When the Apollo astronauts returned from the Moon, they brought a puzzle back with them. Some of the rocks they collected were so strongly magnetic, it implied that the Moon’s magnetic field must have been stronger than the Earth’s when the rocks formed 3.9‒3.5 billion years ago. “That doesn’t make any sense with the physics that we understand about how planets generate magnetic fields,” says Claire Nichols, a planetary geologist at the University of Oxford, UK. Nichols and her Oxford colleagues Jon Wade and Simon N Stephenson have now identified a possible explanation. The key, they say, lies in the rocks’ composition, which happens to provide ideal spacecraft landing sites, leading to sampling bias. “It was a proper kind of Eureka moment,” Nichols says. ### The lunar dynamo The magnetic fields of planets and moons stem from convective currents in their largely iron cores. Scientists expect that objects with smaller cores, such as the Moon, will have lower magnetic field strengths.…