Once upon a time, it rained on Mars — and the downpours cut channels in the Red Planet.
That’s the theory being advanced by scientists in Switzerland who have studied the shapes of the ancient river valleys on Mars.
In a recent paper published in the journal Science Advances, the scientists, from the Swiss university ETH Zurich, said they looked at the valley networks on Mars and found that the angles they branched were similar to the narrow angles that are found in arid landscapes on Earth.
That implies, according to the lead author of the study, Hansjorg Seybold, a situation similar to arid landscapes on Earth, where sporadic heavy rainfall events and the rainwater running off quickly over the surface create the valley networks, the university said in a statement.
“We find that valley networks on Mars generally tend to branch at narrow angles similar to those found in arid landscapes on Earth. This result supports the inference that Mars once had an active hydrologic cycle and that Mars’ valley networks were formed primarily by overland flow erosion,” the study said.
The Swiss researchers, citing river networks in Florida, said seepage from the ground, another theory that has been advanced for the source of the water, creates networks with wider angles than those created by stormwater runoff.
One theory is that long ago the northern third of Mars was covered by an ocean and that water from the ocean evaporated, then condensed around the volcanoes of the highlands to the south, causing heavy rain that carved river valleys, the university said.
It may have been a long time since it rained there. Conditions like those found in Earth’s arid landscapes probably existed on Mars about 3.6 billion to 3.8 billion years ago, the university said.
The question of whether it once rained on the planet is important, the researchers noted, because of it what it says about whether the planet was once habitable.
“Fluvial runoff erosion would require very different conditions” from the cold, forbidding conditions on Mars today, the study said. (The average temperature on Mars now is minus-81 Fahrenheit.)
“These different scenarios have profoundly different implications for Mars’ climatic history,” the study said. “Correctly interpreting Mars’ climatic history is important because Mars is the only planet known to have undergone a major transition in planetary habitability, from more habitable in the past to less habitable today.”
Seybold said in an e-mail, “We conclude that if the physics which forms channel networks is the same on Earth and Mars, Mars’ valley networks must have been PRIMARILY formed by fluvial surface runoff rather [than] by re-emerging groundwater flow.” He noted that groundwater had been the dominant explanation from the late ’80s to the early 2000s.
He also said “some kind of precipitation is needed. Thus the climatic conditions of Mars must have been very different to the cold and dry climate we see today.”
Last year, scientists from the Smithsonian Institution and Johns Hopkins University Applied Physics Laboratory looked at how intense Martian rain could have been and found it could have been strong enough to “exceed the infiltration capacity of most soils, which would be necessary to initiate martian valley network formation.”
The ETH Zurich study “does a nice job of quantifying in a systematic way the visual impression that many geologists get instinctively — ‘well, they look like rain-fed valleys,’ ” said Ralph D. Lorenz of the Johns Hopkins lab, one of the authors of last year’s study.
“A whole range of processes must have acted on Mars’ surface as its climate has evolved over time, and it seems hard to imagine that rainfall wasn’t among them,” said Lorenz, who is writing a book on the history of planetary climate studies.