Opportunity, the indefatigable robot that has been exploring Mars for seven and a half years, arrived three weeks ago at the edge of a 13.6 mile wide crater named Endeavour and has been sending back images of the surrounding environment.
The first rock it examined was a flat-topped object about the size of a foot stool that apparently was cast up by an impact that left an impression the size of a tennis court on the rim of the crater.
Called Tisdale 2, the rock "is different from any rock we've ever seen on Mars," said Steve Squyres, a Cornell University scientist who is the principal investigator for Opportunity.
"It has a composition similar to some volcanic rocks, but there's much more zinc and bromine than we've typically seen," he said at a news conference.
The observations and measurements taken by the American Martian orbiters leads scientists to believe that the rocks on the rim of the crater contain clay minerals that form in wet conditions and which are less acidic and possibly more favourable for life, they said.