The US space agency (Nasa) has reported that its Curiosity rover has made another significant discovery on Mars.
The robot has drilled into a rock that contains clay minerals - an indication of formation in, or substantial alteration by, neutral water.
Scientists say the find is one more step towards showing conditions on the Red Planet in the distant past could have supported life.
Many rocks studied previously were probably deposited in acidic water.
While this would not have precluded the possibility of micro-organisms taking hold on Mars, it would have been more challenging, scientists believe.
Identifying clays shows there were at least some locations on the planet billions of years ago where environments would have been much more favourable.
Curiosity made the assessment after studying a powdered sample drilled from a fine-grained rock at its exploration site in Gale Crater, a deep impact bowl on Mars' equator.
It found the rock sample to be 20-30% smectite - a particular class of clay minerals. Scientists say the rock may have formed in a lake.
The rover has already seen plenty of evidence for past running water in Gale. Last year, it identified the remains of an ancient riverbed system where water once flowed, perhaps a metre deep.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
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