Where is the magnetized material located on the lunar surface?

Magnetometers onboard spacecraft have detected magnetic field signals originating from the lunar crust. These signals are known as magnetic anomalies and are generated by rocks that are permanently magnetized. Lunar magnetic anomalies are distributed heterogeneously over the lunar surface and the geological processes that gave rise to them is under debate. Thus, the Moon's geological history can be further assessed by inferring the shape of the underlying magnetized material. Up to now, these sources were not fully described for such geological assessment studies.

Joana Oliveira and her colleagues, in a recently published work, evaluated the ability of a methodology up to now used to infer the direction of the magnetization, called the method of Parker, to recover the location and shape of the magnetized material by using orbital magnetic field data only.

Lunar magnetic field map at 30 km altitude using Tsunakawa et al. 2015 model.

Through a series of tests, the authors of this study have shown that the Parker’s method can constrain the shape of the source of a magnetic anomaly, provided that the respective part of the crust is magnetized along a common direction.

"We tried to take it a step further to crack the unidirectional assumption by testing complex bodies with different directions, and we were surprised by how this method was still able to recover most of the magnetized structure”, Joana said.

The authors also applied the method to two lunar magnetic anomalies related to two visible geological features an impact crater and an albedo anomaly, also know by swirls. Results show that the inferred shape and location of the magnetized material are in good agreement with the associated geological features and suggest that one originated by an impact event and the other by volcanic activity.

Parker inversion results for the Mendal-Rydberg basin. The magnetized material (related to the dipole moments) is correlated with the inner depression in blue color of the topography map, despite the magnetic field signal being shifted to the southwest from the center of the basin. Figure adapted from Oliveira et al. 2024.

Future applications can focus on constraining the origin of the many lunar magnetic anomalies that are not associated with visible geological features.

Joana S. Oliveira is an archive scientist working for the European Space Agency (ESA) JUICE and Heliophysics missions, with a background in planetary sciences. She is interested in learning about the history of rocky planets through their magnetic field signals.


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