Day to day, it’s easy to lose sight of an astonishing fact: Since 2012, humankind has been driving a nuclear-powered sciencemobile the size of an SUV on another planet. This engineering marvel, NASA’s Mars rover Curiosity, has revolutionized our understanding of the red planet. And thanks to the intrepid rover, we now know that ancient Mars had carbon-based compounds called organic molecules—key raw materials for life as we know it. A new study published in Science on Thursday presents the first conclusive evidence for large organic molecules on the surface of Mars, a pursuit that began with NASA’s Viking landers...