NASA's Curiosity rover has found remnants of an ancient freshwater lake on Mars that may have supported tiny life forms, scientists say.

There is no water left in it, but drill tests and a chemical analysis of its fine-grained rocks by Curiosity's instruments suggest microbial life could have thrived there billions of years ago.

A series of sedimentary deposits in the Glenelg area of Gale Crater, Mars.

The rocks contain signs of carbon, hydrogen, oxygen, nitrogen and sulphur, and "would provide perfect conditions for simple microbial life", said the report in the journal Science.

Small bacterial life forms known as chemolithoautotrophs are known to thrive under similar conditions on Earth, and are typically found in caves and under the sea in hydrothermal vents.