Science: Curiosity Rover Finds Evidence of Ancient Stream on Mars

Polished, smooth rocks found by NASA's Curiosity Rover suggest the planet once had a swift moving streambed, with depths of up to 3 feet and water that moved at speeds of up to 1.6 miles per hour.

We know it was a streambed because it takes a fast flow to move pebbles of this size, and they're rounded, says Dawn Sumner, a researcher at the University of California, Davis, one of the authors of the study.

The pebbles are believed to be at least two billion years old. Researchers are unsure how long the stream was, but Sumner says in order for rounding to take place, the stream must have been "flowing for a long period of time over a long distance. You aren't going to get rounding with transient water or a flash flood, she says.

The team found hundreds of pebbles that suggested movement by water. Other discoveries have found what is known as a mudstone, which likely once sat in standing water. Orbiting satellites have captured other evidence of ancient rivers and streams.

In January, the European Space Agency found a striking river with numerous tributaries that ran for nearly 1,000 miles and may have been as deep as 1,000 feet at some points. Sumner says discoveries such as those are important, but the Curiosity Rover is able to tell scientists things from the ground that photos from satellites cannot.