The Mars (searchrover Spirit (search), after a standstill caused by a cold shadow, rolled another 80 feet toward a crater that it should reach within the next three weeks.

Controllers also figured out how to drive its twin, Opportunity (search), so that it won't slip while scooting along a rock formation that's stumped scientists, NASA (search) said Thursday.

"I'm very pleased to report that we have two very busy rovers on the surface of Mars," said Art Thompson, a robotics engineer at NASA's Jet Propulsion Laboratory.

NASA had been unable to send commands to Spirit through its high-gain antenna after the rover's mast shaded the motors used to turn the lollipop-shaped antenna so it faces Earth. The cold forced the motors to stop, leaving the antenna pointing off target.

Eventually, the sun warmed the motors and they resumed working. By then it was too late to begin moving again, forcing a one-day delay before Spirit could resume its 1,122-foot trek to a crater.

Thompson said Spirit is in "outstanding health" and hasn't shown any lingering effects from a computer memory problem that interrupted its mission last month.

On the other side of Mars, Opportunity surveyed most of the rocky inner rim of a small crater. The rover had run into trouble driving up and down the sandy slopes, but engineers studied the problem on Earth using dry sand and learned to command Opportunity to take into account slippage.

New photographs from Opportunity, including microscopic images, reveal the geology of its landing site is more complicated than initially thought.

The composition of the finely layered rock spied by Opportunity remains unknown. Nor do scientists know what the round, BB-sized granules that pepper it might be -- or how they originated.

"I am stumped. I have no idea how they got there," said scientist Wendy Calvin of the University of Nevada, Reno.

Photographs of a rock dubbed "El Capitan" show its layers aren't always parallel to one another, scientists said. That suggests the layers were laid down in a dynamic environment.

Scientists believe volcanic ash blown across the landscape or dust, transported by water or wind, accumulated to form the angled layers visible in the sulfur-rich rock.