Mars rover finds signs of microbial life (clicky)
Researchers are now trying to determine what produced the patch of nearly pure silica - the main ingredient of window glass.
They believe the deposits came from an ancient hot-spring environment or an environment called a fumarole, in which acidic steam rises through cracks.
On Earth, both of these types of settings teem with microbial life, said rover chief scientist Steve Squyres.
"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," he said.