Mars is a frigid desert, but we know in the past that it had rivers, lakes, and even maybe an ocean. Those features are still carved in its rocks today. Most of the valleys and lakes were carved before 3.7 billion years ago, but something that is not clear is how long these water features were around. A new study argues that the rivers flowed for only a fraction of that epoch.

Previous work estimated that it took at least tens of thousands of years for the valleys to form – that's the minimum amount of time for those rivers to have flowed. But what is the maximum time? Planetary Science Institute Research Scientist Alexander Morgan has estimated just that. He found that, at most, water was carving the rocks for 100 million years, but it was not a constant flow.

"In this study, I used craters that predate and postdate valley systems to place maximum bounds of hundreds of millions of years on the era over which these systems formed. Previous work had only determined minimum timescales, so these new results provide an upper bound on the timescale over which Martian valleys were active," Morgan said in a statement.

"Given what we know about erosion rates on early Mars, longer timescales imply that conditions permitting rivers were highly intermittent, with long arid periods interspersed with brief episodes of fluvial activity."

The reason why the activity appears to be intermittent is not clear. Maybe erosion was impeded by boulders and terrains that were more difficult to carve out. Or maybe, rivers only began to flow when the weather got warm enough, or after some particularly strong volcanic activity. The climate of a planet can change, like the changes responsible for Earth’s periodic glacial periods.

The work adds some nuance to the past history of Mars. Was the Red Planet "warm and wet" with an ocean? Or was Mars "cold and icy" with massive ice sheets? It could have had its periods of warmth over longer frigid and dry epochs.

"Over the past decade or so we've come to realize that these descriptors are far too general, and it doesn't really make sense to try to condense hundreds of millions of years of climate history into a two-word description," Morgan said.

"Like Earth, early Mars was complex, and the conditions permitting surface water likely varied considerably. Earth has undergone massive climatic changes throughout its history – for example, 20,000 years ago, the area that is now Chicago was beneath half a mile of ice – and surface conditions permitting rivers on early Mars likewise probably waxed and waned."