Nasa's ageing Opportunity rover on Mars has just made what may be one of its most significant discoveries to date.
The nine-year-old robot has identified rock laden with what scientists believe to be clay minerals.
Their presence is an indication that the rock, dubbed Esperance, has been altered at some point in the past through prolonged contact with water.
Opportunity has seen a clay-bearing outcrop before but scientists say this is by far the best example to date.
"It's very rich," said Steve Squyres, the rover's principal investigator.
"We've been discovering evidence for water on Mars since we first landed back in 2004. What's different here?
"If you look at all of the water-related discoveries that have been made by Opportunity, the vast majority of them point to water that was a very low pH - it was acid.
"We run around talking about water on Mars. In fact, what Opportunity has mostly discovered, or found evidence for, was sulphuric acid.
"Clay minerals only tend to form at a more neutral pH. This is water you could drink. This is water that was much more favourable for things like pre-biotic chemistry - the kind of chemistry that could lead to the origin of life."
