Soil that China's Chang'e-6 mission brought back from the Moon contains iron in a form never before found in natural lunar samples. Researchers at the Chinese Academy of Sciences describe it in a paper in PNAS, published on September 16.
The find matters because the Moon once had a magnetic field, and scientists want to know how it changed over time. Particles that keep a stable magnetic record could act as witnesses.
Vortices in glass
The team studied two pieces of impact glass, which forms when a meteorite hit melts the soil. Inside the glass they found tiny particles of γ-iron, a form of metallic iron with a different crystal structure from ordinary iron. In both samples it was the main form of iron.
Using electron holography, a method that maps magnetic fields at very small scale, the researchers saw that the larger particles form a single magnetic vortex. That state is stable and responds to outside fields in a consistent way.
"This tiny magnetic fossil may help us better understand the Moon's ancient magnetic history," says Long Li of the Hefei Institutes of Physical Science. The study shows the particles can keep a record. It does not yet read one.
