An episodic high-intensity lunar core dynamo

Magnetizations within lunar rocks indicate that the ancient Moon produced an internally generated magnetic field 1 – 4 . Yet the long-lived field intensities of 40–120 μT inferred for the ancient Moon 1 are unsustainably high and cannot be reconciled with the small lunar core size, requiring three orders of magnitude more energy than deemed possible 5 . Here, we report that a 1 Gyr protracted foundering of cool, dense Ti-rich material initially emplaced at shallow depths can generate the high-intensity magnetic fields (≥50 μT) preserved within returned lunar samples through temporary episodes of elevated superadiabatic heat flow (≥50 mW m − 3 ) out of the lunar core. Our results can simultaneously explain both the high magnitude and large-scale variability in field intensities before at least 3.5 Gyr ago 1 while still permitting lower fields to have persisted thereafter.