The results, published Tuesday in Science, show just how much we still have to learn about Mars' atmosphere. Readings from the rover found methane levels to be around half of what scientists had expected, based on all the natural processes that should be creating methane on the surface.

But periodically during their 20 month observation, the NASA scientists saw methane levels spike by an order of 10 -- sometimes in as little as 60 Martian days (just over two months on Earth).

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Methane gas should stick around for hundreds of years, but Mars apparently keeps having spikes and drops of the gas. Months after a spike, the methane would be back to those unexpectedly low levels.

That probably means that it's being vented out somewhere near the crater, then quickly dispersing. It's likely that some process beneath the surface -- whether it's biological or geological -- occasionally floods the Gale Crater with methane by burping it out. Then the winds of Mars scatters the gas, and UV radiation destroys much of the methane.

In other words, Mars is still a very active planet.

It's exciting that methane is definitely in Mars' atmosphere, Curiosity Project Scientist John Grotzinger said during a news conference at the American Geophysical Union Meeting on Tuesday, even though it's not proof of life on its own.

“We now have full confidence that there is methane," Grotzinger said. "And that there are organics preserved in rocks [around the planet]... those things are both consistent with the former or existing presence of life."