The Moon no longer generates a global magnetic field, yet evidence of its ancient magnetism lingers in lunar rocks and soil. By studying the magnetic minerals in these materials, scientists can piece together the Moon’s magnetic history.
Recently, researchers analyzing metallic iron in impact glass from the Chang’e-6 lunar soil discovered face-centered cubic γ-Fe for the first time in natural lunar samples. This notable finding, led by Prof. Haifeng Du, was published in the Proceedings of the National Academy of Sciences (PNAS) on September 16.
Dr. Long Li, a team member, stated that this “tiny magnetic fossil may help us better understand the Moon’s ancient magnetic history.” Using advanced techniques such as focused ion beam preparation and transmission electron microscopy, the team identified nanoscale iron particles throughout the glassy material, with γ-Fe being the dominant form in the samples.
Typically, γ-Fe is stable only at high temperatures, transitioning to α-Fe as it cools. However, the researchers suggest that unique impact conditions on the Moon may enable γ-Fe to persist at the surface, helped by factors like small amounts of carbon, rapid cooling of molten material, and protection from surrounding glass.
Further studies using off-axis electron holography revealed that larger γ-Fe particles can create a stable single-vortex magnetic state, maintaining a consistent magnetic response in external fields. This stability implies that γ-Fe may serve as an important recorder of magnetic information about the Moon’s past.
Overall, the discovery expands the range of magnetic minerals found in lunar samples. Since γ-Fe and α-Fe form under distinct conditions and exhibit different magnetic behaviors, they could provide insights into different stages of lunar impact events. Future research will be crucial to understanding how these minerals can illuminate the Moon’s ancient magnetic field evolution.
