When the Artemis program returns humans to the lunar surface, astronauts may want to make sure they’re up to date on their tetanus shots. The Moon is covered in rust.
In 2020, data from India’s Chandrayaan-1 orbiter revealed hematite, a kind of rust, scattered around the Moon’s poles. Like other forms of rust, hematite is an iron-rich mineral which forms when iron interacts with water and oxygen. While there’s good evidence for the existence of water at the lunar poles, scientists were puzzled at how rust could form in an ostensibly oxygen-free environment.
A new study published in Geophysical Research Letters offers a suspect: Earth.
How Earth oxygen flows across space to rust the Moon’s surface
We typically think of the Moon as an isolated rock in space, a quarter million miles from here. It’s a barren place, once described by Apollo 11 astronaut Buzz Aldrin as “magnificent desolation.” Despite all appearances, the Moon is seemingly more active and more connected to the Earth than we previously realized.
When scientists first saw rust on the lunar surface, they were confused. Hydrogen from the solar wind is a reducer, adding electrons to the materials it interacts with. To make rust, you need an oxidizer to remove electrons. Earth is protected from the solar wind by the magnetosphere, but it should have made rust production on the Moon impossible. Yet, the hematite was there, begging for an explanation.
The solar wind is the dominant weather phenomenon in the solar system. On any average day, Earth and the Moon (as well as the rest of the solar system) are bathed in charged particles let loose from the Sun. But for about 5 days every month, around the full Moon, Earth acts as a shield, blocking most of the solar wind from reaching the lunar surface. With the solar wind blocked out, a flow of ionized atoms from Earth, known as Earth wind, takes the lead.
During that period, Earth’s magnetosphere (which trails behind it like a windsock) carries oxygen from the upper atmosphere all the way to the lunar surface. Data from Chandrayaan-1 also suggests there’s more hematite on the Moon’s near side, consistent with an Earth oxygen origin.
To test the idea, researchers took crystals of iron-rich minerals similar to those found on the Moon and exposed them to the type of high-energy hydrogen and oxygen ions found in the magnetosphere. When iron interacted with oxygen, it turned into hematite rust. When it reacted with hydrogen, it turned back into iron.
The experiments demonstrate that influence from Earth is sufficient to form hematite at the lunar poles when combined with the water preserved there. Of course, to find out for sure, we’d need to gather samples and bring them back home for analysis. That’s something that could be achieved by the Artemis program, when astronauts return to the Moon for the first time in more than 50 years.
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