A research team led by Prof. DU Haifeng from the High Magnetic Field Laboratory of the Hefei Institutes of Physical Science, Chinese Academy of Sciences, and their collaborators have discovered face-centered cubic γ-Fe in lunar soil returned by the Chang' e-6 mission. This is the first time that this iron phase has been identified in natural lunar samples.
"This tiny magnetic fossil may help us better understand the Moon' s ancient magnetic history," said Dr. LI Long, a member of the team.
The study was published online in Proceedings of the National Academy of Sciences (PNAS).
Although the Moon no longer has a global magnetic field, lunar rocks and soils still contain records of ancient magnetism. Studying the magnetic minerals in these samples can help scientists understand how the Moon' s magnetic field evolved over time.
In this study, the team studied impact-glass particles containing metallic iron from Chang' e-6 lunar soil samples.
Using focused ion beam preparation, transmission electron microscopy and chemical analysis, they found many nanoscale iron particles distributed inside the glassy material. Further analysis confirmed that some of these particles were face-centered cubic γ-Fe, which was the dominant iron phase in the two impact-glass samples examined.
Normally, γ-Fe is stable only at high temperatures and transforms into α-Fe during cooling. The researchers found that the unique conditions associated with lunar impacts could allow γ-Fe to be preserved under lunar surface conditions.. They suggested that trace amounts of carbon and other elements, rapid cooling of impact-generated melts, and the surrounding glassy matrix may have helped preserve the γ-Fe structure.
Using off-axis electron holography, the team further examined the magnetic structure of individual γ-Fe nanoparticles. They found that relatively large γ-Fe particles formed a stable single-vortex magnetic state and maintained a stable magnetic response when exposed to an external magnetic field. This suggests that γ-Fe may serve as a previously unknown recorder of magnetic information in lunar materials.
The discovery expands the known range of magnetic minerals in lunar samples. Since γ-Fe and α-Fe form under different conditions and have different magnetic properties, they may preserve information from different stages of lunar impacts. Future studies will help clarify how these minerals contribute to our understanding of ancient lunar magnetism.
Lunar topography and magnetic anomalies. (Image by LI Long)