Contrary to what was thought to be a scientific impossibility, scientists detected rust—a product that requires oxygen, water, and oxidative conditions—on the surface of the Moon, a famously oxygen-poor, liquid water-less, and reducing environment that prohibits oxidation.
The scientists speculated that the oxygen needed for the reaction that forms rust had been carried to the poles of the Moon by wind from the Earth, and a paper detailing the discovery was published on Wednesday in the journal Science Advances.
Rust is the reddish-brown material left behind when iron atoms react with oxygen and water in what is known as an oxidizing, or electron-losing, reaction. The Moon’s very thin atmosphere does not trap much oxygen, and solar winds constantly blast the surface of the Moon with charged hydrogen, causing it to have highly reducing, or electron-gaining, conditions.
So while rust is common on Earth, its discovery on the Moon caught researchers by surprise.
“I don't think anyone expected this on the Moon's surface,” said Shaui Li, the first author of the paper and a researcher at the University of Hawaiʻi at Mānoa. “This is basic chemistry—we all know that the lunar surface is highly reducing, so there is no reason you would be able to see a high-valence iron like hematite.”
By comparing reflectance data collected by the Indian Chandrayaan-1 mission to pure samples of rust, Li’s group identified material at latitudes above 60 degrees on the Moon’s surface as hematite, or iron (III) oxide. Li said that the comparison was fairly straightforward, and he is very confident that the reflectance spectra were of hematite.
“The methodology was quite simple,” he said. “When we compared the data to measurements of pure hematite under lab conditions, we could see that the spectra were so similar to each other, and so different from other minerals.”
The grouping of rust at the poles reminded Li of his own discovery of water ice in the polar regions of the Moon; unlike that 2018 finding, however, the hematite didn’t collect uniformly around the poles but were distributed more densely and sparsely at different directions. One candidate that could oxidize iron and provide the rust’s irregular distribution was Earth wind, which transports oxygen to the Moon and shields it from solar wind for a few days in its cycle.
This discovery serves as a reminder of how intimately connected the Earth and Moon are, even though they are over 200,000 miles apart, Li said. Additionally, the Earth wind has likely transported material to the Moon that is billions of years old—sampling this material in locations demarcated by the rust could help scientists understand the conditions of early Earth.
Finally, the research could even affect space exploration and colonization of the Moon. Areas of the Moon’s surface that have hematite might also have higher levels of oxygen and water, making these regions comparably better than the rest of the Moon for agriculture. The rust could even help lunar miners, Li said.
“In the future, if you want to use the resources on the surface of the Moon, this type of iron could be one type that humans could use.”