Chinese scientists have, for the first time, identified micrometer-sized crystals of hematite and maghemite in lunar soil samples brought back by the Chang'e 6 mission from the moon's far side.
This finding, published in the latest issue of the journal Science Advances, reveals a previously unknown oxidation process on the moon. It provides direct sample evidence for the origin of magnetic anomalies around the South Pole-Aitken Basin and challenges the long-standing view that the lunar surface is entirely in a reduced state with minimal oxidation, according to the China National Space Administration.
The research, conducted by Shandong University, the Institute of Geochemistry of the Chinese Academy of Sciences, and Yunnan University, identified these iron oxides in the Chang'e 6 samples collected from the SPA Basin, the largest and oldest known impact basin in the solar system.
Researchers used multiple analytical techniques, including micro-area electron microscopy, electron energy loss spectroscopy, and Raman spectroscopy. These methods confirmed the crystal structure and unique characteristics of the hematite particles, verifying that they are native to the moon and not contaminants from Earth.
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Unlike Earth, where rust forms through water and oxygen interactions, the moon has long been considered a strongly reducing environment with minimal oxidation. Previous lunar samples lacked evidence of high-valent iron oxides like hematite. The new study shows that lunar "rust" originates from violent impacts: when massive asteroids struck the moon, they created transient high-oxygen-fugacity gas environments. In this extreme setting, iron in troilite minerals was oxidized, releasing sulfur and forming hematite through vapor-phase deposition at 700 to 1,000 C.
A key byproduct of this process is magnetic minerals — magnetite and maghemite — which may serve as the mineral carriers for the magnetic anomalies observed around the SPA Basin. This addresses a long-standing mystery about the moon's magnetic features, as these intermediate products of the impact-induced oxidation could have retained magnetic properties from ancient impact events.
According to researchers, the findings have enriched knowledge about the moon's evolutionary history and provided an important scientific basis for future lunar studies.
The Chang'e 6 robotic mission, the world's first attempt to bring samples from the far side of the moon, was launched in May 2024 from the Wenchang Space Launch Center in Hainan province. The mission successfully concluded after 53 days of maneuvers, retrieving a total of 1,935.3 grams of samples from the far side.
