Testing the Robustness of Parker's Method Against Complexly Magnetized Sources and Implications for Lunar and Planetary Paleopole Determinations

Magnetic field measurements by the Lunar Prospector and Kaguya spacecraft have revealed the presence of numerous magnetic anomalies in the lunar crust. Inverse approaches such as Parker's method have been used to estimate magnetization directions and assign paleopole locations for isolated anomalies. The resulting paleopoles are scattered around the Moon and are not consistently found at the geographic poles as would be expected for an axially dipolar field. Possible explanations for this behavior include a multipolar field, a non-axial dipole field, true polar wander, transient impact-generated fields, or systematic errors in paleopole inversion methods. We investigated the extent to which Parker's method may produce inaccurate paleopole determinations due to non-uniformly magnetized source bodies. We modeled the cooling histories of hypothetical, variably sized, lunar magnetic bodies to investigate whether they could record complex magnetization directions during primary cooling. We found that portions of large (>1.5km thick, >10km wide) cooling bodies such as impact melt sheets and impact ejecta deposits may stay above the Curie temperature of metallic Fe long enough to record reversals of the lunar dynamo field, assuming that the ancient Moon had a similar reversal frequency to the Earth as an upper limit. We found that Parker's method will generally obtain an accurate magnetization direction for some portion of a radially cooled body with a uniform distribution of magnetic carriers, even if one or more magnetic reversals took place during cooling. In our models, Parker's method recovered the magnetization direction of the innermost region of uniform magnetization. However, other initial conditions (e.g., a body experiencing spatially asymmetric cooling, asymmetric distribution of Fe content within the body, or a juxtaposition of multiple bodies spaced close together each having the same magnetization direction) may produce an incorrect direction of magnetization using Parker's method. In sum, our results indicate that lunar magnetic anomalies with high field strengths and limited geographic extents are the most likely to yield accurate magnetization directions.