Curiosity's stream discovery raises hopes of finding life on Mars
Discovery by Curiosity rover of evidence of ancient strean bed on planet is step towards finding if micro-organisms could have survived
Nasa's Mars rover, Curiosity, has discovered gravel carried by an ancient stream that once "ran vigorously" on the red planet.
The news bodes well for the mission to investigate whether Mars could sustain life, or if life ever existed there.
Pictures beamed back by Curiosity show a "jack-hammered-up slab of city sidewalk, but it's really a tilted block of an ancient stream bed", said project scientist John Grotzinger.
Finding evidence of water is the first step towards learning whether the environment on Mars could have supported microbes. While scientists have found evidence of the former presence of water, this is the first time stream-bed gravel has been discovered.
Previous images of Mars indicate it may have been part of a broader network of streams in the area.
Professor Sun Kwok, of the International Astronomical Union's Bioastronomy Commission 51, said: "If you asked [if water or life existed on Mars' frozen surface] 20 years ago, it would have been pretty far-fetched. Now that we're closer to proving there's water, some concrete evidence may come about soon [for the existence of life]."
Kwok said that judging by the ubiquity of micro-organisms on earth, in the air, on the surface and even in the deep reaches of oceans, it was possible micro-organisms could have lived, or are living, underground in Mars in areas that are yet to be explored. "It's a big step forward. I'm excited, but we're not there yet," he said.
Pebbles in the stream bed ranged in size from a sand grain to a golf ball. This means they could not have been carried by wind but must have been transported by water, said mission scientist Rebecca Williams.
The Curiosity rover, which has been exploring Mars since early August, also investigated a second outcrop known as Link.
The pictures transmitted by Curiosity show the pebbles have been cemented into layers of conglomerate rock at a site between the north rim of the Gale Crater and the base of Mount Sharp, where Curiosity is heading.
The sizes and the shapes of the rocks give an idea of the speed and the depth of the stream, Nasa said.
Scientists estimate the water moved at a brisk pace of around one metre per second, and was somewhere between ankle and hip deep.
It is unclear how long the water stayed on the surface, but it could have been for "thousands to millions of years", said mission scientist Bill Dietrich.
Imagery previously captured from Mars' orbit show an alluvial fan of material washed down from the rim, with many apparent channels uphill of the areas the rover has explored since early August.
Curiosity is on a two-year mission to investigate whether it is possible to live on Mars and to learn whether conditions there might have been able to support life in the past.
The US$2.5 billion craft landed in Gale Crater on August 6, opening a new chapter in the history of interplanetary exploration.