Pebbles on Mars found by the Curiosity rover are smoothly rounded, suggesting they were shaped by running water. By carefully studying 515 specimens, researchers were able to conclude that the rocks were transported for a significant distance by water, suggesting an ancient river.
Curiosity found several patches where rounded pebbles, much like those found near running water on Earth, abounded. The patches were located between the north rim of Gale Crater and Mount Sharp, within a few hundred feet of the spot where the rover landed.
The pictures were taken by Curiosty's mast camera, or MastCam, during the rover's first 40 days on the Red Planet. When the accumulations of rocks were first spotted, initial ideas about their formation centered around an ancient river, but those ideas were not proven until now. One of the rocks, named Link, was also analyzed using Curiosity's Chemistry and Camera (ChemCam), which shot it with a laser.
The rocks ranged in size from 0.08 inches to 1.6 inches in diameter and some of the objects were cemented together. The rocks in these patches included streambed gavels - the first found on Mars. The size, shape and number of these rocks allowed researchers to calculate the depth of the ancient river, as well as how quickly the water flowed.
"At a minimum, the stream was flowing at a speed equivalent to a walking pace... or three feet per second, and it was ankle-deep to hip-deep," Rebecca Williams of the Planetary Science Institute said.
From the structure of the rocks, it is likely they were carried at least a few miles over a period of time lasting several months or longer. The natural cement holding together some of the rocks was likely formed from a combination of sand, mud and smaller pebbles. At one location, Hottah, Curiosity discovered layers of the small rocks alternating with layers of sand, a familiar sight near rivers on Earth.
"For decades, we have speculated and hypothesized that the surface of Mars was carved by water, but this is the first time where you can see the remnants of stream flow with what are absolutely tell-tale signs," Williams said.
This is not the first time that NASA has encountered evidence that water once flowed on Mars, but this is the best evidence yet that rivers helped to shape the surface of the Red Planet.
Results of the study were published in the journal Science May 31.