Did it rain or snow on ancient Mars? New study suggests it did

Visit ancient Mars—a surprisingly temperate planet where snow or rain falls from the sky, and rivers rush down valleys to feed hundreds of lakes.

A new study from geologists at the University of Colorado Boulder paints this picture of a red planet that was relatively warm and wet, much different than the frigid wasteland we know today. The team’s findings suggest that heavy precipitation likely fed many networks of valleys and channels that shaped the Martian surface billions of years ago—adding new evidence to a long-running debate in planetary science.

The researchers, led by Amanda Steckel, who earned her doctorate in geological sciences at CU Boulder in 2024, published their findings April 21 in the Journal of Geophysical Research: Planets.

“You could pull up Google Earth images of places like Utah, zoom out, and you’d see the similarities to Mars,” said Steckel, now 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 the new research, Steckel and her colleagues set out to investigate the warm-and-wet versus cold-and-dry theories of Mars’ past climate. The team 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,” Steckel said. “But we see these valleys beginning at a large range of elevations. It’s hard to explain that with just ice.”

A tale of two red planets

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 Brian Hynek, senior author of the study and a scientist at the Laboratory for Atmospheric and Space Physics (LASP) at CU Boulder.

To study that ancient past, he and Steckel, who now serves on the Perseverance science team, created, essentially, a digital version of a portion of Mars.

The team drew on a computer simulation, or model, originally developed for Earth studies by study co-author Gregory Tucker, a professor at the Department of Geological Sciences at CU Boulder. Matthew Rossi, a research scientist at the Cooperative Institute for Research in Environmental Sciences (CIRES) at CU Boulder, also served as a co-author.

The researchers used the software to model the evolution of the landscape on synthetic terrain that resembles Mars close to its equator. In some cases, the group added water to that terrain from falling precipitation. In other cases, the researchers included melting ice caps. Then, in the simulation, they let the water flow for tens to hundreds of thousands of years.

The researchers 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 11,000 feet high.

“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.”

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 Hynek said the study provides scientists with new insights into the history of another planet: our own.

“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,” he said.