Methane doesn’t last forever in the Martian atmosphere, however. Exposure to the sun’s radiation, combined with reactions with other gases, breaks down the gas molecules within a few centuries. This chemistry is what makes the spike that Curiosity found so intriguing. If methane is present in the Martian atmosphere right now, it must have been released fairly recently. Detectable quantities might be a sign that something is alive on Mars, capable of replenishing the supply.

Or not. Natural interactions between rock and water can also produce the gas. The methane might have been forged deep beneath the Martian surface—where reservoirs of ancient water chafe against sediment—escaping into the atmosphere through a narrow crack in the ground. The whiff Curiosity caught might have been billions of years old.

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Scientists make sure to point out this dichotomy, as they did in the wake of the Times leak, whenever the question of life on Mars comes up. The story of methane on the red planet is complicated, and the search for the elusive gas was fraught well before Curiosity caught this spike. Ask one camp, and they’ll tell you that spacecraft have detected significant amounts of methane more than once. But ask another, and they’ll say NASA hasn’t found any at all.

The first evidence of methane in the Martian atmosphere came in 2004, from a spacecraft orbiting the planet and ground-based telescopes on Earth. There was debate right away. The scientists behind the discoveries said they were “99 percent confident” that the methane was there. Outside researchers said the signals weren’t strong enough. The ground observations, in particular, presented the uncomfortable possibility that molecules in Earth’s own atmosphere might have become scrambled in the measurements.

“The ground-based observations had been controversial, to say the least,” Paul Mahaffy, the NASA scientist who leads the instrument team on Curiosity that measured the recent uptick, said at a conference in June. “And so we were going to go to Mars and understand whether it was really there or not.”

To settle the debate, and even maybe solve the mystery, they needed to put instruments right inside the atmosphere. In its first year of operations, the Curiosity rover came up empty. But in 2013, it registered a spike. The puff seemed to remain for some weeks before vanishing. Subsequent detections followed the same pattern—short-lived signals that seemed to coincide with seasonal changes.

These were exciting findings, but some scientists weren’t convinced. “All the measurements that have been reported have been very tiny compared to the background signals that they have to sift through,” says Kevin Zahnle, a planetary scientist at NASA who studies Mars but isn’t part of the agency’s rover mission, and the field’s most vocal methane skeptic. “None of them really are convincing. But if you’re an investigator whose instrument it is, you’re much more likely to be convinced.”