Chang'e-6 soil yields first natural lunar gamma iron, a possible magnetic recorder

The Moon no longer has a global magnetic field, but lunar rocks and soils still hold records of ancient magnetism. Studying the magnetic minerals in these samples can therefore help scientists understand how the Moon's magnetic field evolved over time.

Researchers examining impact-glass particles containing metallic iron from Chang'e-6 lunar soil samples have discovered face-centered cubic gamma-Fe, the first time this iron phase has been identified in natural lunar samples.

The study, led by Prof. Du Haifeng of the High Magnetic Field Laboratory at the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences (CAS), was published in the Proceedings of the National Academy of Sciences (PNAS) on September 16.

"This tiny magnetic fossil may help us better understand the Moon's ancient magnetic history," said Dr. Li Long of HFIPS, a member of the team.

The researchers used focused ion beam preparation, transmission electron microscopy and chemical analysis to identify numerous nanoscale iron particles distributed within the glassy material. Further analysis confirmed that some of these particles were face-centered cubic gamma-Fe, the dominant iron phase in the two impact-glass samples examined.

Normally, gamma-Fe is stable only at high temperatures and transforms into alpha-Fe during cooling. The researchers found that the conditions associated with lunar impacts could allow gamma-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 gamma-Fe structure.

Using off-axis electron holography, the team then examined the magnetic structure of individual gamma-Fe nanoparticles. They found that relatively large gamma-Fe particles formed a stable single-vortex magnetic state and kept a stable magnetic response when exposed to an external magnetic field. This suggests that gamma-Fe may serve as a previously unknown recorder of magnetic information in lunar materials.

According to the researchers, the discovery expands the known range of magnetic minerals in lunar samples. Because gamma-Fe and alpha-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 the understanding of ancient lunar magnetism, the researchers said.

CONTACT: Zhao Weiwei, Hefei Institutes of Physical Science, Chinese Academy of Sciences, [email protected]