The Opportunity rover that NASA sent to Mars almost ten years ago has just made one of the most interesting discoveries about the planet yet, evidence of water, reported the BBC.

The rover looked at a rock that researchers have called Esperance, scientists determined the rock had been physically changed by prolonged exposure to water.

NASA has discovered plenty of evidence of liquid once existing on Mars, but it has always proved to be acid, not water.

"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," said Steve Squyres, Opportunity's principal investigator.

This time was different, there seemed to be conclusive evidence that the rock had once been exposed to real, drinkable water.

"We run around talking about water on Mars. In fact, what Opportunity has mostly discovered, or found evidence for, was sulphuric acid," Squyres said. "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."

The clay in the rock were found to contain aluminum, possibly a type called montmorillonite, but the rover doesn't have the technology to tell for sure.

The fact that there's clay at all suggests that Mars once had running water, and was much warmer than the frigid condition it is today.

The Curiosity, which landed more recently, also found clay where it first landed almost halfway around the planet. On the rover's next journey researchers hope it will find an even deeper stack of rocks to continue studying the evidence of water.

"Maybe [we can] try to reconstruct the actual depositional environment of these materials and whether they were lacustrine - that is, formed by a lake - or fluvial (river) or an alluvial fan (network of streams), or whatever," said deputy principal investigator Ray Arvidson, of Washington University, St Louis.

The Opportunity is on the verge of breaking down, the rover has many problems physically and the flash memory is full of glitches.

"Remember, the rover continues in a very hostile environment on Mars," said John Callas, Nasa's Opportunity project manager. "The rover could have a catastrophic failure at any moment. So, each day is a gift."

See what the Curiosity rover, which in better shape, will be up to next: