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Scientists announced today they now have confirmation that water once flowed vigorously across the surface of Mars. The evidence comes from pictures taken by NASA's 1-ton, six-wheeled rover Curiosity that's been crawling around the floor of a Mars crater named Gale since mid-August.

NPR's Joe Palca has more on what it's found.

JOE PALCA, BYLINE: In a way, today's announcement is no surprise. When scientist chose Gale crater as the rover's landing site, they did so because pictures from orbit showed what appeared to be an alluvial fan on the floor of the crater. As the name suggests, an alluvial fan is a fan-shaped formation that forms when rapidly moving water suddenly pours down a slope or the side of a mountain.

This formation occurs in several places on Earth. The pictures taken from Mars' orbit show a channel leading to the edge of Gale crater and then the distinctive fan shape formation spreading out onto the crater floor.

The rover is still in its checkout phase, so scientists weren't expecting any major discoveries, at least until they reached a spot about 500 yards from the landing site called Glenelg.

JOHN GROTZINGER: As we were driving along on the way to Glenelg, we encountered some really interesting outcrops that were surprising to the team.

PALCA: John Grotzinger of the California Institute of Technology is the lead scientist on the Curiosity mission. He spoke at a news conference held at NASA's Jet Propulsion Laboratory in Pasadena. Curiosity's landing site is on the bottom edge of the alluvial fan. Scientists were hoping to find rocks that might have been carried along the crater floor by the alluvial fan's streams of water.

The rock outcrop presented a perfect opportunity for study. Rebecca Williams is at the Planetary Science Institute in Tucson and a member of the Curiosity science team.

REBECCA WILLIAMS: When a geologist goes into the field, what they want to do is see a fresh exposure of rock to look at things like the grain size, the shape, the color and the arrangement of those grains, and that tells you a lot about the formation history of that rock.

PALCA: Williams says the pebbles in the rock have rounded edges, something that's consistent with being tumbled around by rapidly moving water.

Michael Malin of Malin Space Science Systems is the chief scientist for two of the rover's cameras. He says pebbles and larger rocks got smushed together as they rolled down the side of the crater and across the crater floor.

MICHAEL MALIN: Other materials were deposited on top of them. Eventually, they were cemented together by salts or some other material to act as a means of holding the rock together.

PALCA: This formed basically a new rock made out of other rocks.

Caltech's Grotzinger says it would be nice to have chemical or mineralogical evidence that confirmed water played a role in the formation of this rock. That's something the rover will be able to do later. But there are times in geology when the picture say it all, and this is one of them.

GROTZINGER: This is a rock that was formed in the presence of water. And we can characterize that water as being a vigorous flow on the surface of Mars.

PALCA: This isn't the first time scientists have seen signs of water on the now cold and dry planet. But evidence like this that there was once huge floods of water increases the chance that life might have once existed on Mars.

Seeing this rock may not be a surprise, but Rebecca Williams says it's gratifying all the same.

WILLIAMS: This is just wonderful ground truth confirmation of water-transported material that was predicted based on analysis of orbital images.

PALCA: Now, scientists will go back to testing instruments on the rover, pleased to have their first scientific discovery under their belts.

Joe Palca, NPR News.

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