After almost 10 years of roaming the red planet, the Opportunity rover has made a new discovery that offers scientists more clues about Mars's watery history.

NASA researchers announced Friday that a rock that was examined by the rover last month, and which they named Esperance 6, contains clay minerals, pointing to the existence of water on ancient Mars.

"What we have found at Esperance has vaulted into my  top five discoveries for the whole nine and a half years," said Steve Squyres, principal investigator for Opportunity.

The chemical composition of Esperance 6, located near the rim of the Endeavour Crater where Opportunity has been conducting experiments since August 2011, suggests that it had been in contact with water with a neutral pH (acidic) level during the planet's early history.

Previous discoveries had only indicated the presence of a liquid with a low pH level — closer to sulfuric acid, Squyres explained during a teleconference Friday. "This is the most powerful evidence of a neutral-chemistry water, found by Opportunity," said Squyres.

Earlier this year, a chemical analysis by the rover Curiosity also suggested that water had once existed on the planet.

Opportunity in 'remarkably good health'

Launched in July 2004, the Opportunity rover's initial mission was planned to last only 90 solar days (a Martian sol is equivalent to about 24 hours and 40 minutes).

While the rover has had minor problems with its flash memory cells, project manager John Callas says that Opportunity is "in remarkably good health considering her age."

NASA lost communication with Opportunity's twin rover, Spirit, in 2010.

"I don't think anyone on this planet imagined that this rover would have lasted this long," Callas said.

As Opportunity enters its tenth year on the planet, it is embarking on a new mission by making its way toward Solander Point, an elevated area comprising multiple rock layers.

Researchers aim to have Opportunity arrive at the new study area by August, before the start of winter in the planet's southern hemisphere.