An enormous crater near the northern plains of Mars once harboured an ancient lake that could have supported microbial life, Nasa scientists claim.
The freshwater lake stood for more than one hundred thousand years at the base of Gale Crater, a 150km-wide formation that was created when a meteor punched into the red planet around 3.7bn years ago.
Tests on rock samples by Nasa's Curiosity rover revealed the presence of fine clay minerals that formed in a standing body of water, and coarse-grained sandstones laid down by river flows that drained into the lake.
"The presence of these minerals tells us the water was likely to be fresh water, which means it's much more conducive for microbial life," said Sanjeev Gupta, a geologist at Imperial College, London, and a member of the Curiosity science team.
"These rocks are similar to those we would find if we walked along the Dorset or Devon coast line," he added.
The Nasa team is not sure how deep or wide the lake was, but suspect it was deep enough not to have dried out periodically, as this would have left traces of crack marks in the rock samples.
The $2.5bn (£1.6bn) rover landed in on Mars in August 2012 on a mission to explore whether the planet may once have been habitable, though not to look for signs of ancient life itself.
Curiosity's main objective is to trundle up nearby Mount Sharp, a 5km-high mountain that sits in the middle of Gale Crater. Through measurements of its exposed rock faces at different altitudes, researchers hope to piece together the geological history of the planet.
But Curiosity did not make for Mount Sharp immediately. After relaying details of the Martian soil near its landing site, the rover was steered towards a 5m-deep trough in the crater called Yellowknife Bay. Here, the robotic science laboratory drilled into a rock formation called Sheepbed mudstone and examined the powder with its instruments.
Though a combination of x-ray diffraction experiments and analyses of gases given off when the powder was baked in an onboard oven, researchers identified so-called smectite clay minerals that formed in water and elements crucial for life, including carbon, hydrogen, oxygen, sulphur, nitrogen and phosphorus.
The chemical makeup of the minerals showed that they formed in water with low salinity and a neutral pH, suggesting it was neither too acidic nor too alkaline for life to exist.
Writing in the journal, Science, researchers explain that the conditions in the lake were well suited to support a type of microbial life called chemolithoautotrophs. These organisms are found on Earth and can survive by breaking down rocks and minerals for energy.
The lake may have persisted on Mars for tens of thousands of years, said John Grotzinger, project scientist on the Nasa mission at Caltech in Pasadena.
Monica Grady, professor of planetary and space sciences at Open University, said the images of rocks and minerals beamed back from Mars by the Curiosity rover were "spectacular, beautiful and absolutely compelling."
Grady said the findings were shifting scientists' views on how life might have existed on Mars. "The life we know on Earth is largely based on photosynthesis. Grass photoynthesises, cows eat the grass, we eat the cows. We know there are other platforms for life, but they are mostly at the bottom of the ocean and in really odd places."
Instead of relying on the sun's energy, life in the Martian lake could have survived on energy liberated by chemical reactions. "This is really, for the first time, showing that absolutely all the ingredients are there for life based on chemistry, and that would be reactions between sulphur and iron. It says that there were absolutely environments on Mars where energy from chemical reactions could have been harvested to make nutrient pathways work. It's so beautifully laid out."
Grady said that even though the Martian lake may have been around for only tens of thousands of years, that may have been long enough for life to emerge there. "We don't really know how long it took for life to get going on Earth. We don't know if it got going once or lots of times.
"The more we know about how life developed on Earth, the more we're beginning to understand that it didn't take very long at all. The fact the lake might have been relatively short lived, in terms of hundreds of thousands of years, doesn't mean that life couldn't have got going there. It doesn't mean that at all."
Curiosity has found some evidence for carbonate rocks on Mars. Grady said that she and many other scientists are keen to see the rover find some of these and analyse them. The rocks form in the presence of carbon dioxide, which on Earth is a product of respiration, and is important for photosynthesis and the carbon cycle.