The rover was to use its rock abrasion tool Sunday to grind away at a martian outcropping dubbed "Flat Rock," but the tool left "no discernible impression on the rock," NASA's Jet Propulsion Laboratory reported.
National Aeronautics and Space Administration scientists now hope to learn more about the rock's makeup by scraping its exterior. All indications are that the tool is working, NASA said.
Researchers also plan to have Opportunity use its alpha particle X-ray spectrometer to identify the rock's chemical elements before attempting to grind away at it again later this week.
Meanwhile, on the other side of Mars, Opportunity's twin rover, Spirit, traveled nearly 86 feet Saturday, bringing its total odometer reading to more than 822 feet. But because Spirit had to maneuver around several obstacles on its journey, it made a net gain of just 72 feet toward its ultimate destination, a large depression scientists have nicknamed the "Bonneville Crater."
Spirit was expected to use an array of scientific tools to take measurements of its surroundings before continuing toward the crater.
The twin rovers' $820 million mission was designed to seek geological clues to whether ancient Mars had enough water to have supported life. Both rovers have now found evidence of past water activity on the planet, NASA announced last week.
The Spirit rover's instruments found signs that water may have altered a volcanic rock in a region called Gusev crater. That discovery, announced Friday at NASA's Jet Propulsion Laboratory, followed a study of a rock called "Humphrey."
"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.
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.
The amount of water suggested by the data is far less than what Opportunity earlier found at its landing site.
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.
JPL scientists have been trying to identify an object seen in film of the martian surface that resembles a pair of "bunny ears." It appears to have been part of the rovers' landing equipment.