Harvard scientists have suggested that methane on ancient Mars may explain why it was once warm enough to support liquid water on its surface. The findings are published in Geophysical Research Letters.

What made Mars warmer in its history is a bit of a mystery. Previous theories have leaned towards carbon dioxide, which, as we know from Earth, is a decent greenhouse gas. But even though CO2 makes up 95 percent of the thin Martian atmosphere today, it can’t account for a much thicker atmosphere necessary to support significantly warmer temperatures in the past.

So, the team modeled how methane might behave in the atmosphere of Mars billions of years ago, when the Sun was 30 percent fainter than it is today (known as the faint young Sun paradox). Various geological processes are thought to have been releasing much more methane into the atmosphere back then than today.

This methane would have been slowly converted to hydrogen and other gases, a process that is sort of happening on Saturn’s moon Titan today. When the effects of methane, hydrogen, and carbon dioxide were combined, the team found they did a better job at warming early Mars than carbon dioxide alone by trapping incoming solar radiation.

“This research shows that the warming effects of both methane and hydrogen have been underestimated by a significant amount,” said Robin Wordsworth from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) in a statement.

In 1977, the late astronomer Carl Sagan suggested Mars could have been warmed by hydrogen in its past. But this is the first time we’ve got a good estimate of the effect it might have had, alongside methane.

And we might get a better insight into methane on Mars soon, too. ESA’s Trace Gas Orbiter (TGO) is currently in orbit around the Red Planet, with the goal of working out where the methane in the Martian atmosphere is coming from. And this may have implications for life existing on other planets.

“Early Mars is unique in the sense that it’s the one planetary environment, outside Earth, where we can say with confidence that there were at least episodic periods where life could have flourished,” said Wordsworth. “If we understand how early Mars operated, it could tell us something about the potential for finding life on other planets outside the Solar System.”