NASA's Curiosity rover has found evidence that the area in Gale crater it is traveling through once had vigorously flowing water for perhaps thousands or millions of years. Scientists don't know exactly how long ago this river would have flowed but it would likely be billions of years in the past.

Several pieces of evidence attest to this hypothesis, but the key feature was a strange rock sticking out of the Martian regolith that Curiosity spotted two weeks ago. The rock (above), nicknamed Hottah, "looked like someone came to the surface of Mars with a jackhammer," said John Grotzinger of Caltech, project scientist for the mission, during a NASA press conference on Sept. 27, adding that it wouldn't look out of place as a slab of concrete in downtown L.A.

Detailed observation of the rock showed that it was a composite, essentially a rock made of other rocks. The tiny rocks, called clasts, were embedded in the overall structure and appeared rounded, eroded after being transported by wind or water. The rocks were too large to have been moved by wind, suggesting that flowing water was the cause for their roundness.

In another sandy light-colored rock that Curiosity passed, named Link (above), the rover spotted clasts that had fallen to the ground, creating a pile of stones that had been liberated from the rock.

Both Hottah and Link were made of "a different type of material, one we hadn’t seen on Mars before," said Rebecca Williams of the Planetary Science Institute in Tucson, Arizona during the briefing.

Along with material seen at the Goulburn scar, which was uncovered when Curiosity's lander rockets disturbed the ground, these rocks add up to a picture of an ancient stream bed that once flowed through this area. Judging from the size of the gravel that was carried, scientists estimate that water likely ran at a steady two miles per hour and came to about ankle or hip height.

Above Curiosity's landing site, orbiters have photographed an alluvial fan (below) – a fan-shaped area where debris once spread out. This is taken as evidence of a watershed that would have flowed downslope through this area. Given that there are multiple stream channels seen in the orbital photos, scientists don't think this was just one enormous burst of water but a more sustained flow that occurred over thousands or millions of years.

Curiosity's next step is to pull out its vast array of sophisticated instruments to chemically analyze this area on Mars. The rover is currently on its way to an area named Glenelg, where it will search for further confirmation of water. Along the way, it will stop and scoop some Martian sand to run through its internal laboratory. It will look for evidence of organic carbon that might indicate the presence of ancient life.

Images: 1) and 2) NASA/JPL-Caltech/MSSS 3) NASA/JPL-Caltech/UofA