Scientists have discovered pebbles on Mars, showing that a stream has flowed on the planet – and giving more weight to the theory that it was once able to support life.

The rounded pebbles found by the team from Nasa’s Curiosity rover mission, and published in Science, are only formed when they are carried through water over long distances, according to the researchers.

It is the first time that evidence of sustained water flows on Mars has been uncovered.

Dawn Sumner, a geologist at University of Cali-fornia, Davis, said the discovery of the pebbles – which were laid down more than two billion years ago – was due to choosing the right place for Curiosity to land.

She said: “The main reason we chose Gale Crater as a landing site was to look at the layered rocks at the base of Mount Sharp, about five miles away.

“We knew there was an alluvial fan in the landing area, a cone-shaped deposit of sediment that requires flowing water to form.

“These sorts of pebbles are likely because of that environment. So while we didn’t choose Gale Crater for this purpose, we were hoping to find something like this.”

The report authors noted that conditions on Mars at the time the pebbles were deposited were vastly different to the cold, dry environment of the present day if water was to flow over several kilometres.