A study of lunar soil samples delivered by the Chinese Chang'e-5 mission linked fragments of oxidized iron to glass particles produced by falling meteorites. Apparently, it is their impact that triggers the reactions that cause iron oxidation and the appearance of "rust" without the participation of oxygen and water.
The samples delivered from the Moon by the Apollo missions' astronauts had previously found compounds of iron (III) oxide, hematite - or simply rust. Even then, this finding has puzzled experts, because the lunar soil has not oxidative but reductive properties, there is virtually no oxygen on the satellite, there is very little water, and all of it is in a solid state.
Subsequent research has only complicated the mystery. Recent imaging conducted by the Indian probe Chandrayaan-1 showed that there is much more hematite on the Moon than could be imagined, especially in the pole regions. Then there were hypotheses explaining the formation of rust by weak "leaks" of oxygen from the Earth's atmosphere. Part of this gas could reach the surface of the satellite, oxidizing the iron.
Another sample of lunar iron (III) fell into the hands of scientists recently, thanks to the work of the Chinese mission Chang'e-5. The apparatus delivered about 1,700 grams of regolith collected in the north of the Ocean of Storms. The area was still volcanically active a couple of billion years ago, and the local surface is much younger than where the Apollo missions visited.
Analysis has shown that this material contains fairly large amounts of hematite. Its accumulation in such a short time can not be explained by the weak impact of terrestrial oxygen. That's why scientists who studied the samples suggested and justified another way of "lunar rust" occurrence.
The authors have studied sample CE5C0400YJFM00408 and found that hematite particles in it are associated with tiny glass fragments. Such glass agglutinates occur during meteorite impact. This suggests that the "lunar rust" is formed during rapid reactions occurring during the impact of micrometeorites. Recall that the Moon has no atmosphere, and such bodies reach its surface much more frequently, bombarding it almost continuously.
Such chemical reactions are called disproportionations. When they occur, the same element acts as both a reducing agent and an oxidizing agent, giving rise to two different compounds containing the element in different oxidation states. Apparently, it is these transformations that the iron contained in the lunar regolith undergoes, which leads to the accumulation of "rust" in it.