“This is a brand new mission, brand new landing site for all intents and purposes, geologically,” Dr. Squyres said. “A whole new set of puzzles for us to go off and solve.”

Opportunity and a twin rover, Spirit, arrived on Mars in January 2004, landing on different sides of the planet with the goal of exploring the surface for signs of past water. Spirit got its wheels stuck in a sand trap in May 2009 and could not get its solar panels pointed toward the sun; unable to generate enough electricity, it stopped communicating in March 2010 and is not expected to be heard from again.

But Opportunity, about the size of a golf cart, continues rolling on. It has now driven 20 miles. It had been designed to travel about two-thirds of a mile.

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When it landed in 2004, it rolled by chance into Eagle Crater, which is 70 feet in diameter, and over the years explored a series of progressively larger craters. Three years ago, Opportunity — which had already far outlived its original mission — set off on a journey that no one was confident it would complete. It drove 13 miles to Endeavour, arriving on Aug. 9.

Opportunity is no longer in pristine condition, however. It is now usually driven backward to even out the wear on the gears. One of the joints on the robotic arm is stuck. Care is taken to minimize the movement of the camera to avoid wearing out the motor.

“All in all, we have a very senior rover that’s showing her age,” said John Callas, the project manager. “She has some arthritis and other issues, but in general, she’s in good health.”

Both Spirit and Opportunity have discovered evidence of liquid water, albeit water that is highly acidic, like sulfuric acid, that made parts of ancient Mars potentially habitable, at least intermittently.

The Endeavour rock that Opportunity looked at, named Tisdale 2, is a breccia, and was smashed apart by the impact, then fused back together. That was not unexpected for a rim of an impact crater.

The high levels of zinc, however, were unexpected. Dr. Squyres said it was a clue that this rock might have formed in a hydrothermal system, although it was much too early to speculate about whether it could have been geysers like those found at Yellowstone Park or something much less dramatic, like water vapor percolating through the rocks.

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The two rovers were not designed to look for signs of life, past or present. Even NASA’s next Mars mission, which involves an S.U.V.-size rover named Curiosity that is scheduled for launching later this year, does not carry any life-detection experiments. But it will be able to identify some of the molecular building blocks for life.