Researchers uncover faint evidence that ancient Mars experienced rainfall as well.
For the past four years, NASA’s Perseverance rover has traveled over a region of Mars where scientists believe a strong river once flowed into a crater, forming a large delta. However, for this to have occurred, a significant amount of water would have had to be present—an idea that poses challenges if the Red Planet has always been frozen. Computer simulations suggest that ancient Mars likely experienced regular rainfall and snowfall, which contributed to the development of extensive networks of river valleys and lakes. According to a new paper published in the Journal of Geophysical Research: Planets, the distribution of these features aligns more closely with precipitation models than with the effects of merely melted ice caps.
The research, conducted by geologists at the University of Colorado in Boulder, posits that our neighboring planet, averaging 140 million miles away, was warm and wet billions of years ago, countering the long-held belief that early Mars was predominantly cold and icy. While most scientists agree that some water was present on the surface approximately 4 billion years ago, the source of that water remains a topic of debate, as noted by Amanda Steckel, who led the study during her doctoral research. “We see these valleys beginning at a large range of elevations,” Steckel explained. “It’s hard to explain that with just ice.”
The researchers developed a digital representation of a section of Mars and tested various climate scenarios. In some simulations, they included widespread rain or snow, while in others, they focused solely on melting polar ice. They then used the simulations to observe the outcomes of water flow over thousands of years.
Their goal was to explore whether ancient Mars might have had a climate more similar to Earth’s for at least a period. The results indicated that when precipitation was integrated into the climate, valleys and streams formed across various regions and elevations. Conversely, when water was sourced only from melting ice, valleys mainly emerged in the highlands, adjacent to the ice caps. The team contrasted the simulated data with actual images from NASA spacecraft that have observed Mars from orbit. The patterns created by rain or snow more accurately reflected what is observed on the Martian surface.
“Water from these ice caps starts to form valleys only around a narrow band of elevations,” Steckel remarked. “Whereas if you have distributed precipitation, you can have valley heads forming everywhere.” Today, Mars experiences occasional snowfall, but only in its coldest areas—at the poles and under cloud cover at night. While there is no photographic evidence of Martian snowfall due to camera obstructions from clouds, other instruments can detect it. For instance, the Mars Reconnaissance Orbiter, equipped with the Mars Climate Sounder, has gathered data on falling carbon dioxide snow—also known as dry ice. Additionally, the Phoenix lander utilized a laser-based tool to identify water snow near the Martian north pole in 2008.
The research team still does not completely understand how Mars maintained warmth sufficient for rain or snow, particularly considering that the young sun was about 25 percent dimmer than it is today. Despite this mystery, geologist and study coauthor Brian Hynek asserts that without rain, landforms like Perseverance’s Jezero Crater would not make sense. The dry delta, for instance, contains scattered boulders. “You’d need meters deep of flowing water to deposit those kinds of boulders,” he stated.
