Why is there rust on the moon, and other lunar mysteries

These samples have been inventoried, distributed, scrutinised, studied in great detail and stored in protected environments. These samples have helped us understand the composition of the moon as well as piece together the story of the earth and moon’s shared origin.

The earth and the moon have plenty of elements in common, such as oxygen, silicon, aluminium, iron and calcium, albeit in varied forms. But once in a while, the moon will throw up something unique not yet found anywhere on earth.

In September 2022, the Chinese found one such element. A solid, columnar-shaped, colourless, semi-transparent crystal particle, about with a radius of 10 microns, they are calling Changesite-(Y), after the Chinese moon goddess.

Changesite-(Y) was found in samples collected from the far side of the moon by the Chang’e-5 probe, which touched down on the lunar surface in late 2020. It was China’s first lunar sample-return mission, and the first mission by any country to make it to the far side of the moon.

Changesite-(Y) is the third material found on the moon that hasn’t been found on Earth yet; three other materials first identified as unique to the moon were later found on Earth too.

Changesite-(Y) is particularly interesting because of the presence of a Helium-3 isotope in it, this being a potential fuel for nuclear fusion energy.

The rusty puzzle

The Moon Mineralogy Mapper instrument on board the Chandrayaan-1, supplied by NASA and best-known for having confirmed the presence of water ice on the moon, detected something else of dramatic significance too: hematite, a type of iron oxide or rust.

The presence of iron on the moon’s surface is well-known. But iron oxide only forms in the presence of liquid water and elemental oxygen, neither of which was known to be present on the moon. There is oxygen on the moon, for instance, but it is bound to other compounds in the form of oxides of silica, aluminium, magnesium, etc.

So how did the rust form? “It’s very puzzling. The Moon is a terrible environment for hematite to form in,” Shuai Li of the University of Hawaii, lead author of a 2020 paper on the mystery, writes on the NASA Jet Propulsion Laboratory website.

In his paper, Li posits that the moon does in fact contain trace amounts of elemental oxygen deposited there from time to time by Earth’s magnetotail (the tail formed when solar winds swing past Earth’s magnetic field).

As for the water, the theory is that when fast-moving space dust (which could be carrying water molecules themselves) pelts the lunar surface, it could release water molecules. In just the right moments, when the moon is shielded from the Sun, oxygen is present and water is released and mixes with the iron in the soil, a chemical reaction could occur to form rust.

None of this explains another rusty mystery: the presence of hematite on the far side of the moon, which has no direct contact with Earth’s magnetotail. Where did that oxygen come from? That remains a mystery for another time.