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The latest evidence that Mars was once a warmer, wetter world comes from a surprising place—the hidden subsurface depths of Jezero Crater—rather than its surface, which NASA’s Perseverance rover has explored for the past five years. The site of a vast, dried-up lake, Jezero also hosts ancient river deltas. Laid down by flowing water as early as 3.7 billion years ago, these deltas are so sprawling that they can be seen from orbit. Now, however, Perseverance’s ground-penetrating radar has found signs of an even older river-and-delta system at Jezero that is buried deep beneath the surface.

Published today in the journal Science Advances, the findings suggest the Red Planet’s window of habitability stretches even further back in time than many scientists had imagined.

“It extends the window of fluvial deposition on Mars,” says Emily Cardarelli, a scientist at the University of California, Los Angeles, and the study’s lead author. “On Earth, those conditions produce minerals that can preserve fossils.

Jezero is the crash site of an asteroid that slammed into Mars’s surface almost four billion years ago. NASA chose it as Perseverance’s exploration zone because it has a wealth of fluvial features that hint that the crater was once ripe for life—and for preserving life’s telltale traces in stone. The new study relies on data from 78 traverses of the area from September 2023 to February 2024.

The rover used its radar capabilities to study layers of sediment buried more than 35 meters below ground—nearly twice as deep as it had previously probed—where it registered echoes of even older river-carved slopes and sinuous, meandering channels. These subsurface features, the researchers say, formed as early as 4.2 billion years ago, hundreds of millions of years before the water-washed terrain that Perseverance has been studying on the surface. That means there were multiple sustained periods of water flow in the crater’s history—multiple opportunities scattered across Jezero’s deep past in which it might have harbored life.

The result also reinforces that Mars is now a planet that is almost frozen in time, with lands that are far more undisturbed than any on Earth. “The fact that we have this record of this age is remarkable,” Cardarelli says. On Earth, rocks of a similar age lost any clear signature of ancient rivers long ago. “They’ve been heated, they’ve been squished, and they’ve been altered by water,” she says. “They’ve had a rough time.

With this more intact geological record, astrobiologists hope Mars can yield not only the first-ever smoking-gun proof of extraterrestrial life but also clearer data on how that life emerged in the first place. This question, it turns out, could be of equal importance for understanding life’s origins on Earth as well: circumstantial evidence suggests that ancient asteroid impacts much like the one that carved out Jezero Crater could have also exported any early Martian life to our own world.

The newfound buried river delta “is very clear evidence for a long duration of activity,” says Jack Mustard, a planetary scientist at Brown University, who has extensively studied Jezero Crater. “And that’s very exciting to have,” Mustard says the distinct delta beneath the soil isn’t surprising because sporadic periods of flow are common in the formation of rivers and lakes. “If you were to ask someone how the Mississippi Delta formed,” he says, “you would see multiple episodes of overlapping deltas.”

Cardarelli says that we haven’t heard the last about Jezero from Perseverance. “There’s a lot more to say about this particular area—and other areas within the crater,” she says. “We’re still digesting all our data.”

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