Before trekking off last month, Opportunity used a grinder to scrape away the top layer of a light-colored rock for a peek inside. The rock was so lumpy and covered with crud that it took the rover several tries to crack open its secrets.
Unlike other rocks that Opportunity inspected during the past nine years, the latest told a different story: It contained clay minerals, a sign that water coursed through it, and formed in an environment that might have been suitable for microbes.
Previous rock studies by Opportunity pointed to a watery past on Mars, but scientists said the water was acidic.
"This is water you can drink," said mission chief scientist Steve Squyres of Cornell University.
Since landing on opposite ends of the red planet in 2004, Opportunity and its twin, Spirit, have impressed scientists with their longevity. Both outlasted their original, three-month warranty.
While Opportunity continues to plow ahead, Spirit's mission came to an end when it got stuck in sand and stopped communicating in 2010.
Project manager John Callas said Opportunity showed signs of wear, but was otherwise in good health. It recently experienced a bout of amnesia with its flash memory, but Callas said it was not serious.
Opportunity is not the only Mars rover on the move. Earlier this week, NASA said its newest rover, Curiosity, will soon head to a Martian mountain.