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 similar to 50 mu T surface field intensities between 3.85 and 3.56 Ga followed by a period of much lower (<= similar to 5 mu T) 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 mu T could have persisted until 200 Ma, but the similar to 50 mu T 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.