The rover has found and photographed bright veins of a mineral, apparently gypsum deposited by water, evidence of a past wet environment on Mars, a NASA release said Thursday.
"This tells a slam-dunk story that water flowed through underground fractures in the rock," Steve Squyres of Cornell University said.
"This stuff is a fairly pure chemical deposit that formed in place right where we see it," said Squyres, principal investigator for Opportunity.
"That can't be said for other gypsum seen on Mars or for other water-related minerals Opportunity has found. It's not uncommon on Earth, but on Mars, it's the kind of thing that makes geologists jump out of their chairs."
The vein of gypsum, or calcium sulphate, is about the width of a human thumb and 16 to 20 inches long, protruding slightly from the bedrock on either side of it.
"To me, this is the single most powerful piece of evidence for liquid water at Mars that has been discovered by the Opportunity rover," Squyres said.
"This stuff formed right here. There was a fracture in the rock, water flowed through it, gypsum was precipitated from the water. End of story. There's no ambiguity."