Spirit has about two weeks left to reach a sun-facing slope on the northern edge of a plateau known as Home Plate in Gusev Crater. So much dust blankets its solar panels however, that the rover needs to spend a day charging its batteries just to crawl for an hour, project manager John Callas said during a briefing at the American Geophysical Union conference in San Francisco this week.

"We don’t have that much time to drive every day," Callas said.

Global dust storms swept over the planet for two months earlier this year, blotting out 99.5 percent of the direct sunlight. The rovers survived the storms, but Spirit is so coated with debris that its solar power panels are operating at 42 percent of their original capacity.

"It’s almost camouflaged. There’s so much dust on the arrays," Callas said.

Even if Spirit reaches its winter resting spot, surviving its third Martian winter will be tough, Callas said. Engineers estimate Spirit’s power levels will be reduced to 30 percent — about what is needed just to keep its equipment from freezing.

Spirit had 50 percent power during the last winter and 70 percent during its first winter on Mars.

Scientists have complied a list of more drastic measures if power levels fall lower than expected, including turning off the survival heater for the rover’s miniature spectrometer.

That instrument was key to analyzing what principal investigator Steve Squyres with Cornell University said may turn out to be Spirit’s more important find: a patch of nearly pure silica — the main ingredient of window glass — in the Mars soil.

Scientists believe the silica is either a hot spring deposit or the result of acidic steam rising through cracks. On Earth, both settings teem with microbial life.

"This concentration of silica is probably the most significant discovery by Spirit for revealing a habitable niche that existed on Mars in the past," Squyres said.

Spirit’s twin, Opportunity, is exploring the other side of Mars’ equator, having recovered better from the dust storm. Winds cleaned off Opportunity’s dusty solar wing panels and the rover is exploring layers of exposed rock in an area known as Victoria Crater.

The rovers, which landed on Mars in January 2003, already have been operating more than 15 times longer than originally planned.