Later this year, NASA’s Mars Curiosity rover could solve a big mystery that has plagued the scientific community for half a century: does Mars have methane in its atmosphere?

Curiosity celebrates three years of exploring Mars tonight. The car-sized robot landed on the planet at 1:17AM ET on August 6th, 2012 and has since learned a lot about its red home — including the existence of ancient water flows. It's also measured the radiation levels on Mars and found evidence that the planet could have supported life millions of years ago.

Curiosity's biggest discovery came late last year, however, when NASA researchers announced that the rover had detected methane spikes in Mars' atmosphere — which may mean something is producing the gas on the planet. In a study published in Science, the researchers wrote that Curiosity's onboard Sample Analysis at Mars (SAM) laboratory measured the average amount of methane in the air to be 7 parts per billion during late 2013 and early 2014. Measurement just before and after that time detected methane in the atmosphere at just 0.7 parts per billion.

The findings have reignited a decades-long debate over whether or not methane exists on Mars. Back in the 1960s, a NASA spacecraft seemed to have detected methane — but upon closer inspection, it had sniffed out the wrong gas. Since then, researchers have used telescopes and other spacecraft to try to definitively locate methane coming from the planet. Methane on Mars is a big deal: it would end the long-standing debate, of course, but it also has significant implications. Methane is a signature of organic matter — potentially, even, of life. On Earth, about 90 percent of atmospheric methane is produced from the breakdown of organic matter.

"So by extrapolation, if there’s methane on Mars, it's possible that it's coming from biological sources," Chris Webster, lead author of the Science study and senior research scientist at NASA's Jet Propulsion Laboratory, tells The Verge.

However, not everyone in the scientific community is convinced that the methane Curiosity detected is coming from Mars. Kevin Zahnle, a scientist at NASA Ames Research Center, says that the measurements are behaving in a funny way — suddenly appearing and then disappearing. This indicates that the methane is coming from somewhere nearby, but he says it’s not from a Martian source. "There is a known local source of methane on Mars, and it’s the rover itself," says Zahnle. "So just from the Occam's razor perspective, you have to think that the methane is somehow coming from the rover."

The methane saga

Whether or not Mars' atmosphere holds methane has been hotly debated for decades. The drama began in 1969, when researchers at the University of California-Berkeley announced that NASA's Mariner 7 spacecraft flying past Mars had detected methane near the planet’s polar ice caps. Those findings were retracted a month later after it turned out the methane signal had actually been carbon dioxide coming off the polar ice — the spacecraft’s instrument wasn’t sensitive enough to distinguish between CO2 and methane.

This image shows concentrations of methane discovered on Mars by NASA researchers in 2003. (NASA)

Then, NASA researchers using ground-based telescopes said they had mapped multiple methane plumes coming off the Martian surface in 2003. They released an intricate map of these plumes, showing distinct areas of methane measuring 60 parts per billion; Earth's atmosphere is estimated to have nearly 2,000 parts per billion of methane.

Except there was a problem: the methane signatures didn’t last very long. By 2006 they had all but disappeared. That's perplexing since methane has a chemical lifetime of 300 years on Mars, which is how long it takes ultraviolet light from the Sun to break the molecule apart. Methane discovered in 2003 should still have been around a mere three years later.

In 2004, the Mars Express — a satellite orbiting the planet — also detected methane in the atmosphere. The regions where the spacecraft found methane didn’t match up with the locations seen by ground-based telescopes, though. These inconsistencies led Zahnle and a group of researchers to conclude that there wasn’t definitive evidence for methane on Mars.

That’s why part of Curiosity’s mission goal was measuring methane. In theory, evidence of the gas detected on the Martian surface should be more reliable than detection from Earth or Mars orbit. The rover's engineers equipped the bot with the SAM instrument, which is specifically designed to search for chemical compounds of carbon — including methane.

But when Curiosity first arrived on Mars in 2012, SAM didn't pick up any methane coming from the Martian atmosphere for nearly a year.

A plume in bloom

Curiosity's SAM instrument consists of two chambers. One of them, the foreoptics chamber, generates a laser light that shines into the other, called the sample cell chamber. That sample chamber is filled with — yep — samples of Martian air. The laser then bounces back and forth between two mirrors inside the sample cell. The length of time it takes the laser to bounce between the mirrors tells the researchers what compounds — including methane — are found in the sample.

Curiosity's SAM instrument consists of two different chambers — the foreoptics chamber and the sample cell chamber. (NASA/JPL)

When the NASA research team first switched on the SAM instrument after Curiosity had landed, they detected high methane signals right away. But that methane came from Earth — a little bit of Florida’s air had come along with Curiosity for its ride, stowed away in the foreoptics chamber. So the researchers pumped out the chamber, leaving just a little bit of methane to act as a signal for comparison.

After that, SAM picked up very tiny signals of methane, at around 0.7 parts per billion. Webster says that was considered a "non-detection" in the science community. "The community was very disappointed," he says. "There were thousands of news stories claiming there was no life on Mars based on those measurements."

Then starting in late 2013, SAM measured four big methane spikes. Together the measurements averaged at 7 parts per billion — a fairly high value, and 10 times the previous reading. The spikes were detected over a period of two months before they disappeared, and SAM went back to picking up 0.7 parts per billion of methane. This led the researchers to believe that the rover had encountered one of the planet's mysterious methane plumes for a little while.

Was it just a leak?

Curiosity is currently located in Gale Crater. (NASA)

When Zahnle first heard about these findings, he was immediately suspicious. There are a few ways that the rover may be responsible for the methane, he says. It’s possible that the methane is leaking from one of the substances inside Curiosity — or the methane left inside the foreoptics chamber is building up somehow. Zahnle says the Curiosity team is aware of these potential leak sources, but that the researchers are making a "more hopeful" interpretation of where the methane spikes are coming from.

"It would be nice if they were a little less totally optimistic about their approach," says Zahnle."It's documented that methane is building up in the rover. It's coming from the rover. Some place or places are leaking and creating methane."

But Webster rejects the idea that the rover is to blame, saying it's impossible. He notes that 7 parts per billion is just too high of a concentration to be coming from the foreoptics chamber. "The numbers just don't add up for that," says Webster.

We may have a resolution to the debate very soon, according to Webster. It's possible that the high methane is seasonal and occurs every year on Mars in the northern spring. This November will mark one full Martian year since the methane spikes two years ago — so if the plumes return, it means they are cyclical. "If the methane comes back, that will be the biggest story about Mars methane to date, because it's not just seeing high methane; we'll see a pattern in it."

A return of methane could indicate two different explanations for the gas. It’s possible that every Martian spring, the Sun is positioned where it breaks down ancient organic material near where Curiosity is currently located. Or it could mean something much more exciting — that living beings on Mars are actively producing methane, and it takes them a full Martian year to do it.

But if the methane doesn't return, it could mean the measurements were a fluke. So is Mars a barren space rock, or a place where microbes can survive? Three years into its mission, rather than solving the controversy, Curiosity has only deepened it.