According to a study published in the journal Science, rounded pebbles found in September 2012 by NASA’s Curiosity rover indicate that a stream once flowed on the Red Planet.

Rounded pebbles of this size are known to form only when transported through water over long distances. They were discovered between the north rim of the planet’s Gale Crater and the base of Mount Sharp, a mountain inside the crater.

These pebbles represent the first on-site evidence of sustained water flows on the Mars landscape, and support prospects that the Red Planet could once have been able to host life.

“Finding the rounded pebbles, which were deposited more than 2 billion years ago, was a matter of landing in the right place,” said study co-author Dawn Sumner, a co-investigator for NASA’s Mars Science Laboratory team.

“The main reason we chose Gale Crater as a landing site was to look at the layered rocks at the base of Mount Sharp, about five miles away,” she said.

“We knew there was an alluvial fan in the landing area, a cone-shaped deposit of sediment that requires flowing water to form. These sorts of pebbles are likely because of that environment. So while we didn’t choose Gale Crater for this purpose, we were hoping to find something like this.”

The discovery comes from Curiosity’s exploration of the Mars surface during its first 100 Martian days. During that time, the rover traveled about a quarter mile from its landing site, examining multiple outcrops of pebble-rich slabs. Curiosity took high-resolution images of these pebbles at three locations known as Goulburn, Link and Hottah. The grain size, roundness and other characteristics of the pebbles led the researchers to conclude they had been transported by water.

“The discovery involves some of the most basic principles of geology. On the first day of my sedimentary class, I have the students measure grain size and the rounding,” Sumner said. “It’s simple, and it’s important.”

“We could see that almost all of the 515 pebbles we analyzed were worn flat, smooth and round. We have classified them according to their geometry, which can be described using a single number – the ‘Corey shape factor’, where 0 describes rocks that are completely flat like a piece of paper and 1 means they are perfect spheres”, explained co-author Asmus Koefoed, a researcher at the Niels Bohr Institute at the University of Copenhagen.

“These rock formations point to a past on Mars that was warmer, and wet enough to allow water to flow for many kilometers across the surface of Mars,” said co-author Dr Linda Kah from the University of Tennessee, Knoxville.

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Bibliographic information: R. M. E. Williams et al. 2013. Martian Fluvial Conglomerates at Gale Crater. Science, vol. 340, no. 6136, pp. 1068-1072; doi: 10.1126/science.1237317