Constraints on the spatial distribution of lunar crustal magnetic sources from orbital magnetic field data

Abstract

We know from spacecraft measurements that the crust of the Moon is heterogeneously magnetized. With the exception of a few magnetic anomalies related to craters and swirls, the origin of most of the lunar magnetic anomalies is not understood. Here we evaluate the performance of an inversion methodology, initially conceived to infer the direction of the underlying magnetization from magnetic field measurements, commonly referred to as Parker's method, to elucidate the origin of the magnetic sources by constraining the location and geometry of the underlying magnetization. We assess the performance of the method by conducting a variety of tests, using synthetic magnetized bodies of different geometries. These have been chosen such that they mimic  the main geological structures potentially magnetized within the lunar crust. Our test results show that the Parker method successfully localizes and delineates the two-dimensional surface projection of subsurface three-dimensional magnetized bodies, when certain conditions are fulfilled. In particular, the magnetization should be close to unidirectional, and the magnetic field data should have a higher spatial resolution than the smallest dimension of the magnetized body as well as a high signal-to-noise ratio. As an additional evaluation test, we applied this inversion methodology to two lunar magnetic anomalies that are associated with visible geological features, the Mendel-Rydberg impact basin and the Reiner Gamma swirl. For Mendel-Rydberg,  our analysis shows that the strongest magnetic sources are located within the basin's inner ring in agreement with previous studies showing that during an impact, the crust inside the newly formed crater undergoes demagnetization and potentially remagnetization (if an ambient magnetic field is present). For Reiner Gamma, we found the strongest magnetic sources form a narrow dike-like body that emanates from the center of the Marius Hills volcanic complex. The reason that only one such dike emanating from Marius Hills is magnetized could be linked to an atypical iron-metal composition or to the lunar ambient magnetic field being only intermittently present. Future applications of this method can focus on constraining the origin of the many lunar magnetic anomalies that are not associated with visible geological features.