How strong are lunar crustal magnetic fields at the surface?: Considerations from a reexamination of the electron reflectometry technique

Abstract

Despite extensive study, we do not yet fully understand the origins of the unique lunar crustal magnetism. The strength of surface fields and their relation to local geology are crucial pieces of the puzzle. However, only a few surface measurements exist, and spacecraft magnetometers cannot detect magnetization with wavelengths much smaller than the orbital altitude. Meanwhile, electron reflectometry (ER) enables a remote measurement of surface fields, but its sensitivity to magnetization with different spatial scales is not well understood. In this paper, we report on new simulations of the ER technique and its sensitivity to magnetic fields produced by simulated crustal magnetization with various strengths and spatial distributions, utilizing full particle tracing simulations and the same data analysis techniques used for space data. We find that the ER technique reliably detects surface fields from magnetization with wavelengths larger than ∼10 km but has increasingly less sensitivity to smaller wavelengths. Since the few surface measurements we have imply very incoherent near‐surface magnetization, this implies that the ER technique may seriously underestimate the strength of lunar fields in some areas. Our results imply that small‐scale impact‐related crustal magnetization may prove even more important than previously thought.