Scientists have discovered evidence of ancient, shallow lakes on Mars that once had liquid water exposed to the atmosphere, challenging previous theories that all Martian water was covered in ice. Live Science reports: The patterns, which were photographed by NASA's Curiosity rover, are known as wave ripples -- minute ridge-like structures that form along the shores of lakebeds. This means that exposed liquid water must have flowed across Mars' surface at some point in its history. The ripples were present in two separate lakebeds in Gale Crater, which Curiosity has been exploring since Aug. 2012. "The shape of the ripples could only have been formed under water that was open to the atmosphere and acted upon by wind," study first author Claire Mondro, a sedimentologist at CalTech, said in a statement.

The researchers also analyzed the height and spacing of the ripple waves to determine the size of the lake that formed them. The structures are approximately 0.2 inches (6 millimeters) tall and about 1.6 to 2 inches (4 to 5 centimeters) apart, indicating they were left by small waves. Based on these dimensions, the researchers believe the Martian lake must have been less than 2 meters (6.5 feet) deep. Both dry lakebeds appear to have formed around 3.7 billion years ago, indicating that Mars had an atmosphere dense and warm enough to support liquid water for longer than previously thought -- which could have intriguing implications. "Extending the length of time that liquid water was present extends the possibilities for microbial habitability later into Mars's history," Mondro said. In other words: living organisms may have had a longer window in which they could have evolved on the Red Planet. The findings have been published in the journal Science Advances.

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