An ancient network of rivers on Mars once made parts of the planet habitable for microbial life, according to the latest analysis from Nasa's Curiosity rover.
Rock dust drilled from sediments in the giant Gale crater on the red planet were found to contain clay minerals that can have formed only in water, scientists said.
The discovery of other substances alongside the clays, such as calcium phosphate, suggest the soil was neutral or mildly alkaline, making the environment suitable for microbes.
Instruments aboard the Curiosity rover have allowed scientists to build up a gradual picture of the planet's geological past, but the latest analyses are the strongest evidence yet that Mars was once hospitable to life.
"A fundamental question for this mission is whether Mars could have supported a habitable environment," Michael Meyer, a lead scientist on Nasa's Mars Exploration Program, said. "From what we know now, the answer is yes."