Each spherule is about the size of a BB. The Opportunity rover has photographed them on the sandy soil at the its landing site and embedded in rocks at a nearby ledge. The new images include a 3-D view.

The landing site is covered with fine grains of sand, larger irregular-shaped grains that possibly have eroded from the rock ledge, and the "spectacular" spherules, said Mark Lemmon, science team member for the rovers at Texas A&M University.

"There's much more than meets the eye in these images," Lemmon said, but he couldn't say what that was.

Scientists would like to know if the spherules were created by water, or if they were instead born in volcanic eruptions. More observation will be needed to find out, assuming answers are forthcoming.

"We haven't seen anything that leads us to conclude that water has to be involved," Lemmon cautioned at a press conference today.

"Personally I'm really stumped" about the spherical objects, said Wendy Calvin, a geophysicist at the University of Nevada at Reno and member of the rover science team. "I have no idea how they got there." But she added that there was "no reason" to assume biology was involved.

Digging in

Opportunity will further investigate the spherules, along with interesting layering found in rocks protruding from the ledge, in coming days. The rover has yet to use one of its most powerful research instruments, a Rock Abrasion Tool (RAT) that will dig into the rocks to expose their inner compositions.

Opportunity is traversing the small ledge, which is about the height of shoebox, to examine various outcroppings. It started with a rock called Stone Mountain and has since investigated locations dubbed Alpha and Bravo, said Art Thompson, an engineer on the rover project. It is now at the Charlie site.

Each position change involved driving the rover about 13-20 feet (4-6 meters).

Engineers experienced some problems with the rover slipping as it navigated the slope leading out of the subtle depression in which it landed on the Meridiani Planum near the Martian equator. That problem -- which could lead to trouble if the rover were to run into something unexpected -- has apparently been solved, Thompson said, by having the robot intentionally over-drive on the way up and under-drive on the way down.

Once scientists decide they've got all the information they want from the ledge, Opportunity will crawl above it and dig into some soil on the surrounding plain, where higher concentrations of the mineral hematite appear to exist.

Hematite typically forms in the presence of water, but it is sometimes the result of volcanic activity. Scientists want to know which was the case around the Opportunity landing site.

The search for water drives all of NASA's Mars missions, because water is the key ingredient required for life.

Spirits up

On the other side of the planet, in the Gusev crater, the twin rover Spirit continues to drive toward a small impact crater some 800 feet (240 meters) from its landing site. Its last two daily runs covered 89 feet (27 meters) and 79 feet (24 meters).

A glitch earlier this week caused Spirit to miss a day of work. Cold onboard temperatures, caused when the rover shaded parts of itself, prevented it from pointing an antenna earthward. The problem has been solved, Thompson said.

"She's in outstanding health now," he said of Spirit, adding that the rover is experiencing no adverse effects from an earlier memory problem that had put the robot out of commission for several days.

"We have two very busy rovers," Thompson said.