Why Is the Moon Covered in Rust? Even Scientists Are Stumped by This Metal Mystery

Without oxygen and water, how is the substance

moon rust
The blue areas in this composite image from the Moon Mineralogy Mapper (M3) aboard the Indian Space Research Organization's Chandrayaan-1 orbiter show water concentrated at the Moon's poles. Homing in on the spectra of rocks there, researchers found signs of hematite, a form of rust. ISRO/NASA/JPL-Caltech/Brown University/USGS

Puzzling astronomers, the moon appears to have rusty patches on its surface—despite lacking two essential, rust-inducing ingredients: water and oxygen.

According to a new paper published in the journal Science Advances, scientists from the University of Hawaiʻi at Manoa made the discovery after reviewing data collected from Chandrayaan-1’s Moon Mineralogy Mapper instrument (also known as M3), an instrument built by NASA’s Jet Propulsion Laboratory at California State University that was onboard during a Indian Space Research Organisation’s (ISRO) mission. University of Hawaiʻi geologist Shuai Li, the paper’s lead author, says he had been studying imagery captured during the mission and noticed spectra, or light being reflected off of the moon’s surface, present at the lunar poles, revealing that these areas are comprised of compositions that are different from other expanses of the moon.

In a news release published by the Jet Propulsion Laboratory, the team, “homed in on these polar spectra.” While it’s not uncommon for the moon to be covered in iron-rich rocks, what surprised them the most was how closely the material resembled the mineral hematite. Hematite—which is a form of iron oxide, a reddish-brown substance we know as rust—occurs when iron becomes exposed to oxygen and water, which prompts the original question: Without ample supplies of water and oxygen, how did it get there?

“It's very puzzling,” Li said in a statement to NASA. “The moon is a terrible environment for hematite to form in.”

The team's research is part of a larger study that began in 2018, resulting in the discovery of water-ice on the moon’s polar regions, according to a University of Hawaiʻi press release.

To confirm that hematite was in fact present, the team recruited Jet Propulsion Laboratory researchers Abigail Fraeman and Vivian Sun to give their findings a second look.

“At first, I totally didn't believe it,” Fraeman says in a NASA release. “It shouldn't exist based on the conditions present on the moon. But since we discovered water on the moon, people have been speculating that there could be a greater variety of minerals than we realize if that water had reacted with rocks.”

So, what exactly is causing the moon to rust like an old jalopy sitting in a junkyard? The scientists have come up with a few ideas, but the one that makes the most sense to them is all thanks to Earth. They found that the surfaces of the moon most affected by oxidation are the ones facing our planet. The scientists theorize that oxygen could be making the 239,000-mile journey onboard Earth's magnetotail, a particle-packed magnetic wake trailing our planet like a windsock.

“Our hypothesis is that lunar hematite 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 solar wind when the Moon is in Earth's magnetotail during the past several billion years,” he says.

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