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It's been almost two months since NASA's newest Mars rover landed at the bottom of a large crater on the red planet. The Curiosity Rover has been rumbling over the frigid dusty landscape, snapping photos and sending them to Earth. Mission scientists chose the crater as a landing spot because they suspected that water might once have flowed there. And as NPR's Nell Greenfieldboyce reports, it now looks like they were right.

NELL GREENFIELDBOYCE, BYLINE: During its slow trek across Gale crater, Curiosity spotted some rocky outcroppings that jut up out of the alien soil. They immediately caught the attention of mission scientists.

JOHN GROTZINGER: And to us it just looked like somebody came along the surface of Mars with a jackhammer and lifted up a sidewalk.

GREENFIELDBOYCE: John Grotzinger is with the California Institute of Technology. He says the exposed slabs of rock are about four inches thick. They're made of rounded bits of gravel in a sandy matrix. The rock face has eroded a little bit. Some of the smooth pebbles have gotten loose and have fallen down into a little pile. They're about the size of M&M candies. Scientists looked at all this and came to this conclusion.

GROTZINGER: This is a rock that was formed in the presence of water.

GREENFIELDBOYCE: Flowing water. It looks like part of an ancient stream bed. Now, scientists have known for a long time that Mars once had liquid water on its surface. Orbiting spacecraft can see canyons that must have been carved by water. In fact, NASA deliberately picked Curiosity's landing site because it looked like a place where a canyon stream had spilled water onto a plain.

But guessing there was once water is different from really seeing that. Jim Bell is a planetary scientist at Arizona State University who's working on this mission.

JIM BELL: Being down on the ground and seeing these rocks and soils up close and seeing rounded pebbles like you see in a streambed and kind of these cemented sandstones that you'd find if you were, you know, walking up a stream in your favorite little canyon somewhere.

These are all telling us that, you know, there was water really flowing across the surface there, and probably pretty deep water - ankle-deep, knee-deep water - like you'd have in an occasional desert flood on the Earth, in the Southwest, for example.

GREENFIELDBOYCE: Other researchers say this is convincing evidence of flowing water. Peter Doran is at the University of Illinois at Chicago. He says the rounded grains in that rock are big enough that they couldn't have been carried and smoothed by the wind.

PETER DORAN: Before, we never really saw a rock on Mars where we could tell whether it was wind or water that was doing the transport. And now we have a clear sign of flowing water on Mars and we can get estimates of the size of the flow and so on. It's really fascinating.

GREENFIELDBOYCE: Other Mars rovers have seen evidence of water, but not like this. Andrew Knoll is a planetary sciences professor at Harvard University. He says earlier rovers saw things that could be associated with groundwater that might occasionally bubble up. These latest findings show something different.

ANDREW KNOLL: Something happened on Mars that simply doesn't happen today. And that is, there was water flowing at high rates over the Martian surface. That's really what they've found.

GREENFIELDBOYCE: Water is important, of course, because it's needed for life. And Curiosity's main mission is to search for evidence that Mars was once capable of supporting life. Its mission is expected to last about two years.

Nell Greenfieldboyce, NPR News.

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