But this was not a lazy river. This was likely a larger and faster-moving channel than many scientists had accounted for, perhaps comparable to medium-size rivers on Earth.
Before NASA sent the Perseverance rover to Jezero Crater, geologists already believed the basin once held a bygone body of water. Orbital images showed classic delta landscapes on the surface, with fanlike deposits where a river might have emptied into a lake.
Perseverance's findings add compelling direct evidence from below the surface. They suggest the Martian river, existing between 3.7 and 4.2 billion years ago, was part of a larger system than space satellites could see. Its currents may have been capable of carrying sand and small rocks downstream.
The new data, published in the journal Science Advances, also show the river was likely stable over a long stretch of time, not merely a flash flood with sudden mudflows. That matters because it bolsters the idea that Mars once had better conditions for supporting simple life.
Mars today is cold and dry, with a thin atmosphere that lets water quickly evaporate or freeze. But billions of years ago, the planet probably had a thicker atmosphere and a warmer climate. Rivers and lakes may have lasted long enough to carve valleys, move sediment, and reshape entire regions.
The rover collected the underground data as it drove across the floor of Jezero Crater. Its instrument, the Radar Imager for Mars Subsurface Experiment, or "Rimfax" for short, works by sending radio waves into the ground and interpreting the echoes that bounce back. Different materials -- sand, rock, or ice -- reflect the signals in distinct ways. By studying those patterns, researchers can map hidden ancient landscapes.
During 78 traverses, the car-sized lab on six wheels collected data from more than 115 feet underground, according to the new paper. That's nearly two times deeper than the rover has peered below the surface in the past.
The radar images show steep, slanted layers that usually form when water has shuttled sediment and dropped them in stacks. Over time, those stacks build up into recognizable shapes. On Mars, they now sit buried under dust and volcanic debris.
China's now-defunct Zhurong rover, part of the 2021 Tianwen-1 mission, also used underground radar to discover evidence of a vast ancient Martian ocean, which may have covered about one-third of the Red Planet's surface at one time. Its location is about 3,000 miles away from Perseverance at Utopia Planitia -- the place where NASA's Viking 2 lander touched down in 1976.
The Chinese radar revealed ancient beaches that extended nearly a mile, buried 30 to 115 feet below the surface. Several U.S.-based scientists, including Michael Manga at UC Berkeley, helped analyze the data.
"The fact that you can go to Mars with a rover and move over the surface and look underground is kind of mind-boggling to me," Manga told Mashable last year.
Researchers say the new Perseverance work may have implications for where scientists look for biosignatures in the future. Sediments formed in water are prime targets because they can trap and preserve chemical clues about the past.
Perseverance is collecting Martian rock and soil samples to send back to Earth, though the future of that mission remains doubtful. Mars Sample Return has been in limbo since a review found it would cost upward of $11 billion and take nearly two decades to achieve. NASA leaders say they're trying to salvage it with a new approach.
In September, NASA announced a sample collected by Perseverance contained fossilized material that ancient alien microorganisms could have created. Though the evidence is strong, NASA scientists say they can't rule out other non-biological explanations for what the rover found.
Perseverance scientists say they've exhausted what they can learn about the sample on Mars, but advanced tools on Earth could probe it for complex organic molecules, cell structures, and DNA.
"This finding by our incredible Perseverance rover is the closest we've actually come to discovering ancient life on Mars," said Nicky Fox, NASA's associate administrator for science.
