New Delhi: Iron is one of the most common elements on Earth. On the Moon, however, it can behave in ways that are far less familiar.
Scientists studying soil brought back by China’s Chang’e-6 mission have identified a form of metallic iron that had never before been found in a natural lunar sample. The material, known as γ-Fe or gamma iron, was discovered inside tiny pieces of glass created by ancient impacts on the Moon.
The finding matters because this unusual form of iron appears capable of preserving information about magnetism. Researchers say it could act as a tiny record of the Moon’s ancient magnetic environment, offering another way to investigate a major mystery: when the Moon had a global magnetic field, how strong was it, and what happened to it?
So, what is strange about this iron?
The newly identified material is not a new element. It is a different crystal structure of ordinary iron.
Iron can arrange its atoms in different ways depending on temperature and other conditions. The form scientists found, γ-Fe, has a face-centred cubic structure and is normally stable only at high temperatures. As it cools, it ordinarily transforms into another form, known as α-Fe.
Yet researchers found nanoscale particles of γ-Fe that had remained stable in lunar material at surface temperatures.
How?
The leading explanation involves the violent impacts that have battered the Moon for billions of years. A large impact can generate enormous heat, melt or vaporise rock and then cool the resulting material extremely rapidly. The researchers propose that trace amounts of elements that stabilise the gamma phase, combined with this rapid cooling, allowed the unusual iron structure to become trapped inside impact glass before it could transform.
In other words, the ancient bombardments on the Moon’s surface may have created a kind of natural freezer for an iron structure that normally should not survive.
The widespread cratering on the lunar surface was caused by a massive influx of asteroids, comets, and protoplanetary debris during the chaotic early history of our solar system.
It came from the Moon’s far side
The iron was found in samples collected by Chang’e-6, which in 2024 became the first mission to bring material back from the Moon’s far side.
The spacecraft landed in the South Pole-Aitken Basin, one of the oldest, deepest and largest impact basins on the Moon. It returned about 1.94 kg of material, giving scientists an unusually fresh look at a part of the lunar surface that has never been sampled directly before.
The location is important. The South Pole-Aitken Basin has experienced enormous impacts over lunar history, making its rocks and soil a record of violent events that occurred billions of years ago.
The researchers examined impact-glass fragments and found numerous tiny iron particles embedded inside them. Advanced microscopy and chemical analysis confirmed the presence of γ-Fe. The study, published in Proceedings of the National Academy of Sciences in September, describes this as the first identification of the phase in a natural lunar sample.
The real surprise is magnetic
This is where the discovery gets more interesting.
The Moon does not have a global magnetic field today. But lunar rocks contain remanent magnetisation, meaning they preserve evidence that they were exposed to magnetic fields in the past.
That has led scientists to conclude that the young Moon probably had a dynamo, with movement of electrically conducting material inside its core generating a global magnetic field. Exactly how that dynamo worked, how strong the field was and when it disappeared remain open questions.
The newly discovered γ-Fe could provide another clue.
Using electron holography, researchers found that the iron nanoparticles can form a magnetic vortex, in which the magnetic moments arrange themselves in a circular pattern. The team argues that this makes the particles capable of retaining magnetic information from their formation and subsequent history.
That is why one researcher described the particles as a possible “magnetic fossil” of the Moon’s past.
What could it tell us about the Moon?
Scientists already use lunar rocks as records of ancient magnetism. But different minerals respond to magnetic fields in different ways, and each can preserve information under different conditions.
The discovery of γ-Fe expands the list of possible magnetic recorders.
It could help researchers distinguish between magnetism generated by the Moon’s ancient core dynamo and magnetic signatures produced during major impacts. The γ-Fe and more familiar α-Fe forms also form under different conditions, meaning they could preserve information from different stages of lunar events.
That could eventually help answer a deceptively simple question: what was happening inside the Moon when it still had a magnetic field?
The discovery also fits into a broader change in scientists’ understanding of lunar iron. Earlier work on China’s Chang’e-5 samples found metallic iron, magnetite and unexpectedly high levels of ferric iron produced through impact-related processes. Chang’e-6 has since provided evidence of hematite and maghemite, despite the Moon generally being considered a chemically reducing environment.
So the Moon’s iron story is becoming considerably more complicated.
The latest discovery does not mean scientists have solved the mystery of the lunar magnetic field. But it gives them a new microscopic tool with which to investigate it.
And that is what makes a few almost impossibly small particles of an unusual form of iron potentially important to understanding the history of an entire world.