Perhaps the biggest surprise resulting from missions to Earth’s Moon has been the discovery that all lunar rocks are magnetized to some degree. This is especially surprising since surface and orbiting satellite data find no current global field (<1nT, or a magnetic dipole moment more than five orders smaller than Earth’s). The strongest magnetizations in returned samples approach 1% of terrestrial mid-ocean ridge basalt values, although most are much weaker. Lunar paleointensities have been interpreted to indicate terrestrial-strength magnetizing fields (100’s μT) at 3.6 to 4.3 Ga, and there is new evidence for a dynamo at only 0.1 – 2.5 Ga. But the debate continues on the validity of the paleointensities. In short, the origin(s?) of lunar magnetism remains unsolved after its discovery more than four decades ago.
Fundamental planetary science questions arise from this ongoing saga. For example, what were the characteristics of the magnetizing field(s)? How did the rocks become magnetized? What is the source(s) of the numerous anomalies? And, what (if any) are the implications of lunar magnetism for the origin and evolution of the Earth-Moon system, including the Earth’s dynamo? This talk surveys hypotheses and evidence for a lunar core and dynamo action therein, briefly reviews lunar sample paleomagnetism, discusses possible magnetic effects associated with hypervelocity impacts, and touches on lunar formation and how its orbital characteristics may have affected terrestrial thermal evolution and the geodynamo.