After the Mars Global Surveyor (MGS) settled into orbit around the red planet in 1997, a magnetometer on board began sending back measurements that have puzzled planetary scientists ever since. A section of the martian crust appeared to consist of long 'stripes' of iron-bearing minerals permanently magnetized with alternating orientations. Clearly, an ancient dynamo imprinted its field in the rock during the planet's early history. But why the stripes?