We have known for decades that the Moon once generated a strong magnetic field — comparable in strength to Earth's — throughout the period from about 4.25 to 3.5 billion years ago. Only in the past few years have we learned that the field didn't simply switch off then: it weakened dramatically but lingered on, faintly, until as recently as 1.5 billion years ago, before disappearing entirely. As Sonia Tikoo explains in the podcast, we don't really understand either how the early field grew so strong or how any field could last so long — and no single mechanism seems able to account for both the intense early epoch and the long, weak tail that followed.

Sonia Tikoo studies the history of magnetic fields on the Moon and other small solar system bodies using paleomagnetism and fundamental rock magnetism.  She is an Assistant Professor in the Department of Geophysics at Stanford University.