For the past four years, NASA's Perseverance rover has rambled over a region of Mars where scientists say a strong river once emptied into a crater, creating a large delta.
But for that to have happened, a lot of water would have gushed — something difficult to explain if the Red Planet were always frozen.
Computer simulations show that ancient Mars likely experienced regular rain and snow, which helped shape vast networks of river valleys and lakes. The distribution of these land features lines up better with precipitation models than merely the effects of melted ice caps, according to a new paper published in the Journal of Geophysical Research: Planets.
The research, performed by geologists at the University of Colorado in Boulder, argues that our planetary neighbor, on average 140 million miles away in space, was warm and wet billions of years ago, challenging a long-held belief that early Mars was mostly cold and icy.
Most scientists agree that at least some water existed on the surface during a period about 4 billion years ago, but where the water came from has been up for debate, said Amanda Steckel, who led the study as a doctoral student.
"We see these valleys beginning at a large range of elevations," Steckel said in a statement. "It’s hard to explain that with just ice."
The researchers created a digital version of part of Mars and tested different climate scenarios. In some computer simulations, they added widespread rain or snow. For others, they only tried melting down polar ice. Then, they used the software to simulate what would happen if that water flowed for thousands of years.
Their goal was to determine whether ancient Mars may have had a more Earth-like climate, at least for a while.
The results showed when precipitation was part of the climate, valleys and streams formed in many different regions at varying elevation points. When water came only from melted ice, the valleys mostly formed in the highlands, in close proximity to where the ice caps would have been.
The team compared the simulation data to real images from NASA spacecraft that have observed Mars from orbit. The patterns created by rain or snow more closely matched what is actually seen on the Martian surface.
"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."
Today, snow occasionally falls on Mars, but only in the coldest extremes, according to NASA: at the poles and under cloud cover at night. So far there's no photographic evidence of Martian snowfall — clouds obscure the cameras on spacecraft — but other instruments are capable of detecting it.
The Mars Reconnaissance Orbiter, for example, has the Mars Climate Sounder, which has collected data on carbon dioxide snow — aka dry ice — as it fell to the ground. The Phoenix lander also used a laser-based tool to spot snow made of water near the Martian north pole in 2008.
The team still doesn't fully grasp how Mars could have stayed warm enough for rain or snow, especially since the young sun was about 25 percent dimmer than it is today. Despite the mystery, geologist and study coauthor Brian Hynek says without rain, landforms like Perseverance's Jezero Crater just don't make sense. The dried delta, for example, features scattered boulders.
"You’d need meters deep of flowing water to deposit those kinds of boulders," he said.
