Nasa rover Curiosity has beamed back pictures of bedrock that suggest a fast-moving stream, possibly waist-deep, once flowed on Mars - a find that the mission's chief scientist called exciting.
There have been previous signs that water existed on the red planet long ago, but the images showing pebbles rounded off, probably 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 did not come as a complete surprise. Nasa decided to put Curiosity down inside Gale Crater near the Martian equator because photos from space hinted that the spot possessed a watery past.
The six-wheeled rover landed safely on August 5 after a nail-biting plunge through the Martian atmosphere. It is on a two-year, 2.5 billion-dollar mission to study whether the Martian environment could have been favourable for microbial life.
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 once warmer and wetter.
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 - ranging from a sand grain to a golf ball - indicates that they could not have been carried by wind, said mission scientist Rebecca Williams of the Planetary Science Institute in Tucson, Arizona.
Though Curiosity did not use its high-tech instruments to drill into the rocks or analyse their chemical make-up, Mr Grotzinger said scientists were sure that water played a role based on just studying the pictures.
It is unclear how long the water persisted on the surface, but it easily could have lasted "thousands to millions of years", said mission scientist Bill Dietrich of the University of California, Berkeley.
Curiosity chanced upon the dried-up streambed while driving to Glenelg, an intriguing spot where three types of terrain meet. Its ultimate destination is Mount Sharp, a mountain rising from the centre of crater floor, but it was not expected to travel there until the end of the year.