IN 1974, the spacecraft Mariner 10 measured a magnetic field around the planet Mercury. Although it is 100 times weaker than Earth’s field, the puzzle has always been how such a small planet can be generating any magnetic field at all. Now it seems the answer may lie within the planet’s crust.
Earth’s magnetic field is thought to be produced by the flow of material in its molten core. But Mercury is so small, at just 4879 kilometres across, that its core should have cooled and solidified long ago. Most scientists trying to explain its magnetic field have been searching for reasons why the core might…
