Scientists examining data from NASA’s Curiosity Mars rover have concluded that imagery from three sites on the floor of Gale Crater represents rocks deposited there by water flowing down from the crater wall, probably billions of years ago.

The conglomerate rock first turned up in an outcropping near Curiosity’s landing site, exposed by the downwash from the retro-rockets on its landing “sky crane.” A second site produced more of the same, and closer examination of a third rock called Hottah with the 34mm and 100mm lenses on the rover’s mast camera allowed the science team to reach a consensus.

“It was really when we got to Hottah where we saw this again most clearly that it was [a] very easy-to-reach consensus,” said John Grotzinger, the Curiosity chief scientist, in releasing the findings Sept. 27.

In all three cases the rover’s cameras imaged conglomerate rocks formed of rounded pebbles too large to have been transported by the Martian wind, but very similar to rock found in alluvial fans on Earth. The consensus, after more than 50 Martian days of study, is that the conglomerates were formed of pebbles rounded in a “vigorous stream” of water flowing down through a long valley now formally designated Peace Vallis.

Once the stream reached the crater floor, it deposited the pebbles in the familiar fan-like feature, where they were later buried and “cemented” into the conglomerate layer. Scientists speculate that parts of the conglomerate layer were returned to the surface by the action of a “small” meteor impact that fractured and displaced it.

William Dietrich of the University of California, Berkeley, a Curiosity co-investigator, estimated the stream could have been hip-deep, moving at about 3 ft. per sec. Until its chemical composition can be studied with some of Curiosity’s other instruments, it is not possible to estimate how long, or how long ago, the water flowed, he says, although the best guess is that the stream was “ancient” and probably ran for “thousands of years,” at least intermittently.

The nuclear-powered rover is most of the way to the feature known as Glenelg, where three different types of surface material come together. But before it reaches that scientifically interesting area, it will park for two or three weeks to give scientists time to check out the chemistry laboratories inside the rover’s body by pouring loose sand or other material into them for analysis. Science team members are looking for a good sample target for that work, Grotzinger says.

Ultimately the team hopes to use data from Curiosity’s instruments to make judgments about the past habitability of the Martian environment. Finding evidence of running water is an important first step, Grotzinger says, but closer chemical analysis will allow the scientists to learn more about whether the energy levels and possible carbon concentrations when the water was present could have supported microbial life.

The discovery also validates the broader exploration strategy that has been practiced over the past two decades, with rovers providing ground truth to orbital data.

“Now that we’re down on the ground we can see the textural evidence, where you see the individual pebbles, the rounding, the geometric relationship that they have to each other,” Grotzinger says.