Scientists studying lunar soil collected by China's Chang'e-6 mission have identified an unexpected form of iron preserved inside microscopic impact glass.
Researchers detected gamma-iron featuring a face-centred cubic crystal structure, an unusual variety of metallic iron that had never previously been confirmed in natural lunar samples. The microscopic particles trapped within the glassy material may hold crucial clues about the ancient magnetic history of the Moon.
The discovery is particularly surprising to planetary scientists because gamma-iron normally exists only at extremely high temperatures. As molten metal cools down under ordinary conditions, it quickly transforms into a different crystalline arrangement.
Preservation inside impact glass
The study was led by Professor Haifeng Du from the High Magnetic Field Laboratory of the Hefei Institutes of Physical Science, an organisation under the Chinese Academy of Sciences. Working alongside fellow researchers, Du examined glassy debris created during meteorite impacts that contained numerous iron nanoparticles.
To inspect the microscopic structures, the team used focused ion beam sample preparation, transmission electron microscopy and detailed chemical analysis. These diagnostic techniques confirmed that gamma-iron was the dominant form of iron present across the two glass samples selected for the investigation. The team published their findings in the journal Proceedings of the National Academy of Sciences.
Impact glass forms on the Moon when meteorites collide with the lunar surface at hypervelocity speeds. Because the Moon has no atmosphere to burn up incoming space rocks, surface impacts generate intense localized heat and pressure that melt local soil and rock into liquid, which then solidifies rapidly in the cold vacuum of space.
Conditions behind the iron transformation
Under ordinary terrestrial and planetary conditions, gamma-iron converts directly into alpha-iron, or alpha-Fe, whenever temperatures drop. The research team suggested that the violent, extreme environment generated during lunar impact events prevented this natural transformation from taking place.
Several factors appear to have combined to preserve the unstable metallic structure on the surface of the Moon. The researchers noted that the rapid cooling of the molten material, the presence of small quantities of carbon and other chemical elements, and the enclosing matrix of glassy material all helped stabilize the unusual crystal form.
While alpha-iron has a body-centred cubic atomic structure and is the common form of iron found at ambient temperatures on Earth, gamma-iron possesses a face-centred cubic lattice. In industrial metallurgy, maintaining gamma-iron at room temperature usually requires specific chemical alloys or rapid quenching methods.
Magnetic vortex in nanoparticles
Beyond identifying the mineral, the researchers investigated whether these metallic particles could retain ancient magnetic records. To examine the magnetic behaviour of individual gamma-iron nanoparticles, the team applied off-axis electron holography, an advanced microscopy method that allows scientists to map magnetic fields at an extraordinarily small scale.
Their measurements showed that relatively large nanoparticles displayed a stable single-vortex magnetic state. Within these particles, the internal magnetization was arranged in a distinct circular pattern. When exposed to an external magnetic field during laboratory testing, the particles maintained a steady and resilient magnetic response.
Dr Long Li of the Hefei Institutes of Physical Science, a co-author of the study, said that this microscopic magnetic fossil could help researchers gain a clearer understanding of the ancient magnetic history of the Moon.
Unlocking ancient lunar magnetism
Today the Moon lacks a global magnetic field generated by an active internal dynamo, but its surface rocks and regolith retain remnants of magnetism acquired billions of years ago. By studying magnetic minerals locked within lunar samples, scientists can piece together how the Moon formed, cooled and evolved over geological time.
The discovery of gamma-iron provides planetary researchers with a new material to evaluate when reconstructing the magnetic evolution of the Moon. Because gamma-iron and alpha-iron crystallize under different physical conditions and display distinct magnetic qualities, the two forms may preserve records from completely different stages of lunar impact events.
The authors concluded that further research will be required to establish how reliably these minerals retain ancient magnetic signals over billions of years and to determine what they can reveal about the history of lunar magnetism.
