The Spirit rover's instruments found signs that water may have altered a volcanic rock in a region called Gusev crater, halfway around the planet from where the Opportunity rover earlier uncovered evidence that its landing site was once drenched.
The discovery announced Friday at NASA's Jet Propulsion Laboratory followed a study of a rock called "Humphrey" that Spirit encountered while driving from its landing site toward a big crater named "Bonneville."
"That's my favorite rock so far," said Ray Arvidson, the mission's deputy principal investigator, of Washington University in St. Louis.
"If we found this rock on Earth we'd say, 'Well, this is a volcanic rock that's had a little bit of fluid move through it, either when it formed or shortly thereafter, and it's been modified,"' Arvidson said.
The amount of water suggested by the data is far less than what Opportunity found at its site.
Spirit used its rock abrasion tool to grind below the surface and reveal cracks filled with apparent minerals, an indicator of water action.
The water was most likely present when the magma was crystallizing into rock. It could have come up with the volcanic magma or the magma could have interacted with ground water, becoming infused with it, Arvidson said.
Scientists making the historic announcement about Opportunity's discovery earlier this week could not say whether there had been standing surface water or even an ocean there, but data showed water had flowed or percolated through those rocks.
JPL also released photographs of magnets placed on the rovers to analyze the pervasive martian dust. Morten Madsen, a team member from the Center for Planetary Science in Copenhagen, Denmark, said that most, if not all, of the dust particles in the martian atmosphere are magnetic.