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NASA's Curiosity rover found evidence for what scientists believe was an ancient, flowing stream on Mars at a few sites, including the rock outcrop pictured here. The key evidence for the ancient stream comes from the size and rounded shape of the gravel in and around the bedrock, according to the Jet Propulsion Laboratory/Caltech science team. The rounded shape leads the science team to conclude they were transported by a vigorous flow of water. The grains are too large to have been moved by wind. Curiosity arrived on Mars on August 6 and began beaming back images from the surface. See all the images here as they are released. <a href='http://www.cnn.com/2012/07/17/tech/gallery/mars/index.html' target='_blank'>Check out images from previous Mars missions.</a> NASA's Curiosity rover found evidence for what scientists believe was an ancient, flowing stream on Mars at a few sites, including the rock outcrop pictured here. The key evidence for the ancient stream comes from the size and rounded shape of the gravel in and around the bedrock, according to the Jet Propulsion Laboratory/Caltech science team. The rounded shape leads the science team to conclude they were transported by a vigorous flow of water. The grains are too large to have been moved by wind. Curiosity arrived on Mars on August 6 and began beaming back images from the surface. See all the images here as they are released. Check out images from previous Mars missions.If the Curiosity rover had been sent to Mars in ancient times, it might have found itself sinking in a stream.

The 2,000-pound super-rover, which made its now-legendary landing on Mars on August 6, has come across stones in conglomerate rock suggesting that water must have flowed there in the past.

One such rock outcrop is called Hottah, after Hottah Lake in Canada's Northwest Territories. It looks like someone took a jackhammer and lifted up a sidewalk, said John Grotzinger, lead scientist for the Curiosity mission, at a press conference Thursday.

The consensus is that "this is a rock that was formed in the presence of water," Grotzinger said. "We can characterize that water as being a vigorous flow."

In and around this bedrock, Curiosity has come across rounded gravels. The rocks appear to have been subjected to a sediment transport process, carried by either water or wind, said scientist Rebecca Williams of the Planetary Science Institute in Tucson, Arizona.

The gravels seem too large to have been transported by wind, meaning it's likely that this is a stream bed.

Curiosity completed its longest drive to date on September 26. The rover moved about 160 feet east toward the area known as "Glenelg." The rover has now moved about a quarter-mile from its landing site. Curiosity completed its longest drive to date on September 26. The rover moved about 160 feet east toward the area known as "Glenelg." The rover has now moved about a quarter-mile from its landing site.

A second rock outcrop, called Link, holds similar evidence. Scientists used data from Curiosity and the orbiters at Mars to enhance their understanding of the area.

The water flowing in these rock formations was probably somewhere between ankle and hip deep, said Curiosity science co-investigator William Dietrich of the University of California, Berkeley.

What we've done on Mars, and what's next

Scientists believe the water and sediment flowed down the crater into an alluvial fan -- a geological formation created by material that water transports. At least intermittently, this fan appears to have extended down to the area where the rover landed.

It's hard to say how long ago this water flowed -- an estimate would be "thousands to millions of years," Dietrich said.

Previously, scientists have used data from past Mars missions to speculate about channels on the planet, and whether water could have flowed in them. This is the first direct observation of streambed material, Dietrich said.

This image combines photographs taken by the rover's Mars Hand Lens Imager at three distances from the first Martian rock that NASA's Curiosity rover touched with its arm. The images reveal that the target rock has a relatively smooth, gray surface with some glinty facets reflecting sunlight and reddish dust collecting in recesses in the rock. This image combines photographs taken by the rover's Mars Hand Lens Imager at three distances from the first Martian rock that NASA's Curiosity rover touched with its arm. The images reveal that the target rock has a relatively smooth, gray surface with some glinty facets reflecting sunlight and reddish dust collecting in recesses in the rock.

There are no plate tectonics on Mars, meaning the planet does not have moving plates underneath the surface that cause quakes. So why does this Hottah rock formation look the way it does?

Grotzinger hypothesizes that "Somewhere near this outcrop, a small impact occurred and lifted the beds up and rotated them," he said.

The rover did not employ any of its chemistry tools to examine the area; rather, scientists made their judgments about the rocks based on photos.

Curiosity is now three-quarters of the way between Hottah and Glenelg, its next official stop. Glenelg was chosen as a target because it has three types of terrain, including layered bedrock, making it a potentially interesting place for Curiosity to try out its drill.

An unsolved mystery is whether life could have been supported on Mars. Water is a necessary ingredient, but an energy source and carbon are also essential.

Curiosity can detect organic molecules and tear them apart to deliver details about them back to Earth, but they would still not be definitive evidence that life existed. Such molecules can come from nonliving sources, and Curiosity doesn't have the technology to make that distinction. That will have to wait for another mission.

This rover's ultimate destination is Mount Sharp, a 3-mile-high mountain with layers of sediment that will provide more opportunities to search for organic molecules.

An image released Monday, August 27, was taken with Curiosity rover's 100-millimeter mast camera, NASA says. The image shows Mount Sharp on the Martian surface. NASA says the rover will go to this area. An image released Monday, August 27, was taken with Curiosity rover's 100-millimeter mast camera, NASA says. The image shows Mount Sharp on the Martian surface. NASA says the rover will go to this area.

Curiosity recently tested out its contact instruments on a rock called Jake Matijevic. It used its Alpha Particle X-Ray Spectrometer (APXS) instrument to touch the rock, and the Mars Hand Lens Imager (MAHLI) to take close-up photos. Another instrument, ChemCam, shot laser pulses at the rock and finished its testing on Monday.

The $2.6 billion mission is slated to last for two years, but previous rovers have far outlasted their estimated lifetimes.

The Spirit rover operated from 2004 to 2010, and the Opportunity rover has been chugging along since 2004. These twin rovers had initial mission periods of only 90 days.

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