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Great floods once poured down a towering Martian mountain.

And NASA's dust-covered Curiosity rover has proof.

The ?????? ??????? ??????? ?????? ????car-sized NASA robot has spent much of 2024 exploring the Gediz Vallis channel, a dried-up waterway that travels down the three-mile-high Mount Sharp. Although Mars today is 1,000times drier than the driest desert on Earth, the rover has spotted clues that long ago the Red Planet experienced momentous floods. It was a wet world.

"This was not a quiet period on Mars," Becky Williams, a scientist at the Planetary Science Institute who researches Mars using the rover's Mast Camera, said in a statement. "There was an exciting amount of activity here. We’re looking at multiple flows down the channel, including energetic floods and boulder-rich flows."

The images below show what Curiosity has recently found.

Below is a wide-view photo of a section of Gediz Vallis as it winds down Mount Sharp. You can see prominent buildups of rocks and boulders, such as those in the foreground on left. "This area was likely formed by large floods of water and debris that piled jumbles of rocks into mounds within the channel," NASA explained. Impressively, this debris pile-up extends some two miles down the mountain (though some of this was likely caused by landslides, too).

Curiosity also closely examined these water-tumbled rocks. A number of them contain telltale "halo" markings, as seen in the image below. "Finally, water soaked into all the material that settled here," the space agency explained. "Chemical reactions caused by the water bleached white 'halo' shapes into some of the rocks."

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Unlike Earth, Mars no longer harbors an insulating atmosphere. The Red Planet's hot metallic core deep below its surface cooled long ago, and without a heated interior to generate a protective magnetic field, the once water-rich world was exposed to a relentless flow of particles from the sun, called the solar wind. The solar wind progressively stripped Mars of its thick atmosphere, leaving it the frigid, callous, irradiated desert we see today.

The Curiosity rover, which landed on Mars in 2012, continues to scour Mars to determine if the planet could have ever harbored habitable conditions for microbial life. Meanwhile, NASA's Perseverance rover, which landed in 2020, is equipped with instruments that sleuth for hints of past life called "biosignatures" — elements, substances, or features providing evidence of ancient organisms. This could mean telltale chains of molecules or structures that were almost certainly produced by single-celled Martians.

Although it's clear that Mars once hosted bounties of water, robotic Martian explorers have spotted no evidence, so far, that this rocky world ever hosted life.