Old Opportunity Mars rover makes discovery

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."

Prof Squyres, who is affiliated to Cornell University, Ithaca, New York, said he was inclined to put Esperance in his personal top five discoveries made on the Red Planet by Opportunity and her twin rover, Spirit, which stopped working in 2011.

The clays are aluminium-rich, possibly of the type montmorillonite. However, because Opportunity's X-ray spectrometer can only discern the atomic elements in a rock, and not their mineralogical arrangement, no-one can say for sure.

Nonetheless, the mere occurrence of clays is further proof that Mars was much warmer and wetter billions of years ago; a very different place to the cold, desiccated world it has become.

And these results complement nicely those of Nasa's newer rover Curiosity, which has also identified clays at its landing site almost half-way around the planet's equator.