Scientists have discovered pebbles on Mars, indicating that a stream has flowed on the planet and bolstering the theory that it was once able to support life.
The rounded pebbles found by the team from Nasa's Curiosity rover mission, whose findings were published in journal Science today, were laid down more than two billion years ago.
Researchers said the pebbles could only have been formed by being carried through water over long distances.
It is the first time evidence of sustained water flows on Mars has been uncovered.
The report authors said:
These ancient fluvial deposits indicate sustained liquid water flows across the landscape - a finding that raises prospects for the former presence of habitable environments on Mars.
They said the conditions on Mars at the time the pebbles were deposited must have been vastly different to the cold, dry environment of the present day if water was to flow over several kilometres.
However, experts need to learn more about radiation risks to astronauts ahead of any future missions to Mars, according to a separate study published in Science today.
Scientist Cary Zeitlin said astronauts venturing to the red planet are subjected to a dose of radiation representing a large fraction of the amount that is acceptable over a lifetime.