The Paradox of Lunar Magnetism.

Models of lunar magnetism need to explain the following observations. (1) The Natural Remanent Magnetization (NRM) in some of the returned Apollo samples, with ages from about 3.85Ae to 3.65Ae, is stronger than in any older, or younger samples. (2) Magnetic anomalies are found antipodal to the young basins of a similar age, i.e. Orientale (~3.82Ae), Imbrium (~3.83Ae), Serenitatis, (Early Imbrium ~3.75-3.85Ae) and Crisium (Early Imbrium ~3.75-3.85Ae). (3) Major magnetic anomalies are not found over these same basins, although there are minor anomalies associated with central uplifts, or uplifted rings. (4) Strong fields are found over the Cayley Formation and similar material with scale lengths of coherence of the order of kms, or less. Hood's model of magnetization at the antipodes of giant basins in the plasma fields generated by the impacts explains observation (2) and may account for observation (4). It is consistent with observation (3) given that terrestrial craters, such as the Vredefort, do not have strong anomalies over the basin, despite exhibiting strong local magnetizations in shocked material (Carpozen et al., 2005). The model does not account readily for observation (1), which has frequently been taken to require the presence of a lunar dynamo. However, this presents a dilemma. If there were a lunar dynamo at this time, why were the inevitable basin melt sheets not magnetized in the field to give anomalies over the basins. A possible explanation of observation (1) may be that the melt rocks and the few basalts that show the stronger magnetizations were magnetized in the highly variable local fields of the shocked and strongly magnetized material and not in a dynamo field. These local fields would also account for the minor anomalies seen over central uplifts and uplifted rings in some craters. The critical knowledge, which we lack to clarify lunar magnetism, is the detailed form and coherence scale of lunar crustal magnetization.