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 93-mile-wide impact basin is believed to have supported the lake around 3.6 billion years ago.
Analysis indicates that the lake was 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.
Professor Sanjeev Gupta, a member of the Curiosity team from Imperial College London, said: "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 could hold the key whether life did exist on the red planet."