Evidence of a freshwater lake that may once have supported life has been discovered on Mars.
Mudstones from Gale Crater, the landing site of the Curiosity rover, were formed in a lake that may have existed on the planet for hundreds of thousands of years, say scientists.
The 150 kilometre-wide (93-mile) impact basin, with a mountain at its centre, is believed to have supported the lake - and possibly more than one - around 3.6 billion years ago.
Analysis indicates that the lake was calm and probably filled with fresh water containing key biological elements necessary for life such as carbon, hydrogen, oxygen, nitrogen and sulphur.
Conditions within the lake would have been perfect for simple microbial life forms such as chemolithoautotrophs, which on Earth live in caves and around hydrothermal vents, according to the researchers. The organisms break down rocks and minerals to obtain energy.
Professor Sanjeev Gupta, a member of the Curiosity team from Imperial College London, said: "It is important to note that we have not found signs of ancient life on Mars. What we have found is that Gale Crater was able to sustain a lake on its surface at least once in its ancient past that may have been favourable for microbial life, billions of years ago. This is a huge positive step for the exploration of Mars.
"It is exciting to think that billions of years ago, ancient microbial life may have existed in the lake's calm waters, converting a rich array of elements into energy. The next phase of the mission, where we will be exploring more rocky outcrops on the crater's surface, could hold the key whether life did exist on the red planet."
The findings are reported in the latest issue of the journal Science.
On Earth, mudstones generally form in calm conditions. They are created by very fine sediment grains settling layer-by-layer on each other in still water.
Previously, the Mars Science Laboratory scientists operating Curiosity have found evidence of water on the Martian surface in other rocks such as conglomerates.
But the new results provide the strongest evidence yet that Mars could once have been habitable enough to support life.
Drill samples of the mudstones were analysed in Curiosity's on-board laboratory, which is remotely operated by technicians at the American space agency Nasa's Jet Propulsion Laboratory in Pasadena, US.
The rover will now conduct a search for further evidence of ancient lakes or habitable environments in the thick pile of sedimentary rocks scattered across the crater's surface.