Nasa's robotic rover on Mars, known as Curiosity, has found clear evidence that water once flowed over the surface of the planet.
Researchers believe the robot landed at the head of a network of streams several billions of years old.
After only seven weeks of exploration the rover sent back spectacular photographs of rounded pebbles that scientists say were given their shape by water erosion, the BBC reports.
The rocks, which were described in a media briefing at Nasa's Jet Propulsion Laboratory in California, were likely laid down "several billion years ago".
But the actual streams themselves may have persisted on the surface for long periods, Curiosity science co-investigator Bill Dietrich of the University of California, Berkeley, says.
"We would anticipate that it could easily be thousands to millions of years," he told reporters.
The discovery site lies between the northern rim of the equatorial Gale Crater Curiosity landed in and the huge mountain that rises up from the crater's central plain.