Observations of large valley networks on today’s Mars suggest formation by flowing water. However, most climate models cannot sustain temperatures above freezing. To understand this contradiction, a team of planetary researchers modeled the two leading theories for valley formation from precipitation (a warm wet climate) or temporarily melted ice from the edge of an ice cap (an icy cold climate). They found that the main difference between these scenarios was the location of the origin of the valleys that formed. In a warm wet setting, valleys start at many different elevations. In the icy cold scenario, valleys start only near the elevation where ice melted. The authors then examined a region of Mars with many large valley networks, focusing on the location and elevation of valley heads. Their findings showed that the distribution of valley heads matches predictions for a climate that includes precipitation rather than just runoff from melting ice caps. This suggests that precipitation played a significant role in forming these valleys, indicating that ancient Mars likely had a climate warm enough to support rain.
Heavy precipitation likely fed many networks of valleys and channels that shaped the Martian surface billions of years ago. Image credit: M. Kornmesser / ESO.
“You could pull up Google Earth images of places like Utah, zoom out, and you’d see the similarities to Mars,” said Dr. Amanda Steckel, a researcher at the California Institute of Technology.
Most scientists today agree that at least some water existed on the surface of Mars during the Noachian epoch, roughly 4.1 to 3.7 billion years ago.
But where that water came from has long been a mystery
Some researchers say that ancient Mars wasn’t ever warm and wet, but always cold and dry.
At the time, the Solar System’s young Sun was only about 75% as bright as it is today.
Sprawling ice caps may have covered the highlands around the Martian equator, occasionally melting for short periods of time.
In new research, Dr. Steckel and her colleagues set out to investigate the warm-and-wet versus cold-and-dry theories of Mars’ past climate.
The researchers drew on computer simulations to explore how water may have shaped the surface of Mars billions of years ago.
They found that precipitation from snow or rain likely formed the patterns of valleys and headwaters that still exist on Mars today.
“It’s very hard to make any kind of conclusive statement,” Dr. Steckel said.
“But we see these valleys beginning at a large range of elevations. It’s hard to explain that with just ice.”
This image shows a suite of fluvial ridges on Mars (at –67.64 °E, 43.37 °S). Image credit: J. Dickson.
Satellite images of Mars today still reveal the fingerprints of water on the planet.
Around the equator, for example, vast networks of channels spread from Martian highlands, branching like trees and emptying into lakes and even, possibly, an ocean.
NASA’s Perseverance rover, which landed on Mars in 2021, is currently exploring Jezero crater, the site of one such ancient lake.
During the Noachian, a powerful river emptied into this region, depositing a delta on top of the crater floor.
“You’d need meters deep of flowing water to deposit those kinds of boulders,” said Dr. Brian Hynek, a researcher with the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder.
To study that ancient past, the scientists created, essentially, a digital version of a portion of Mars.
They used the software to model the evolution of the landscape on synthetic terrain that resembles Mars close to its equator.
In some cases, they added water to that terrain from falling precipitation. In other cases, they included melting ice caps.
Then, in the simulation, they let the water flow for tens to hundreds of thousands of years.
The authors examined the patterns that formed as a result and, specifically, where the headwaters feeding Mars’ branching valleys emerged.
The scenarios produced very different planets: In the case of melting ice caps, those valley heads formed largely at high elevations, roughly around the edge of where the ancient ice sat.
In the precipitation examples, Martian headwaters were much more widespread, forming at elevations ranging from below the planet’s average surface to more than 3,350 m (11,000 feet) high.
“Water from these ice caps starts to form valleys only around a narrow band of elevations,” Dr. Steckel said.
“Whereas if you have distributed precipitation, you can have valley heads forming everywhere.”
The team then compared those predictions to actual data from Mars taken by NASA’s Mars Global Surveyor and Mars Odyssey spacecrafts.
The simulations that included precipitation lined up more closely with the real Red Planet.
The researchers are quick to point out that the results aren’t the final word on Mars’ ancient climate — in particular, how the planet managed to stay warm enough to support snow or rain still isn’t clear.
“But our study provides scientists with new insights into the history of another planet: our own,” Dr. Hynek said.
“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.”
The study was published in the Journal of Geophysical Research: Planets.
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Amanda V. Steckel et al. 2025. Landscape Evolution Models of Incision on Mars: Implications for the Ancient Climate. JGR Planets 130 (4): e2024JE008637; doi: 10.1029/2024JE008637
