The Paleomagnetic Record of the Apollo Samples

Introduction: The burst of early work on the magnetism of the Apollo samples was followed by a quiescent period until recently. It had been thought that the strong magnetization in samples, whose ages ranged from approximately 3.65 to 3.9 Ga, was evidence of a lunar dynamo at that time. New results have yielded evidence for a possible early dynamo at ~4.2 Ga giving surface fields of the order of μT’s [1], a better understanding of Shock Remanent Magnetization [2] and new paleointensity determinations [3]. As recognized [3], to address the question of a lunar dynamo, it is essential to separate more convicingly effects of impact related shock magnetization and other contamination from any possible primary NRM recording a lunar dynamo field. The key to this work will be (1) the determination of the origin of the Natural Remanent Magnetization (NRM) by comparison of its demagnetization characteristics with those of the various possible origins of the magnetization and (2) classical paleointensity methods to obtain estimates of the ancient lunar surface fields.