Water could have flowed intermittently through Martian valleys for hundreds of millions of years.

Using impact craters as a dating tool, Alexander Morgan, a researcher at the Planetary Science Institute, determined the maximum time frames for the formation of Martian valley networks shaped by running water.

“Mars today is a global desert, but its surface preserves much evidence of past water flows, including what appear to be river valleys. The timescale on which these valleys formed has large implications for the habitability of early Mars, because long periods of stable liquid water would be more conducive to life,” said Morgan, sole author of “New maximum constraints on the era of Martian Valley network formation” which appears in Scientific letters from the Earth and planets.

Martian valley systems formed more than 3 billion years ago and have long been considered some of the strongest evidence for the presence of liquid water on early Mars. Previous work has shown that it takes at least tens of thousands of years to erode these valleys, but the frequency of flow events, and thus the total duration of valley formation, has not been limited.

“In this study, I used craters before and after the valley systems to place maximum limits of hundreds of millions of years on the era in which these systems formed. Previous work had not determined only minimal timescales, so these new results provide an upper bound on the timescale that Martian valleys were active,” Morgan said.

“Given what we know about erosion rates in early March, longer time scales imply that conditions supporting rivers were very intermittent, with long arid periods interspersed with brief episodes of river activity. “

Scientists studying Mars have historically tended to fall into one of two camps: either Mars was “hot and humid” with an ocean, or it was “cold and icy” with huge ice caps. glacial.

“Over the past decade, we’ve realized that these descriptors are far too general and that it doesn’t really make sense to try to condense hundreds of millions of years of climate history into a two-word description .” » said Morgana.

“Like Earth, Mars’ early history was complex, and conditions allowing surface water likely varied widely. Earth has undergone massive climate changes throughout its history. For example, 20,000 years ago, the The area that is now Chicago was under a half-mile of ice. And the surface conditions that allowed rivers to exist in early Mars probably also ebbed and flowed.”

The results suggest that Mars’ rivers were eroding at a very slow rate, similar to parts of Chile’s Atacama Desert. One explanation is that erosion could have been inhibited by the accumulation of large boulders on the river bed, which could not be further destroyed.

Another explanation is that rivers flowed very rarely, perhaps as little as 0.001% of the time. This would imply that rivers on Mars were generally dry but could become active when volcanic activity or variations in the planet’s axial tilt and its orbit around the sun warmed Mars’ surface. These long-term climate changes also occur on Earth (where they are called Milankovitch cycles) and are responsible for Earth’s recent ice ages.

“Over short periods of time, river flow is controlled by precipitation or snowmelt upstream. Over longer periods of time, Earth’s rivers are affected by climate change,” Morgan said. “For example, 20,000 years ago there were large lakes and larger rivers in what is now Nevada. Martian rivers would have functioned similarly, with short-term variability due to storms or snowmelt, and longer-term variability due to changes in the planet’s rotation and orbit around the sun.