Researchers using NASA’s twin Mars rovers are sorting out two possible origins for one of Spirit’s most important discoveries, while also getting Spirit to a favorable spot for surviving the next Martian winter. The puzzle is what produced a patch of nearly pure silica — the main ingredient of window glass — that Spirit found last May. It could have come from either a 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.