Somewhere inside a speck of glass from the far side of the Moon, a particle of iron too small to see without a powerful electron microscope may be holding a message from the distant past.

Scientists studying soil brought back by China's Chang'e-6 mission say that this tiny particle could act as a microscopic fossil of the Moon's long-lost magnetic field. It is a find that has not been made in natural lunar samples before.

A Moon that has lost its magnetism

Earth has a global magnetic field. The Moon no longer generates one. But traces of its ancient magnetism remain locked inside lunar rocks and soil, according to an account of the work from the Chinese Academy of Sciences, published by ScienceDaily.

By studying the magnetic minerals preserved in those materials, scientists can reconstruct how the Moon's magnetic environment changed over time. That is why each new clue matters.

A rare kind of iron

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

The team examined impact glass in Chang'e-6 lunar soil. This is glassy material formed when molten rock created by an impact cools and hardens. According to the paper's abstract, the fragments came from the South Pole-Aitken Basin.

Using focused ion beam preparation, transmission electron microscopy and chemical analysis, the researchers found numerous nanoscale iron particles embedded throughout the glass. A nanoscale particle is measured in billionths of a metre.

Some of those particles turned out to be a form of iron called gamma-iron, or γ-Fe, which has a "face-centred cubic" arrangement of atoms. This was the first time it had been identified in natural lunar samples. In the two impact-glass samples studied, it was the dominant form of iron.

Why that is surprising

Under normal conditions, gamma-iron is stable only at high temperatures. As the material cools, it typically changes into another form known as alpha-iron (α-Fe).

The researchers found evidence that the unusual conditions created by impacts on the Moon may allow gamma-iron to survive at the lunar surface. They proposed several factors that could help lock it in place: small amounts of carbon and other elements, the rapid cooling of molten material during an impact, and protection from the surrounding glass.

A stable magnetic signature

The team also used a technique called off-axis electron holography to study the magnetic behaviour of individual gamma-iron nanoparticles. They found that relatively large particles could form a stable "single-vortex" magnetic state, in which the magnetism circles around inside the particle.

The particles also kept a consistent magnetic response when exposed to an external magnetic field. According to the report, that stability suggests gamma-iron could act as a previously unrecognised recorder of magnetic information in lunar material, preserving clues about conditions in the Moon's past.

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

What comes next

The discovery broadens the known variety of magnetic minerals found in lunar samples. Because gamma-iron and alpha-iron form under different conditions and behave differently magnetically, each could potentially record information from separate stages of lunar impact events.

In the paper's abstract, the authors say their findings provide a basis for understanding how the lunar core dynamo, the process deep inside the Moon that once generated its magnetic field, evolved over time.

It is early days, though. Future research will be needed to determine exactly how much these minerals can reveal about the Moon's ancient magnetic field and how that field changed.

For now, the finding is a reminder of how much can be hidden in a pinch of moon dust. The study, "Magnetic vortex state of natural lunar γ-Fe", appears in PNAS volume 123, issue 38.