The moon is getting rusty.
Scientists had the same reaction you probably did when they reached this conclusion. It shouldn't be possible -- after all, there's no oxygen on the moon, one of the two essential elements to create rust, the other being water.
But the evidence was there.
India's lunar probe, Chandrayaan-1, orbited the moon in 2008, gathering data that has led to numerous discoveries over the years -- including the revelation that there are water molecules on its surface. The probe also carried an instrument built by NASA that could analyze the moon's mineral composition.
When researchers at NASA and the Hawai'i Institute of Geophysics and Planetology analyzed the data recently, they were stunned to find hints of hematite, a form of iron oxide known as rust. There are plenty of iron-rich rocks on the moon -- but rust is only produced when iron is exposed to oxygen and water.
"At first, I totally didn't believe it. It shouldn't exist based on the conditions present on the Moon," said Abigail Fraeman, a scientist at NASA's Jet Propulsion Laboratory, in a press release.
Not only is there no air on the moon, but it is flooded with hydrogen that flows from the sun, carried by solar wind. Rust is produced when oxygen removes electrons from iron; hydrogen does the opposite by adding electrons, which means it's all the harder for rust to form on the hydrogen-rich moon.
"It's very puzzling," said Shuai Li of the University of Hawaii, the lead author of the study, which was published on Wednesday in the journal Science Advances. "The Moon is a terrible environment for hematite to form in."
After months of research, Li and the NASA scientists think they've cracked it -- and the answer to the mystery lies in our very own planet.
Here's their theory
One major clue was the rust was more concentrated on the side of the moon that faces Earth -- suggesting it was somehow linked to our planet.
Earth is encompassed in a magnetic field, and solar wind stretches this bubble to create a long magnetic tail in the downwind direction. The moon enters this tail three days before it's full, and it takes six days to cross the tail and exit on the other side.
During these six days, Earth's magnetic tail covers the moon's surface with electrons, and all sorts of strange things can happen. Dust particles on the moon's surface might float off the ground, and moon dust might fly into a dust storm, according to NASA.
And, Li speculated, oxygen from the Earth travels on this magnetic tail to land on the moon, where it interacts with lunar water molecules to create rust.
The magnetic tail also blocks nearly all solar wind during the full moon -- meaning the moon is temporarily shielded from the blast of hydrogen, opening a window for rust to form.
"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," said Li in a press release by the University of Hawaii.
"This discovery will reshape our knowledge about the Moon's polar regions," he added. "Earth may have played an important role on the evolution of the Moon's surface."
This theory could also explain why rust has been found on other airless bodies like asteroids. "It could be that little bits of water and the impact of dust particles are allowing iron in these bodies to rust," Fraeman said.
But some questions remain unanswered -- for instance, though most of the rust was found on the nearside of the moon, some smaller traces were also detected on its far side, where the Earth's oxygen shouldn't be able to reach. It's also still unclear how exactly water on the moon is interacting with rock.
To gather more data for these unsolved mysteries, NASA is building a new version of the instrument that collected all this existing data about the moon's mineral composition. One of these features will be able to map water ice on the moon's craters -- and "may be able to reveal new details about hematite as well," said the NASA release.