Evolution of the Lunar Magnetic Field

The Moon currently lacks a global magnetic field; however, both paleomagnetic analyses of lunar rock samples and orbital magnetic measurements indicate that it once possessed a core dynamo. Magnetic field measurements of some datable impact basins suggest that the lunar core dynamo persists to the Nectarian period (~3.9 to 3.8 billion years ago Ga). Investigations of the Apollo samples using modern methods demonstrate that the field overall was active between 4.25 and 1.92 Ga. During the period prior to 3.56 Ga, the field was sometimes comparable to Earth's but subsequently declined dramatically and ultimately ceased. Several hypotheses have been proposed to explain the dynamo generation and duration. Thermal convection in the lunar core could have provided dynamo energy for the first several hundred million years while core crystallization could have sustained the dynamo for up to 1.5 Ga. Other mechanisms, such as mantle and/or inner core precession, changes in the rotation rate of the lunar mantle caused by impacts, and a basal magma ocean, also hold the potential to power the dynamo during some time of lunar evolutionary history. Impacts related to plasmas are believed to be insufficient for crustal magnetization though they can amplify the pre-existing magnetic field before the impacts. This paper summarizes and reviews the current understanding of lunar magnetic field evolution, including paleomagnetic studies that quantify the timing of the lunar surface strength, global crustal magnetization features derived from recent global magnetic field models based on orbital magnetic measurements, and various models explaining the powering of a lunar dynamo, which can account for most observations. Finally, we propose the outstanding questions and offer guidance for future lunar exploration such as the Chang'E series and lunar scientific observatories.