The Moon is Rusting, and Earth Might Have Something to Do With It

In the last 50 years or so, we have made substantial advancement in space. There are some facts that we know about our close cosmic neighbours—like that there is practically no air nor liquid water on the moon, and that water and oxygen on Mars from its ancient past, combined with iron, is responsible for the rusting that gives the “Red Planet” its moniker.

But now, a recent discovery about our only natural satellite has scientists baffled. Haematite, an oxidised form of iron—or rusted iron—that requires the presence of both air and water to form, was recently discovered on the moon, where there is neither. 

According to a study published today in Science Advances led by Shuai Li of the University of Hawaii, the oxidised iron mineral haematite has been discovered at high latitudes on the Moon. The discovery was made after Li and his team extensively studied data from Moon Mineralogy Mapper (M3) onboard ISRO’s Chandrayaan-1 mission. 

While pristine metallic iron is prevalent on the Moon, highly oxidised iron has not been confirmed in samples brought back from the Apollo missions. Also, hydrogen in solar wind blasts the lunar surface, which is a reducing agent that essentially leads to the opposite of oxidation. The Moon is a terrible environment for haematite to form in, then. So how did rust get on the Moon?

Li supposes that is because of oxygen from the Earth. While the moon lacks an atmosphere, it is home to small amounts of oxygen that can be traced back to the Earth. "We hypothesise that lunar haematite is formed through oxidation of lunar surface iron by the oxygen from the Earth's upper atmosphere that has been continuously blown to the lunar surface by the solar wind when the Moon is in Earth's magnetotail [a part of the Earth’s magnetic field] during the past several billion years," said Li. 

This discovery fits data, as more haematite was found on the Moon’s earth-facing side than its farther side. The Moon has been inching away from Earth for billions of years, so it's also possible that more oxygen hopped across this rift when the two were closer in the ancient past.

“So, Earth's atmospheric oxygen could be the major oxidant to produce haematite,” explains Li. “Water and interplanetary dust impact may also have played critical roles." While most of the Moon is bone dry, Li’s previous discovery of water ice in the Moon’s polar regions in 2018 showed that water ice can be found in shadowed lunar craters on the Moon's far side. The researcher proposes that fast-moving dust particles on the Moon could release these surface-borne water molecules, mixing them with iron in the lunar soil. During just the right moments—namely, when the Moon is shielded from the solar wind and oxygen is present—a rust-inducing chemical reaction could occur.

Earth's magnetotail also has a mediating effect—it also blocks over 99 percent of the solar wind during certain periods of the Moon's orbit. This opens occasional windows during the lunar cycle when rust can form. 

This discovery will reshape our knowledge about the Moon's polar regions, adds Li. The researcher noted that it's an exciting time for lunar science as almost 50 years since the last Apollo landing, the Moon is a major destination again.

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