NEW YORK: NASA's Mars Exploration rover 'Opportunity' is driving out of a giant crater nearly a year after descending into it to examine exposed ancient rock layers, the American space agency has announced.

"We have done everything. We entered Victoria Crater to do and more," said Bruce Banerdt, of NASA's Jet Propulsion Laboratory in Pasadena, California. Banerdt is project scientist for 'Opportunity' and its rover twin, Spirit.

Having completed its job in the crater, 'Opportunity' is now preparing to inspect loose cobbles on the plains. Some of these rocks, approximately fist-size and larger, were thrown long distances when objects hitting Mars blasted craters deeper than Victoria into the Red Planet. 'Opportunity' has driven past scores of cobbles but examined only a few.

"Our experience tells us there's lots of diversity among the cobbles," said Scott McLennan of the State University of New York, Stony Brook. McLennan is a long-term planning leader for the rover science team.

"We want to get a better characterisation of them. A statistical sampling from examining more of them will be important for understanding the geology of the area."

'Opportunity' entered Victoria Crater on September 11, 2007, after a year of scouting from the rim. Once a drivable inner slope was identified, the rover used contact instruments on its robotic arm to inspect the composition and textures of accessible layers.

The rover then drove close to the base of a cliff called "Cape Verde," part of the crater rim, to capture detailed images of a stack of layers 20 feet tall. The information Opportunity has returned about the layers in Victoria suggest the sediments were deposited by wind and then altered by groundwater.

"The patterns broadly resemble what we saw at the smaller craters 'Opportunity' explored earlier," McLennan said. "By looking deeper into the layering, we are looking farther back in time." The crater stretches approximately a half mile in diameter and is deeper than any other seen by 'Opportunity'.

Engineers are programming 'Opportunity' to climb out of the crater at the same place it entered.

NASA said a spike in electric current drawn by the rover's left front wheel last month quickly settled discussions about whether to keep trying to edge even closer to the base of Cape Verde on a steep slope. The spike resembled one seen on Spirit when that rover lost the use of its right front wheel in 2006.

'Opportunity's' six wheels, the space agency said, are all still working after 10 times more use than they were designed to perform, but the team took the spike in current as a reminder that one could quit.

"If 'Opportunity' were driving with only five wheels, like Spirit, it probably would never get out of Victoria Crater," said JPL's Bill Nelson, a rover mission manager.

"We also know from experience with Spirit that if 'Opportunity' were to lose the use of a wheel after it is out on the level ground, mobility should not be a problem."