Pebbles and sand on Mars best evidence that a river ran through it

Pebbles and sand scattered near an ancient Martian river network may present the most convincing evidence yet that the frigid deserts of the Red Planet were once a habitable environment traversed by flowing water. Scientists with NASA's Mars Science Laboratory mission reported May 30 the discovery of sand grains and small stones that bear the telltale roundness of river stones and are too heavy to have been moved by wind. The researchers estimated that the sediment was produced by water that moved at a speed between that of a small stream and a large river, and had a depth of roughly an inch to nearly 3 feet. Co-author Kevin Lewis, a Princeton associate research scholar in geosciences and a participating scientist on the Mars mission, said that the rocks and sand are among the best evidence so far that water once flowed on Mars, and suggest that the planet's past climate was wildly different from what it is today. "This is one of the best pieces of evidence we've seen on the ground for flowing water," Lewis said. "The shape of these rocks and sand is exactly the same kind of thing you'd see if you went out to any streambed. It suggests a very similar environment to the Earth's." The researchers analyzed sediment taken from a Martian plain that abuts a sedimentary deposit known as an alluvial fan.