be a very important, exciting finding that could help discriminate between biological and geophysical sources," Webster said.

Organics at work

In addition to the methane, Curiosity's onboard chemistry lab — known as Sample Analysis at Mars, or SAM — detected other types of organic molecules. The Tunable Laser Spectrometer is part of the SAM instrument suit.

Curiosity's team said the other organics came from powder that was drilled out from a rock called Cumberland and heated up inside SAM. In this context, "organic" doesn't necessarily mean the compounds were created through biological processes on Mars. It just means the compounds contain organic carbon. That carbon could have been brought to Mars by meteorites that fell to the surface, scientists said.

Researchers worked for months to confirm that the organic carbon wasn't carried to the Red Planet by the rover itself.

""This first confirmation of organic carbon in a rock on Mars holds much promise," a member of the research team, Roger Summons of the Massachusetts Institute of Technology, said in a NASA news release. "Organics are important because they can tell us about the chemical pathways by which they were formed and preserved."

Simmons said one of the next steps would be for Curiosity to identify other rocks that might have a wider spectrum of organic compounds.

The Curiosity team's findings about Martian organic compounds were discussed Tuesday at the American Geophysical Union's meeting in San Francisco, and NASA said publication in a scientific journal was pending.

Webster is the principal author of "Mars Methane Detection and Variability at Gale Crater," published online by Science. The paper lists 28 other authors, plus the Mars Science Laboratory science team.