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A sunrise view of Mars

Darkened gullies slice down the edge of a crater in one of the first high-resolution images sent by the Mars Reconnaissance Orbiter.

The sharp edges of the channels suggest that they are no more than a few million years old. NASA scientists say that the braided gullies look as if sediment-rich streams had carved them, supporting the notion that water once flowed across much of the Red Planet.

"This shows a soaking-wet Mars," says Alfred McEwen of the University of Arizona in Tucson. The orange areas, enhanced for greater contrast, show clay-rich soil, which the scientists say could have formed only in the presence of water. The lightest areas in the picture are covered in carbon dioxide frost, which will burn off during the Martian day.

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