The enigma of lunar magnetism



The results of the Apollo program have given us an intriguing but very inadequate glimpse of the pervasive magnetization that characterizes the lunar crust. Returned sample studies, surface magnetic field investigations, and analyses of orbital measurements have provided useful constraints on the nature of the magnetization, but in retrospect, more questions have been raised than have been answered.

Perhaps the single most important of these questions concerns the origin of the magnetizing field. A simple possibility suggested from the outset is that the moon once possessed an intrinsic global magnetic field which originated in a small, formerly molten iron core. The existence of such a field, if verified, would have extreme implications for our understanding of lunar, and hence planetary, thermal evolution. However, clear evidence for more than a superficially magnetized crust has not emerged from the data analysis. Thermoremanent magnetization itself has been difficult to identify unambiguously in samples of the lunar regolith, partly because of their complicated impact histories. Surface and orbital measurements show magnetic anomaly signatures apparently associated with surficial material, such as basin ejecta, and not with deep-seated structures as expected from slow cooling in the presence of a steady magnetic field. Alternative suggestions for the origin of the magnetizing field have primarily involved local generation mechanisms. Of these, those that employ impact processes to briefly but strongly amplify the weak interplanetary magnetic field seem most reasonable in view of (1) the obvious bombardment history of the moon and (2) the association of some mapped anomalies with basin ejecta. In particular, impacts of cometary bodies, which normally possess large partially ionized atmospheres capable of strongly compressing a weak ambient magnetic field, have been mentioned most in this context.