Testing the Fidelity of Paleopole Determinations From Multidirectionally Magnetized Lunar Crustal Anomaly Source Bodies

Abstract

Inversions of satellite observations of the lunar crustal magnetic field do not consistently produce regional magnetization directions consistent with a selenocentric axial dipolar field. We modeled the thermal evolution of anomaly source bodies in the lunar crust and found that large features (impact melt sheets and ejecta deposits) cool heterogeneously, but generally from the outside in. Regions within these bodies may remain above the Curie temperature of metallic iron long enough (10⁶ yrs) that uniform magnetization is unlikely if the lunar dynamo had Earthlike reversal frequency or other orientation changes. We modeled fields produced by non-uniformly magnetized source bodies. Alternate layers within these source bodies may cancel one another's field expression, reducing the satellite altitude field strength by a factor of up to 4 compared to a uniform magnetization. The surface field geometries of multidirectionally magnetized sources have complex features at the length scale of the subsurface magnetizations for each reversal. At 30 km satellite altitude, fields are smoothed and often resemble those of dipolar point sources. We found that Parker's method generally provides accurate magnetization directions for satellite altitude observations of anomalies that record near-antipodal reversals of a selenocentric dipole field. However, inaccurate paleofield directions may be retrieved if the paleofield exhibits non-antipodal dipole motion over time. Smaller lunar magnetic anomalies with high field intensities relative to the local background are the most likely to yield accurate magnetization directions; however, these anomalies are rapidly cooled and may not record the time-averaged behavior of the lunar dynamo.