Curiosity, 2012: NASA says that water shaped the rocks on the left, in a photograph taken by the Mars rover Curiosity. For comparison, the agency released an image of rocks from the Earth (right).
Curiosity 2012: A wider view of the outcrop of a former streambed shows bedrock that scientists believe was likely exposed by meteorites striking the surface of Mars.
Spirit, 2010: When NASA's Spirit rover got stuck in Martian sand, it proved to be a lucky break: The spinning wheel churned up soil that provided evidence of rocks formed in the presence of water.
Opportunity, 2004: Opportunity discovered tiny mineral spheres — nicknamed blueberries — poking out of rocks that were likely formed by water. Researchers using Opportunity's science instruments identified them as concretions rich in the mineral hematite, deposited by water saturating the bedrock.
Pathfinder, 1997: The first mission to land a rover on Mars, Pathfinder touched down in Ares Vallis, an ancient flood plain in the planet's northern hemisphere. Among the 2.3 billion bits of data sent back by the lander and its rover, Sojourner, were 15 chemical analyses of rocks and soil, which suggested Mars had once had liquid water and a thicker atmosphere.
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NASA's Curiosity rover has found definitive proof that water once ran across the surface of Mars, the agency announced today. NASA scientists say new photos from the rover show rocks that were smoothed and rounded by water. The rocks are in a large canyon and nearby channels that were cut by flowing water, making up an alluvial fan.
"You had water transporting these gravels to the downslope of the fan," NASA researchers say. The gravel then formed into a conglomerate rock, which was in turn likely covered before being exposed again.
"A River Ran Through It," Curiosity's operators tweeted Thursday. "I found evidence of an ancient streambed on Mars, similar to some on Earth."
"From the size of gravels it carried, we can interpret the water was moving about 3 feet per second," said Curiosity science co-investigator William Dietrich, "with a depth somewhere between ankle and hip deep."
The rocks have not undergone scientific analysis. But the NASA team says that taken with geographic data from Mars orbiters, the photographs tell a story all their own.
The images show rocks with round, smooth surfaces; many of them have been broken down into sizes smaller than one inch in diameter.
"The shapes tell you they were transported and the sizes tell you they couldn't be transported by wind," co-investigator Rebecca Williams said. "They were transported by water flow."
"There is earlier evidence for the presence of water on Mars," the agency said in a press release, "but this evidence — images of rocks containing ancient streambed gravels — is the first of its kind."
NASA's team has named the rock outcrop that reveals the former streambed "Hottah," after Canada's Hottah Lake.
Scientists have not yet estimated the age of the rocks, which may have been buried beneath the surface. Their age could be several billion years.
The next step will be to find a good spot to drill into the rock, NASA says. And they'll be looking for possible carbon deposits to determine whether the water on Mars once supported life.
The photographs released Thursday are among more than 13,000 raw images Curiosity has captured. The rover took the photos during its mission to Mars' Gale Crater. The rocks in question lie between the crater's north rim and Mount Sharp, a mountain inside the crater.