The rock, found at the rim of the 14-mile-wide crater, is flat-topped and about the size of a footstool and was apparently excavated by an impact that dug a hole the size of a tennis court into the crater's rim. The rock's been informally named Tisdale 2.
"This is different from any rock ever seen on Mars," said Steve Squyres, principal investigator for Opportunity at Cornell University.
"It has a composition similar to some volcanic rocks, but there's much more zinc and bromine than we've typically seen. We are getting confirmation that reaching Endeavour really has given us the equivalent of a second landing site for Opportunity."
Over the last two weeks, researchers have used an instrument on the rover's robotic arm to identify elements at several spots on Tisdale 2, and have also examined it using the rover's microscopic imager and multiple filters of its panoramic camera.
Rrock exposures on Endeavour's rim are believed to date from early on in Martian history, and include clay minerals that form in less-acidic wet conditions - possibly favorable for life.
Discontinuous ridges are all that remains of the ancient crater's rim. The ridge at the section of the rim where Opportunity arrived has been dubbed Cape York."
"On the final traverses to Cape York [a section of the rim], we saw ragged outcrops at Botany Bay [a gap in the rim] unlike anything Opportunity has seen so far, and a bench around the edge of Cape York looks like sedimentary rock that's been cut and filled with veins of material possibly delivered by water," says Ray Arvidson, the rover's deputy principal investigator at Washington University.
"We made an explicit decision to examine ancient rocks of Cape York first."
NASA doesn't know how long Opportunity's useful life will last.
"We have a very senior rover in good health for having already worked 30 times longer than planned," said John Callas, project manager for Opportunity.
"However, at any time, we could lose a critical component on an essential rover system, and the mission would be over. Or, we might still be using this rover's capabilities beneficially for years. There are miles of exciting geology to explore at Endeavour crater."