SAN FRANCISCO — The lame wheel on the NASA Mars rover Spirit has proved an invaluable science tool, turning up evidence of a once habitable environment, scientists said Monday.

Meanwhile, images from the Mars Reconnaissance Orbiter have largely unraveled the mystery of geological patterns called "spiders" that appear each spring around the south pole.

The scientists reported their findings here at a meeting of the American Geophysical Union.

The right front wheel of Spirit stopped turning in March 2006. Since then, the rover has been driving backward, dragging the lame wheel along. This May, scientists noticed a bright spot in the trail of overturned dirt.

They turned Spirit around for a closer look, finding high levels of silica, the main ingredient of window glass. They then aimed the rover at a bright-colored rock to break it apart to determine if the silica was just a surface coating, or if the rock was silica all the way through.

The target rock survived the charge, but a neighboring rock cracked open and turned out to be rich in silica.

Spirit's twin, Opportunity, which has been exploring a spot on the other side of Mars, has found evidence of an environment once steeped in acidic groundwater. The silica discovery is the first time that Spirit has seen signs of widespread water in its surroundings, a 90-mile-wide impact crater known as Gusev Crater.

Gusev was chosen as a landing site, because, at least from orbit, it looks as if it were once a lake with what appears to be river channels flowing away from it. However, until now, the rocks that Spirit has examined have largely been volcanic basalt with little hint of water.

"This shows us a side of Mars we haven't seen before, and my guess is that it's more common than we had thought," said Steven W. Squyres, the project scientist for the rovers. "Whichever of those conditions produced it, this concentration of silica is probably the most significant discovery by Spirit for revealing a habitable niche that existed on Mars in the past."