Moon Rust Baffles Scientists: 'I Totally Didn't Believe It. It Shouldn't Exist'

A type of rust that requires oxygen and water to form has been discovered on the lunar surface, puzzling scientists who discovered it as the moon has neither oxygen or liquid water.

Researchers led by Shuai Li, from the University of Hawaii, were studying water samples from the moon that were taken in 2008 as part of Indian Space Research Organization's Chandrayaan-1 mission. During this research, they discovered the presence of hematite, a type of rust that normally requires water and oxygen to form.

Li had noticed minerals at the Moon's pools were different to the rest of the lunar surface. After examining these minerals closer, he discovered signatures of hematite. "It's very puzzling," Li said in a statement. "The Moon is a terrible environment for hematite to form in."

He asked Abigail Fraeman and Vivian Sun from NASA's Jet Propulsion Laboratory to look at the data to see if they agreed Li had found hematite. "At first, I totally didn't believe it," Fraeman said. "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."

Further analysis confirmed Li's initial suspicions, and the researchers set about explaining how the rust had formed on the moon. Their study, published in Science Advances, suggest the range of conditions necessary for hematite to exist on the moon.

While only in trace amounts, the moon does have some oxygen coming from Earth's magnetic field, which trails behind the planet as it moves. This fits with the findings that more hematite is found on the moon's Earth-facing side, as this is the side that gets hit with tiny amounts of oxygen.

Earth's magnetic field also protects this region of the moon from the solar wind—a stream of charged particles coming from the sun that would normally prevent oxidation from taking place. Hydrogen is a reducer so adds electrons to materials it hits. For rust to form, an oxidizer that removes electrons is required. By occasionally protecting parts of the Moon from the hydrogen-rich solar wind, hematite is able to form.

Finally while water is known to exist on the moon as ice, it is normally found in the lunar craters on the far side. Li and colleagues say the water involved in the formation of rust likely comes from water molecules released by dust particles that hit the lunar surface and release them, with heat from the impact increasing the rate of oxidation. They also say water molecules could be carried in the dust particles.

These conditions combined would account for the presence of hematite, they conclude.

The team says NASA's 2024 Artemis mission to the moon should help shed more light on the hematite and how it got there. One of the instruments being created for the program, the High-resolution Volatiles and Minerals Moon Mapper (HVM3), will map water ice and should provide new details on the moon rust.

Image of the moon. Researchers were puzzled by the presence of a type of rust at the moon's poles. NASA Goddard