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Mars was a rainier, wetter place than planetary scientists previously thought, according to a new study of ancient, inverted river channels that span more than 9,000 miles (14,484 kilometers) in the Red Planet's southern Noachis Terra region.

"Our work is a new piece of evidence that suggests that Mars was once a much more complex and active planet than it is now, which is such an exciting thing to be involved in," study leader Adam Losekoot of the U.K.'s Open University said in a statement.

We've known Mars was once a wet planet ever since the Mariner 9 orbiter mission from the '70s photographed a surface covered in dried-up river channels. These channels were dated back to over 3.5 billion years ago. However, channels cut into the ground are not the only evidence for running water on Mars.