LOS ANGELES — The NASA rover Curiosity has beamed back pictures of bedrock that suggest a fast-moving stream, possibly waist-deep, once flowed on Mars.
There have been previous signs that water existed on the red planet long ago, but the images released Thursday showing pebbles rounded off, likely by water, offered the most convincing evidence so far of an ancient streambed.
There was "a vigorous flow on the surface of Mars," said chief scientist John Grotzinger of the California Institute of Technology. "We're really excited about this."
The discovery wasn't a complete surprise. NASA decided to land Curiosity inside Gale Crater near the Martian equator because photos from space suggested the spot had water in the past.
Present-day Mars is a frozen desert with no hint of water on its radiation-scarred surface, but geological studies of rocks by previous missions suggest the planet was warmer and wetter at one time.
The latest evidence came from photos that Curiosity took revealing rounded pebbles and gravel, a sign that the rocks were transported long distances by water and smoothed out.
The size of the rocks, from a sand grain to a golf ball, indicates they could not have been carried by wind, said mission scientist Rebecca Williams of the Planetary Science Institute in Tucson, Ariz.
It's unclear how long water persisted on the surface, but it easily could have been "thousands to millions of years," said mission scientist Bill Dietrich of the University of California, Berkeley.
Finding past water is a first step to determining whether the environment was favorable for microbial life.