Methane gas was detected on Mars 6 years ago. Scientists might finally know why.

Almost six years after NASA's Curiosity rover first detected hints of methane on Mars, scientists may have uncovered the reason it was there.

On June 16, 2013, the Curiosity rover's sensors picked up a spike of methane gas levels in the Gale Crater, the 96-mile crater where it had landed in 2012, The Guardian explained. This reading immediately raised some questions, since methane is a gas often produced by microbial life forms on Earth (although it can be released in other ways). Since then, many have speculated on whether this methane blip was a sign of Martian life, just a chemical reaction, or even an error caused by Curiosity's equipment.

But now, additional data provides proof that the methane did indeed come from Mars, Science Alert reported. The European Space Agency's Mars Express mission also caught the spike in methane levels on that date in 2013, proving that it wasn't a sensor malfunction or a mistaken reading by Curiosity.

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After reviewing the ESA's data, scientists took a closer look at what might be causing the methane spike for just that one day. By cross-referencing readings from Mars Express and Curiosity, they determined that the methane originated about 300 miles east of the Gale Crater, in a region known as Aeolis Mensae.

In a study published Monday in the journal Nature Geoscience, the scientists present their theory: that the "geological faults" in the Aeolis Mensae region could have broken permafrost nearby and released methane that may have been trapped within it.

This study may ask more questions than it answers — we still don't know how that Martian methane came to be — but it's still "the first step towards an answer," said Marco Giuranna, one of the study's authors. Read more at The Guardian.