Mars’ hidden past: Rainfall may have shaped the ancient landscape

Picture Mars – not as the cold, dry and dusty world we see today – but as a temperate place with cloudy skies, rainfall, and rivers that carved valleys into the land.

This may have been the case billions of years ago, according to a recent study by scientists at the University of Colorado Boulder. The research suggests that Mars was once a planet shaped by weather cycles.

“Our study addresses a longstanding mystery about the climate of ancient Mars. Observations of large valley networks suggest formation by flowing water. However, most climate models cannot sustain temperatures above freezing,” noted the researchers.

Did rainfall feed rivers on Mars?

The experts suggest that the familiar networks of valleys and channels etched across Mars were likely formed not just by melting ice, but by rainfall or snowfall that fed rivers over long periods.

The findings bring new credibility to the idea that ancient Mars may have been warm and wet, offering a possible explanation for the geological features we still observe from space.

“You could pull up Google Earth images of places like Utah, zoom out, and you’d see the similarities to Mars,” said Amanda Steckel, who led the study and now works at the California Institute of Technology.

Mystery of water on Mars

Today, most researchers agree that water once existed on the Martian surface – especially during the Noachian epoch, which spanned from about 4.1 to 3.7 billion years ago. But there’s been disagreement about how that water got there.

Some believe Mars was always a cold and dry planet, where sunlight barely melted the ice that covered its highlands. Others argue the planet experienced a warmer and wetter climate that allowed for ongoing rainfall or snowmelt.

To test these ideas, the team ran a series of computer simulations to see which conditions could best explain Mars’s ancient valley networks. The simulations were built using landscape modeling software originally designed for Earth studies, and adapted for Mars.

Simulations of Martian rivers

The researchers created a digital Mars, mimicking the terrain near the equator where many valley systems are found. They modeled two scenarios: one where the landscape was fed by melting ice caps, and another where water came from falling precipitation.

Over simulated periods ranging from tens of thousands to hundreds of thousands of years, water flowed across the terrain. Each scenario left a distinct mark.

“We found that the main difference between these scenarios was the location of the origin of the valleys that formed,” noted the researchers.

In the melting ice simulation, valleys formed mostly around the edges of where ice would have sat at high elevations. In the precipitation scenario, valley heads emerged across a wide range of elevations – from low basins to areas over 11,000 feet (3,300 meters) high.

Rainfall could have helped valleys form

“It’s very hard to make any kind of conclusive statement,” said Steckel. “But we see these valleys beginning at a large range of elevations. It’s hard to explain that with just ice.”

The results suggest that the precipitation-fed valleys more closely matched the real Martian surface as seen in satellite images captured by NASA’s Mars Global Surveyor and Odyssey missions.

“Water from these ice caps starts to form valleys only around a narrow band of elevations,” Steckel said. “Whereas if you have distributed precipitation, you can have valley heads forming everywhere.”

Reading Mars like a history book

Mars still holds clues to its watery past. Around the equator, images show vast channel networks spreading out from highlands into basins that may once have held lakes – or even oceans.

NASA’s Perseverance rover, which landed in 2021, is now exploring Jezero Crater, a site that scientists believe once contained a lake that was fed by a powerful river.

“You’d need meters deep of flowing water to deposit those kinds of boulders,” said Brian Hynek, senior author of the study and a scientist at CU Boulder’s Laboratory for Atmospheric and Space Physics.

To reconstruct this Martian past, Steckel and Hynek used their simulations to trace where these ancient rivers might have started. The results suggest widespread precipitation was more likely responsible for shaping the valleys than occasional ice melt alone.

Our understanding of Mars is evolving

While the study doesn’t fully explain how Mars stayed warm enough to support snow or rainfall, it does add a new piece to the puzzle. The research gives scientists a more detailed view of the planet’s early climate – and may even help us better understand Earth’s own geological past.

“Once the erosion from flowing water stopped, Mars almost got frozen in time and probably still looks a lot like Earth did 3.5 billion years ago,” Hynek said.

The Red Planet may be dry and desolate now, but its surface still tells stories of a time when water shaped its landscapes. And those stories might bring us closer to understanding both Mars and our own place in the solar system.

The full study was published in the journal Journal of Geophysical Research Planets.

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