As the robotic explorers Spirit and Opportunity approached their first anniversaries on Mars, the rovers, working on opposite sides of the planet, were on the move, heading to their next targets. Opportunity, which has been crawling around for six months inside a stadium-sized hole in the ground called Endurance Crater, climbed out this week to begin a new round of studies. Its final task inside Endurance Crater was to make a close inspection of exposed rock layers on a crater wall called Burns Cliff, according to NASA scientists.