Billions of years ago, when Mars was a frozen ball of ice, massive plumes of methane might have caused mysterious warm spells that let liquid water flow, a new study says. The research could help scientists understand how and when conditions on Mars were suitable for life.

These transient bouts of global warming may be connected to Mars’ wobbly axis, according to a new model of young Mars’ climate published today in the journal Nature Geoscience. The planet’s shifting tilt exposed more of its surface to the Sun’s rays, thawing subsurface reservoirs of frozen methane, the study suggests. The methane gas then escaped into the atmosphere and mixed with carbon dioxide, another powerful heat-trapping greenhouse gas — triggering spurts of widespread warming lasting several hundred thousand years.

“It’s a paradox, an unresolved paradox of Mars.”

Today, Mars is a cold and barren place, but the planet started out relatively warmer and wetter. NASA’s Curiosity rover and Mars orbiters, in fact, have spotted creases and valleys of river deltas that were likely carved over thousands of years by liquid water. Some of these features, however, date to about 3.6 to 3 billion years ago, when Mars was thought to be completely frozen. This evidence of liquid water suggests that Mars might have experienced periodic warm spells during an otherwise icy period — but so far, no one has been able to figure out exactly how.

“It’s a paradox, an unresolved paradox of Mars,” says Kevin Zahnle, a NASA scientist who was not involved in the study. “On the one hand, some people say that it looked warmish and wettish, at least occasionally. On another hand, nobody can figure out how it could have been warmish and wettish.”

One explanation was that volcanoes might have spewed out enough greenhouse gases to warm the planet and melt the ice. But volcanoes couldn’t have produced global warming lasting thousands of years, the study says. Asteroid impacts would have warmed the planet for even less time.

There is another possibility: global warming produced by massive quantities of methane in the atmosphere, says Edwin Kite, a planetary scientist at the University of Chicago and lead author of today’s study. Earlier this year, research led by Robin Wordsworth at Harvard University revealed that methane — when mixed with carbon dioxide — could have been a much stronger greenhouse gas on Mars than previously thought. That strengthened the case Kite and planetary scientist Peter Gao were building for methane’s role in warming up the Red Planet.

“Nobody can figure out how it could have been warmish and wettish.”

Methane on Mars is the subject of a decades-old debate with exceptionally high stakes: if there’s methane on Mars, it could be a sign of life there, too. Here on Earth, most of the methane in the atmosphere is produced when living things die and decay. But the methane that Kite was interested in is different: it wouldn’t have been created by living things, but by chemical reactions between rocks and water deep beneath the planet’s crust. Methane produced by these reactions may have traveled towards the surface, where it became trapped in a layer of ice.

For this methane to be released into the atmosphere, the ice had to thaw. That could have happened when Mars wobbled on its axis, according to Kite’s models — tilting so that the Sun hit more of the planet’s surface. Just like the Earth, Mars spins around a tilted axis as it orbits the Sun. That tilt is the reason for our seasons: the northern hemisphere experiences summer, for example, when the Earth’s tilt exposes the globe’s northern half to more of the Sun’s energy.

Our planet’s tilt is more or less stabilized by our massive, orbiting Moon — which acts like an anchor. But Mars’ two small and misshapen moons aren’t as effective. So sometimes, Mars wobbles wildly. Kite’s calculations suggest that these wobbles could have triggered enough warming to thaw some of the ice beneath the surface. That in turn released the methane trapped inside — kicking off a bout of global warming that could last hundreds of thousands of years. These warm spells would end when the Sun’s light broke down the methane molecules.

“Habitable doesn't mean inhabited!”

Since water is key for life here on Earth, these warm spells could have created a habitable environment on Mars. Of course, “habitable doesn't mean inhabited!” Kite writes in an email to The Verge, but it helps explain the conditions in which life might have existed.

The study is pretty solid, Alberto Fairén, an astrobiologist and visiting scientist at Cornell University, says in an email to The Verge. These methane bursts may have been one piece of a complicated combination of factors that allowed liquid water to flow, he wrote in a commentary published alongside the study. However, Kite points out there are no direct observations that this methane reservoir actually existed. The study is primarily based on computer modeling. “That is probably the biggest weakness of our model,” Kite says. “All our evidence is indirect.”

Zahnle doesn’t think that the paradox of Mars’ warm wet spells has been resolved yet. The problem is that to explain them, the planet’s atmosphere would need to contain a substantial amount of CO2 that isn’t present on Mars today. “It’s a problem because nobody knows where all that CO2 has gone to,” Zahnle says.

Still, Zahnle says, “They’re very clever, this is a good effort.” And the model will be put to the test soon enough. The European Space Agency’s ExoMars Trace Gas Orbiter, which is currently looping around Mars, is equipped with a methane sniffer. The probe is due to settle into a closer, circular orbit around the planet in March 2018. If its instruments don’t detect any methane in Mars’s atmosphere, then “our model is very unlikely to be correct,” says Kite. “If there is a quick way to prove our model, I don’t know what it is — but there is a quick way to disprove it.”