The robotic vehicles landed in January and first found signs in March that Mars had water eons ago.
The Spirit rover has now rolled nearly two miles across the plains of its Gusev Crater landing site and into an area dubbed the Columbia Hills.
Perched about 30 feet above a plain, it recently found indications water had altered an outcropping of bedrock dubbed Clovis, scientists at the Jet Propulsion Laboratory in Pasadena said.
Sulfur, chlorine and bromine found inside the rock were in much greater concentrations than in rocks on the plain. Those elements are commonly emitted from volcanoes and could have combined with liquid water or water vapor, said Doug Ming, a science team member.
"Here, we have a more thorough, deeper alteration, suggesting much more water," said Steve Squyres, principal investigator for science instruments aboard the rovers.
Meanwhile, halfway around the planet, the Opportunity rover has rolled about 32 feet into Endurance Crater, a stadium-sized depression. At the bottom, it found rippled dunes and a bizarre rock with a lumpy, rounded appearance.
Scientists weren't sure how the rock was formed.
"I don't have an explanation for this one," Squires said. "It doesn't look like anything we've seen anywhere."
The team hopes the vehicle can examine the edge of the dunes, although it won't go out in them for fear of bogging down.
"We built a wonderful rover, but we didn't build a dune buggy," Squyres said.
Opportunity found profound differences in rocks that it bored into at different levels of a layered slope. Tiny ripples in a rock dubbed Millstone are clear signs that it had contact with flowing water, Squyres said.
The $280 million mission was designed to seek geological clues about whether ancient Mars had water.
In March, NASA announced that Opportunity found ripples in sedimentary rock that indicated a pool of saltwater — an environment that could have supported life — once existed at the landing site in the vast Meridiani Planum.
The next month, NASA said Spirit had found evidence that limited amounts of water had deposited minerals in a volcanic rock.
While the rovers continue to send back intriguing new photos and scientific data from the Red Planet, Opportunity Mission Manager Chris Salvo said they are showing signs of "aches and pains" as they pass their 200-day mark, more than twice their expected life expectancy.
The twin rovers are facing the arrival of the Martian winter.
Its shorter days will cause hardship for the solar powered rovers, and mission specialists are trying to steer the vehicles toward brighter areas and out of shadows.