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Water on Mars: The secret's in the pebbles

Curiosity rover found rocks made of smooth, round pebbles, adding to the growing pile of evidence for water flowing across the surface of Mars. To smooth the pebbles, they must have bounced along in a stream between 4 inches and 4 feet deep, scientists say.

By Denise / May 30, 2013

The Curiosity rover investigated an area on Mars named Hottah, which appears to be part of an ancient riverbed.

Courtesy of Malin Space Science Systems / NASA


Mars Rover Curiosity Finds Pebbles Likely Shaped by Ancient River

By: Denise Chow 
Published: 05/30/2013 02:06 PM EDT on

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Smooth, round pebbles found by NASA's Mars rover Curiosity provide more evidence that water once flowed on the Red Planet, according to a new study.

The Curiosity rover snapped pictures of several areas with densely packed pebbles, and by closely analyzing the rock images, researchers discovered that the shapes and sizes of the individual pebbles indicate that they traveled long distances in water, likely as part of an ancient riverbed.

The rocks were found near Curiosity's landing site, between the north rim of Gale Crater and the base of Mount Sharp, a peak that rises 3 miles (5 kilometers) above the crater floor. 

Round and smooth

Scientists divided a photo mosaic of an area called Hottah into smaller frames to study the small rocks, which were cemented together and ranged in size from 0.08 inches (2 millimeters) to 1.6 inches (41 mm) across. In total, the researchers examined 515 stones and noticed that their surfaces were round and smooth.

Rocks worn by wind are typically rough and angular, whereas stones in water tend to become smooth over time, as the rocks get churned around with coarse grains of sand.

"We could see that almost all of the 515 pebbles we analyzed were worn flat, smooth and round," study co-author Asmus Koefoed, a research assistant at the Niels Bohr Institute at the University of Copenhagen in Denmark, said in a statement.

The cemented sections of rock were likely formed by a combination of fine sand, mud, gravel and pebbles, the researchers said. This mixture clumped together and hardened, creating the solid formations seen by the Curiosity rover. Over time, as sand particles were blown across the surface of Mars, the tops of these cemented rocks became worn and flat, the researchers added.

Gale Crater

"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," study co-author Dawn Sumner, a geologist at the University of California, Davis, said in a statement. "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."

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