Strange magnetic anomalies on the moon can finally be explained

Small, frozen and silent, the moon has an amazing distribution of magnetic fingerprints across its dusty surface, and it’s not all easy to explain.

A new study led by geoscientist Zhuang Guo of the Institute of Geochemistry of the Chinese Academy of Sciences could help us better understand unusually strong magnetic field readings that don’t match other characteristics of the moon.

Guo’s team analyzed the lunar soil It was returned to Earth in December 2020 by the Chang’e 5 probe, revealing particles of a mineral known as magnetite, which is rarely seen in samples of lunar dust.

“Lunar magnetic anomalies have been a mystery since the time of Apollo,” Guo and colleagues wrote in their published paper.

“Therefore, a deep understanding of the formation mechanism and distribution characteristics of magnetite on the Moon can provide a new perspective to explain the genesis of magnetic anomalies in the lunar crust.”

Magnetite, a strongly magnetic iron ore, is found in near-microscopically spherical iron sulfide grains that resemble molten droplets. Further thermodynamic modeling indicates that the magnetite in these grains is the result of large impacts on the lunar surface.

For planetary scientists, the presence of magnetite is crucial: it can be used to track magnetic fields throughout history, as well as identify possible indicators of life, two of the hottest research discussions about any planet or moon.

Based on their findings, the researchers believe that magnetite may be widely distributed in the best lunar soils as well. Unexplained magnetic anomalies on the Moon may now be easier to understand if our modeling is modified to fit the conclusions of this new study.

Unlike soil on Earth, the lunar regolith is extremely low, which means it has an excess of electrons thanks to the constant bombardment of protons streaming in from the sun. This condition makes it difficult for iron to combine with oxygen to form ores like they do down here.

This does not mean that it cannot happen. Tiny grains of magnetite had previously been found in lunar dust, but those studies suggested that magnetite formed at relatively low temperatures — not under the high-pressure, high-temperature conditions of a collision with the lunar surface, as this new work suggests.

“The features of the iron sulfide grain morphology and oxygen distribution indicate that a gaseous melt-phase interaction occurred during high-impact events,” the researchers explain.

Previous research has suggested that meteorites could have injected ferromagnetic material into the lunar surface upon impact, with the projectiles explaining at least some of the magnetic anomalies near the impact sites.

This new study goes further, finding that the fury of these impacts may also have turned the material into sub-microscopic magnetite, making it “a significant source of ferromagnetic material on the lunar surface.”

In other words, the results indicate that the mineral is present widely across the lunar surface, which in turn changes our understanding of how the moon has evolved over time.

The team suggests that the current magnetization of the lunar surface, along with the presence of these minerals, could help explain how the impacts of large bodies led to a lunar magnetic field.

“These formation conditions lead to a matching relationship between the distribution of magnetic anomalies in the lunar crust and the distant ejecta of large impacts,” the researchers conclude.

Research published in Nature Communications.

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