A Martian crater now being explored by the Nasa rover Curiosity once contained lakes that remained for up to 10,000 years at a time - long enough to support life.
Rock formations photographed by the rover suggest that long ago a transient water system of deltas and lakes dominated the landscape of Gale Crater.
The images show evidence of sediments transported southward by shallow streams to the boundary of an ancient lake.
Scientists estimated that over time individual lakes at this location may have persisted for between 100 and 10,000-year-long periods.
This is potentially long enough for primitive life to establish itself in the crater, said the Curiosity team led by Professor John Grotzinger from the California Institute of Technology at Pasadena.
Writing in the journal Science, the researchers concluded: "One of the key criteria for planetary habitability is the duration for which water might have been accessible to enable microbial origination and evolution.
"The Gale crater floor today is the lowest topographic depression for over 1,000 kilometres in any direction, including the northern plains. Our results show that water pooled there, in surface and subsurface reservoirs, for a geologically and perhaps biologically relevant period of time."