NASA (search) rover got its closest-ever look at bedrock on the surface of Mars (search) on Tuesday and discovered layering that scientists were eager for the spacecraft to examine for any long-ago signs of water.

The Opportunity rover (search) spied the fine-scaled layering just days after it landed in a small crater on the far side of the planet from its twin, Spirit. On Tuesday, NASA unveiled a high-resolution photograph of the horizontally striped and fractured slabs of bedrock.

"Look at the wonderful layer cake-structure in there," said Steve Squyres, the mission's main scientist, as he excitedly narrated a slow pan of the black-and-white image. "It's going to be fascinating beyond words to get up close and personal with this thing."

The find is a significant one: Each roughly half-inch thick layer represents an event in the geologic history of the Red Planet that Opportunity should be able to reconstruct with its complement of cameras and scientific instruments.

Scientists believe the layers were laid down billions of years ago, piling up either as ash spewed by successive volcanic eruptions or as sediments borne by wind or water.

Some scientists believe the 18-inch-high band of layered rocks is cross-bedded in part, suggesting a sedimentary origin that would require the presence of water.

If so, the rocks could provide tangible evidence that Mars once was a wetter place capable of sustaining life. Images taken from orbit already suggest vast amounts of water once flowed across the surface of the planet.

The slabs of bedrock rim a portion of the shallow depression near where the six-wheeled robot sits atop its lander.

The Opportunity rover was in good shape after its weekend landing but had developed a small, 15-watt power loss that worried NASA.

Engineers believe a heater in the shoulder of its robotic arm has been turning on unnecessarily when temperatures drop. Engineers continued to monitor the situation, unsure if it could harm the rover.

"We're very paranoid people," mission manager Jim Erickson said.

The Spirit rover, 6,600 miles around the planet, continued to recover from computer problems that last week brought its science work to a halt. Engineers received additional data they hope will help them understand the problem.

Opportunity should roll off its lander sometime in the next two weeks. This week it is scheduled to put to use its mini-thermal emissions spectrometer, an instrument that measures infrared radiation, to reveal what minerals the rocks contain.

Together, the pair of 384-pound rovers make up an $820 million mission to prospect for geologic evidence that Mars was once a wetter world capable of supporting life.