Scientists and engineers celebrated when they saw the first pictures NASA’s Opportunity sent from the rim of a stadium- sized crater that the rover reached after a six-week trek across martian flatlands. Multiple layers of exposed bedrock line much of the inner slope of the impact crater informally called “Endurance.” Such layers and their thicknesses may reveal what the environment on Mars was like before the salty standing body of water evaporated to produce the telltale rocks that were explored in the tiny “Eagle” Crater. That