Nasa scientists say their Opportunity rover has made new discoveries about early water on Mars which may have been drinkable.
The unmanned solar-powered vehicle, described as "arthritic" as it nears 10 years since its launch, has just analysed 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 the rocks through some sort of fracture, leaving an unusually high concentration of clay.
The analysis reveals traces of a what may have been a drinkable type of water that dates to the first billion years of Martian history.
The clay rocks were forming under a more neutral pH, before conditions became more harsh and water more acidic, Dr Squyres said.
The rover's rock abrasion tool, alpha particle X-ray spectrometer and microscopic imager provided the details to scientists, who can learn about the 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 sulphuric acid," Dr 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 favourable 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".
Dr Squyres said 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 slowly making its way, at about 50 metres a day, towards an area a 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.