Scientists on Friday called NASA's Opportunity rover gimpy and arthritic, but hailed its new discoveries about early water on Mars made almost 10 years after it was launched toward the Red Planet.

The unmanned solar-powered vehicle has just analyzed what may be its oldest rock ever, known as Esperance 6. It contains evidence that potentially life-supporting water once flowed in abundance, leaving clay minerals behind.

"This is powerful evidence that water interacted with this rock and changed its chemistry, changed its mineralogy in a dramatic way," said principal investigator Steve Squyres of Cornell University.

He described the research as "some of the most important" of the decade-long mission because it showcases a very different chemistry than most of the previous discoveries about water on Mars, which is now quite dry.

Scientists believe that a lot of water once flowed through these rocks through some sort of fracture, leaving an unusually high concentration of clay.

The analysis reveals traces of a likely drinkable type of water that dates to the first billion years of Martian history, when clay rocks were forming under a more neutral pH, before conditions became more harsh and water more acidic, Squyres said.

The rover's rock abrasion tool, alpha particle X-ray spectrometer and microscopic imager provided the details to Earth-based scientists, who can learn about the Red Planet's history without bringing its rocks to Earth.

Opportunity and its twin rover Spirit launched in 2003 and landed in January 2004 for what was initially meant to be a three-month exploration. Both discovered evidence of wet environments on ancient Mars.

"What Opportunity has mostly discovered evidence for was sulfuric acid," Squyres told reporters, outlining the major difference detected in the Esperance rock's formation. "This is water you could drink," he said.

The oldest rocks, like Esperance, have a neutral pH, signaling that early Martian water was "probably much more favorable in its chemistry, in its pH, in its level of acidity for things like prebiotic chemistry, the kind of chemistry that could lead to the origin of life."

Squyres said the six-wheeled Opportunity rover "has kind of a gimpy shoulder" and that analysis of Esperance took seven tries over many weeks as the rover endured a dust storm, a lumpy terrain and a period when Mars went behind the Sun and out of contact with Earth.

Now, Opportunity is is slowly making its way -- about 50 meters (yards) per day) -- toward an area 1.5 kilometers (one mile) away known as Solander Point that contains 10 times as many geological layers for study as the area where Esperance was found.

It hopes to arrive by August 1.

Spirit launched on June 10, 2003 and finally stopped working in 2010. Opportunity left Earth on July 7, 2003. Both rovers landed on Mars in January 2004.

"Opportunity is is remarkably good health for her age, especially if you measure that in dog years," said John Callas, project manager for Opportunity at NASA's Jet Propulsion Laboratory in Pasadena, California.

"The rover's health is essentially unchanged since we last reported. There has been some arthritis in a few mechanisms for some time but the drive system is performing well."

In all these years, the rover has traveled 36 kilometers (22 miles) on the surface of Mars. But for the rover, that is the car-equivalent of lasting two million miles without an oil change, scientists said.

The main aging concern is what Callas called occasional "flash memory amnesia," or a wearing out from over use of one of the flash memory cells. Bigger problems loom with Mars' hostile environment and harsh temperature changes, he said.

"The rover could have a catastrophic failure at any moment. So each day is a gift," said Callas.

The Opportunity rover now costs about $14 million per year to operate, NASA scientists said.

Its much bigger cousin, the $2.5 billion nuclear-powered Curiosity rover, arrived on Mars in August 2012 for an anticipated two-year mission and a further hunt for traces that life may once have existed.