Magnetized magma intrusions being sources of two lunar magnetic anomalies revealed by 3D inversion

This work is led by Professor Shuo Yao from China University of Geosciences (Beijing), and her doctoral student Hongyi Wang. The authors have applied a 3D amplitude inversion technique from geophysical survey to study the origin of lunar magnetic carriers. This inversion technique was originally proposed and developed at the University of British Columbia’s Geophysical Inversion Facility (UBC-GIF). After careful checks for validity and stability, the authors believed that the amplitude inversion technique is well-suited for the Moon where the direction of the magnetic anomaly is complex, the Curie depth is unknown, and the grid spacing of surface magnetic anomaly is much larger than that on the earth. Assuming the magnetized body being spherical and using the related depth weighting function, the 3D distribution of magnetization could be reconstructed. The boundary of the reconstructed 3D distribution of magnetization can reveal the depth to the bottom of the magnetic carriers. The depth to the bottom is key evidence to determine the origin of the magnetized materials and the Curie temperature.

This work studied a weak magnetic anomaly in Mare Tranquillitatis and a strong famous one called Reiner Gamma in Oceanus Procellarum. The surface magnetic anomaly model is calculated from the orbit measurements of Lunar Prospector and Kaguya spacecraft. The surface ages of the two studied areas are 3.6 and 3.3 billion years just on either side of the famous intensity drop of the ancient magnetic field. According to the results from model test, the boundary of spherical magnetized body could be estimated by the contour at 20% of the maximum magnetization reconstructed by inversion. From the boundary, the authors derived the depths to the bottom and the thickness of the magnetized body. The depth to the bottom of the magnetic carriers under Mare Tranquillitatis is about 50 km, and that under Reiner Gamma is about 30 km. The results indicated that magnetized magma intrusion rather than impact melt layer is the source of the magnetic carriers in lunar crust. The maximum magnetization reconstructed by inversion is about 3.0 A/m under Reiner Gamma. Since the magnetized materials under Reiner Gamma may be older than the surface materials, the intensity of ancient magnetic field deduced by the magnetization is about some microteslas 3.3 billion years ago.

See the article:

Magnetized magma intrusions being sources of a weak and a strong lunar magnetic anomaly revealed by 3D distribution of magnetization

https://doi.org/10.26464/epp2024040