The Case Against an Early Lunar Dynamo Powered by Core Convection

Paleomagnetic analyses of lunar samples indicate that the Moon had a dynamo-generated magnetic field with ~50-μT surface field intensities between 3.85 and 3.56 Ga followed by a period of much lower (≤~5-μT) intensities that persisted beyond 2.5 Ga. However, we determine herein that there is insufficient energy associated with core convection – the process commonly recognized to generate long-lived magnetic fields in planetary bodies – to sustain a lunar dynamo for the duration and intensities indicated. We find that a lunar surface field of ≤1.9 μT could have persisted until 200 Ma, but the ~50-μT paleointensities recorded by lunar samples between 3.85 and 3.56 Ga could not have been sustained by a convective dynamo for more than 28 Myr. Thus, for a continuously-operating, convective dynamo to be consistent with the early lunar paleomagnetic record, either an exotic mechanism or unknown energy source must be primarily responsible for the ancient lunar magnetic field.