A simulated image shows a lake in the large Gale Crater on Mars, with streams of water flowing into it. NASA researchers believe a lake deposited enough sediment in the crater to form a mountain, Mount Sharp. NASA hide caption

A simulated image shows a lake in the large Gale Crater on Mars, with streams of water flowing into it. NASA researchers believe a lake deposited enough sediment in the crater to form a mountain, Mount Sharp.

Signs of water currents and sediments are seen in the latest photos NASA's Curiosity rover sent home from Mars, the space agency said Monday. The images suggest "ancient Mars maintained a climate that could have produced long-lasting lakes," NASA says.

In the huge Gale Crater where Curiosity has been exploring, the water and sediment flow might have been massive enough to build a mountain the 3-mile-high Mount Sharp NASA researchers say. But they acknowledge that they're still working to solve the mystery of how the mountain formed in a crater.

"If our hypothesis for Mount Sharp holds up, it challenges the notion that warm and wet conditions were transient, local or only underground on Mars," said Curiosity deputy project scientist Ashwin Vasavada of NASA's Jet Propulsion Laboratory in Pasadena. "A more radical explanation is that Mars' ancient, thicker atmosphere raised temperatures above freezing globally, but so far we don't know how the atmosphere did that."

NASA says "cross-bedding" in the layers of this Martian rock is proof that water moved on Mars, leaving waves or ripples of loose sediment. The image is from a site at Mount Sharp that NASA calls "Whale Rock." NASA hide caption

NASA says "cross-bedding" in the layers of this Martian rock is proof that water moved on Mars, leaving waves or ripples of loose sediment. The image is from a site at Mount Sharp that NASA calls "Whale Rock."

This isn't the first time NASA has announced signs of water at Gale Crater.

One year ago, NASA analysts saw sedimentary rocks that led them to believe the Martian lake had held freshwater.

And back in 2012, the space agency said images taken by Curiosity showed gravel-like rocks that were smoothed by water and pushed into the shape of an alluvial fan.

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NASA Photos Show New Signs Of A Lake On Mars