The Mars rover Spirit (search) has developed a problem with one of its six wheels, but NASA officials said Tuesday they believe the robot geologist can continue working.

The right front wheel has become balky, requiring more electrical current to turn, said Mark Adler, mission manager at NASA's Jet Propulsion Laboratory (search) in Pasadena.

Engineers are considering whether to simply continue using the wheel until it fails or drive on five wheels and use the problem wheel only when necessary. The Spirit rover also had a problem last week receiving commands from Earth.

The communication problem was the result of cold temperatures, which caused the rover's receiver to drift out of the frequency range in which the commands were sent. The range was broadened, and the rover received commands after the loss of only one day of work, Adler said.

"So right now what we're seeing is Spirit's gotten a little hard of hearing due to temperature, not due to age, and also she's gotten a little bit of arthritis in one of her joints and that is due to age and that's a real degradation," Adler said.

Spirit and the twin rover Opportunity (search) landed on Mars in January and completed their primary mission. They are now in extended missions.

Spirit is exploring the vast Gusev Crater (search) region and has encountered strange rocks that cascaded down from nearby hills.

Scientists are intrigued with images of a rock dubbed "Pot of Gold," which features round and football-shaped nodules in orderly groupings.

Other photographs revealed what scientists called "rotting rocks." One resembles a decaying loaf of bread, its interior disintegrating and leaving a crust-like shell of material formed by an unknown process.

"It's kind of difficult to imagine that somehow water wasn't involved, but it's too early to tell," said Larry Soderblom, a science team member from the U.S. Geological Survey in Flagstaff, Ariz.

On the opposite side of Mars, Opportunity has entered a deep crater named Endurance to explore stratified rock that could open a window deeper into the geological history of the Red Planet.