A Martian rover has dug up evidence of a historic lake that could have been teeming with life under four billion years ago.
Rocks collected by Curiosity, which has been exploring Mars for about a year, suggest the Red Planet's vast Yellowknife Bay was once "really similar" to environments found on Earth.
Nasa scientists found the rocks had signs of carbon, hydrogen, oxygen, nitrogen and sulphur - a mix of chemicals that "would provide perfect conditions for simple microbial life", according to a report in the Science journal .
Tiny bacterial life forms known as chemolithoautotrophs thrive under similar conditions on Earth, where they are typically found in caves and beneath the sea in hydrothermal vents.
Tests on the samples taken from Yellowknife Bay also revealed they contained smectites - the clay minerals commonly found in lake deposits.
David Vaniman, of the Planetary Science Institute, said: "It's commonly called a swelling clay - the kind that sticks to your boot when you step in it.
"You find biologically rich environments where you find smectites on Earth."
Although previous missions to Mars have suggested lakes were once a feature of the planet's landscape, this is the closest scientists have come to proving their existence.
The results provide "the strongest evidence yet that Mars could have been habitable enough for life to take hold", the study said.
John Grotzinger, a professor of geology at the California Institute of Technology, said Yellowknife Bay was most likely a cold lake connected to various streams and surrounded by a ridge of snow-capped mountains.
It may have supported life for tens of millions of years at roughly the same time as the first life forms are thought to have existed on Earth.
The ancient lake bed is contained within Mars' giant Gale Crater, which was chosen as the landing site for the Curiosity rover because it was believed to contain many layers of rock as well as evidence of water.
The $2.5bn (£1.5bn) rover is the size of a small car and is controlled by engineers at Nasa's Jet Propulsion Laboratory in Pasadena, California.
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