Newly identified lunar iron shows a magnetic vortex

The Moon no longer has a global magnetic field like Earth's, but its materials may still hold clues to ancient magnetism. In two pieces of impact-formed glass returned by the Chang’e-6 mission, Pengfei Liu, Haifeng Du and colleagues identified, for the first time in natural lunar samples, a form of metallic iron called gamma iron. Relatively large particles of this form showed magnetism arranged in a vortex: a magnetic orientation that curves around a center. The study appeared in the Proceedings of the National Academy of Sciences; the Chinese Academy of Sciences covered it in an institutional article with no individually named reporter.

In simple terms, an impact heated and melted material at the surface; cooling left small pieces of glass with iron grains trapped inside. The team took microscopic samples from two of those pieces and examined their structure, chemical makeup and magnetism. The glass preserves evidence of the impact process, not a ready-made, dated record of the Moon's entire magnetic field.

Using electron microscopy, which uses electrons to reveal details smaller than ordinary light microscopes can show, the researchers found many nanoscale iron particles—each nanometer is one billionth of a meter. Some had a face-centered cubic structure: their atoms are arranged as if at the corners and the center of each face of a cube. This is the configuration of gamma iron, or γ-Fe, distinct from the more common alpha form in lunar soil.

To see the magnetic arrangement, the team used electron holography, a technique that reconstructs how electrons are deflected by magnetic fields. In the larger gamma-iron particles, the magnetic orientation curved around a center, forming a vortex; they also responded stably to an applied field. The observation shows this material may retain magnetic information, but it does not by itself identify which field was present when each grain formed.

The authors propose that very rapid cooling after the impact, along with small amounts of other elements and the surrounding glass, helped preserve a form of iron that ordinarily does not remain stable when cool. The discovery expands the set of minerals scientists need to consider when reading the Moon's magnetic past. It supports a possible new carrier of a magnetic record, not a completed timeline of the ancient lunar field; measuring more fragments and distinguishing impact magnetism from ambient magnetism will be decisive.