The blueberry-sized pebbles were found in a rock Opportunity has analyzed for nearly two months inside a small crater in the Meridiani Planum, where the rover landed in January on a mission to find signs of past life on the Red Planet.

Using a spectrometer -- an instrument that detects minerals -- the rover found the pebbles contained hematite, a substance containing iron ore, said Daniel Rodionov, a rover science team collaborator from the University of Mainz, Germany.

Hematite on Earth is usually formed in water.

"The question is whether this will be part of a still larger story," said Andrew Knoll, a science team member from Harvard University, Cambridge, Massachusetts.

"Perhaps the whole floor of Meridiani Planum has a residual layer of blueberries," he said.

The area is the size of the US southwestern state of Oklahoma.

Opportunity will in the next few days travel to a crater 750 meters (yards) from the rock it has been studying.

Earlier this month, NASA's associate administrator, Ed Weiler, told a news conference that Opportunity "has landed in an area of Mars where liquid water once drenched the surface. Moreover, this area would have been a good habitable environment for some period of time."

Meanwhile, the Spirit rover that landed on the other side of the planet in January took a panoramic photo of the Gusev crater.

The picture shows the crater's rocky wall about 80 kilometers (50 miles) from the robot, said Albert Haldemann, a deputy project scientist.

The 820-million-dollar Mars mission ends in April and is overseen by some 300 scientists.

SPACE.WIRE