A false colour aerial image showing river valleys on mars

Fast and furious floods shaped Martian surface

US scientists have discovered that overflowing lakes on Mars caused floods that carved out a quarter of the planet’s river valleys, creating deep chasms and shifting vast amounts of sediment.

Today, the Red Planet is a cold and dry desert, but in its early days it had an active water cycle. Before about 3.5 billion years ago, the planet’s denser atmosphere and higher surface temperatures supported water that flowed across the surface, with lakes bigger than some small seas on Earth.

Example of a lake basin on Mars (center) and associated large outlet canyon (extending to the northwest from the basin margins), which formed from catastrophic lake breach flooding. Credit: THEMIS (NASA/JPL-Caltech/Arizona State University) and MOLA (NASA/GSFC/MOLA Science Team), Timothy A. Goudge.

Evidence for this is written in the rocks: the planet is covered in telltale geological clues, including flood-scoured outflow channels extending for hundreds of kilometres, networks of river valleys, deltas, lake beds, and rocks and minerals that could only have been formed by liquid water.

Now, a new study published in Nature has provided insights into how Mars’ enormous network of river valleys formed.

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The team, led by geoscientist Timothy Goudge from the University of Texas, looked at the importance of flooding from overflowing lakes, specifically lakes that filled craters. When the water became too much to hold, it would spill over the edge of the crater and trigger catastrophic floods.

Goudge and colleagues used global maps of Martian valley systems to study the shape and form of the landscape around 262 these lakes. They compared this with river valleys elsewhere.

A global map of Mars showing river valleys around the Red Planet. River valleys formed by crater lake breaches are in white. River valleys that formed gradually over time are in black. Goudge et al.

The river valleys near crater lakes were found to punch far above their weight – with a median depth of 170 metres they are more than twice as deep as river valleys formed elsewhere, which have a median depth of 77m.

In total, the team found that nearly a quarter of river valleys on the whole planet were carved out by lakes overflowing.

“If we think about how sediment was being moved across the landscape on ancient Mars, lake breach floods were a really important process globally,” says Goudge. “And this is a bit of a surprising result because they’ve been thought of as one-off anomalies for so long.”

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Some of these floods, he says, may have only lasted weeks – yet left impressions still visible billions of years later. This is very different to Earth, where ongoing dynamic processes have wiped away most evidence of ancient geological features.

“When you fill [the craters] with water, it’s a lot of stored energy there to be released,” Goudge says. “It makes sense that Mars might tip, in this case, toward being shaped by catastrophism more than the Earth.”

Previous research has only focused on the scale of individual ancient lake systems, but this new study looks at the far-reaching impact of flooding lakes across the whole planet.

The researchers suggest that these floods also had an important role in shaping the rest of the surface, influencing the formation of nearby river valleys.