Ancient Mars had planet-wide rainstorms so intense they breached its lakes

Billions of years ago, rain once fell on the Martian plain, and not always softly. 

New research on the Red Planet's now-empty lakes suggests a huge amount of liquid water must have spilled from the skies roughly 3.5 to 4 billion years ago, enough to sculpt river-like channels and breach several lake basins.

"This is extremely important because 3.5 to 4 billion years ago Mars was covered with water. It had lots of rain or snowmelt to fill those channels and lakes," says planetary scientist Gaia Stucky de Quay from the University of Texas.

Modelling what Mars' climate looked like all those years ago is incredibly difficult, but studies on the geomorphology and chemistry of the planet certainly suggest it was once home to an abundance of water, fed by both rainfall and snowmelt.

Scientists aren't sure how long these downpours lasted or whether the weather was torrential, a drizzle or a mix, but marks on the surface of Mars suggest there were once heavy enough showers to leave a lasting impression.

"Now it's completely dry," says Stucky de Quay. 

"We're trying to understand how much water was there and where did it all go." 

Using satellite images and topography, researchers examined 96 lake basins on Mars that are thought to have formed all those billions of years ago. Some of the basins had ruptured from overflowing water, known as open basins, while others remain intact, known as closed basins. 

By measuring these lakes and their watersheds, the team was able to show how much rainfall and snowmelt would have been needed to fill the intact basins without breaching them, while simultaneously overflowing the open basins. 

In cases where a closed and open basin were fed by the same river, researchers could predict both the maximum and minimum rainfall that might have fallen in a single event.

In just one rainstorm, which could have lasted for days or even thousands of years, researchers estimate precipitation on Mars fell somewhere between 4 and 159 meters (13 and 520 feet).

While the effects can be seen planet-wide, not all areas were impacted equally. Some open-basin lakes were in regions that would be considered 'semi arid' on Earth, so they probably received less water than more humid parts.

"We again stress that our constraints are based on a threshold - not cumulative - event (i.e., lake overflow) that must have occurred during a single, quasi-continuous runoff episode, which may have recurred multiple times," the authors write.

"Indeed, the inlet valleys' large erosional volumes require cumulative water volumes that generally exceed lake basin volumes, thus suggesting repeated runoff episodes… "

In other words, the deeper channels being driven to the lakes were probably chiselled out over several downpours, which would probably have flooded the lakes on several occasions. 

Recently, however, some scientists have suggested these valleys were not carved simply by water, and by overestimating the impact of rainfall, we might be miscalculating the rainfall itself.

Nevertheless, the authors think these new insights into precipitation and aridity could help improve and test our climate models for the Red Planet, but they admit their findings are just a piece in the bigger puzzle.

Understanding the climate evolution of Mars will be key to assessing its potential for harbouring life, and that's why the Mars 2020 Perseverance rover is making its way to a lake bed right now. 

The study was published in Geology.

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