The evolution of the lunar dynamo is crucial for understanding the Moon's deep interior structure, thermal history, and surface environment. A recent study by Chinese scientists conducted paleomagnetic analyses on basalts returned by the Chang'e-6 mission and revealed a significant reinforcement of the lunar dynamo approximately 2.8 billion years ago (Ga).

This groundbreaking work was published in Nature.

Previous paleomagnetic studies of nearside lunar samples have established a general timeline for the evolution of the Moon's magnetic field. However, limited spatial and temporal constraints have left the evolutionary trajectory of the lunar dynamo unclear.

The Chang'e-6 mission, which returned the first farside basalt samples dated to approximately 2.8 Ga, provides a unique opportunity to fill this critical gap in our understanding of the lunar dynamo's spatiotemporal evolution.

Led by Prof. ZHU Rixiang at the Institute of Geology and Geophysics of the Chinese Academy of Sciences (CAS), Associate Professor CAI Shuhui and her colleagues measured the ancient magnetic field strength from the Chang'e-6 basalts, obtaining values ranging from approximately 5 to 21 microteslas (µT).

These findings revealed an unexpected resurgence in field strength at 2.8 Ga, following a sharp decline around 3.1 Ga. This challenges the prevailing hypothesis that the lunar dynamo entered a low-energy state after 3 Ga and remained in this condition until its cessation.

The researchers proposed that the lunar magnetic field during this period may have been driven by a basal magma ocean and/or powered by precessional forces. Additional mechanisms, such as core crystallization, may have also a role.

These findings suggest that the Moon's deep interior remained hot and geologically active well into its mid-early history.

This study represents the first ever paleomagnetic data obtained from the Moon's farside, providing critical insights into the intermediate stages of the lunar dynamo's evolution. By synthesizing this data with existing findings, the researchers suggested significant fluctuations in the lunar magnetic field between 3.5 and 2.8 Ga, indicating a highly unstable dynamo during this period.

These results offer valuable guidance for future lunar exploration missions, including the search for potential magnetic reversals.

The research was conducted in collaboration with the National Astronomical Observatories, CAS. Lunar samples were supplied by the China National Space Administration, and the study was supported by the National Natural Science Foundation of China, the Strategic Priority Research Program (Category B) of CAS, and the Key Research Program of the Institute of Geology and Geophysics, CAS.

Figure 1: The ancient strength of the lunar magnetic field, as recorded by Chang'e-6 basalt clasts, demonstrates the presence of a relatively active lunar dynamo at 2.8 Ga. (Image by ZHANG Min, QI Kaixian, and SHI Pingyuan).