It’s a mystery that has puzzled scientists pondering the red planet: evidence suggests that Mars sustained liquid water even at a time when the planet’s atmosphere was thin and its climate extremely cold.

NASA’s Curiosity rover, which landed on Mars in 2012, has found that the planet’s vast Gale Crater contained a series of lakes 3.5 billion years ago during the "Hesperian” period—marked by a transition to a very cold, icy environment—and after the "Noachian" period, when Mars’s thicker atmosphere would have allowed it to sustain liquid water on the surface.

Mars is about 4.6 billion years old in total.

But new research published in Nature Geoscience could help solve the quandary. Edwin Kite, a planetary scientist at the University of Chicago, and his co-authors say that liquid water could have been sustained during this period thanks to bursts of methane gas. Their mechanism would allow for short periods during which Mars could sustain liquid water, despite an overall freezing climate.  

The authors say that the dramatic variations in the angle of Mars’s tilt on its access could have sometimes exposed new patches of ice to the sun. As these patches retreated, explosive bursts of methane could have been released on the ground, which would then have prompted hundreds of thousands of years of warming, allowing for the temporary presence of liquid water.

“The resulting scenario for Hesperian Mars may have included a generally very cold and icy climate, similar to the continental Antarctica, with ice-sealed lakes and little or no melting occurring across vast expanses of surface ice over tens of millions of years,” wrote Alberto G. Fairén from the Centro de Astrobiología, Madrid, in a commentary article in the paper, which he did not work on.

“At times, however, the changes in atmospheric conditions proposed by Kite et al. may have perturbed the climate for... years, briefly warming the surface to a cold and wet environment similar to that of the Antarctic dry valleys, which today host seasonal rivers fed by ice melt that debouch into large ice-covered lakes,” Fairén continued.

“Although it seems unlikely that a single mechanism can explain not only the presence of liquid water, but its recurrence and persistence, the methane burst hypothesis provides a means to episodically tip the Hesperian climate over the edge,” Fairén concluded.