By a stroke of luck, the Mars rover Opportunity landed literally a stone’s throw away from a scientific gold mine. Its primary task now will be to explore a rock outcropping that appears to be Martian “bedrock,” primeval rock that has been there since it formed. In images transmitted to Earth, the outcropping resembles the horizontal layers of rock familiar to any terrestrial hiker who, wandering through a valley, spies bedrock exposed by erosion or landslides.