We know that the Moon has ice water on it, but not liquid water, and we know that it has practically no air — two fundamentals when it comes to oxidization on Earth. So why has hematite, a common iron oxide on Earth, been discovered on the Moon?
The discovery has scientists puzzled, though they think they it might have something to do with the Earth's magnetic field.
A lunar quandary
A new study was recently published in Science Advances, which presented an analysis of data from the Indian Space Research Organization's Chandrayaan-1 orbiter. In it, the researchers claimed to have discovered that the rock at the moon's poles has a different composition compared with other areas of our celestial neighbor.
The lead author of the study, planetary scientist Shuai Li of the University of Hawaii at Manoa, decided to take a closer look and discovered hematite, which is essentially rust.
"It's very puzzling," Shuai Li said in a NASA press release. "The Moon is a terrible environment for haematite to form in."
The Moon is continuously bombarded by a stream of hydrogen from our Sun's solar wind. This is known to 'donate' its electrons to the materials it interacts with. As oxidization occurs due to a loss of electrons, this solar wind should cancel it out, even if liquid water and oxygen were present in large quantities on the Moon.
As the findings were so baffling, Shuai Li reached out to NASA JPL scientists Abigail Fraeman and Vivian Sun to confirm what he had discovered.
"At first, I totally didn't believe it. It shouldn't exist based on the conditions present on the Moon," Fraeman said. "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."
Earth's magnetotail transports oxygen to the Moon
The research paper outlines three key factors that might account for the rusting on the Moon. Firstly, though the Moon lacks an atmosphere, it is in fact home to trace amounts of oxygen. Where does that oxygen come from? Our planet.
Earth's magnetic field — which is officially called a magnetotail — trails behind the planet as it speeds through space, orbiting our Sun. In 2007, Japan's Kaguya orbiter discovered that oxygen from Earth's upper atmosphere can actually travel through this magnetotail, allowing it to traverse the 239,000 miles (385,00 kilometers) of space from Earth to the Moon.
Then there's all that hydrogen being delivered to the Moon by the solar wind. Though it should cancel out any oxidization on the Moon, Earth's magnetotail has also been shown to block over 99 percent of the solar wind during certain periods of the Moon's orbit. So that opens specific windows during which rust can form.
Lastly, the paper posits that scarce water molecules found on the lunar surface could be released by fast-moving dust particles that regularly pelt the Moon. These, in turn, could mix with iron in the lunar soil. The heat from the dust particle impacts might also increase the oxidization rate.
So the scientists' theory relies on some very specific conditions to be met for the oxidization to take place. It is worth remembering though that these processes would have taken place over millions of years — the rust was only just discovered on the Moon but it most certainly didn't appear overnight.
NASA's Project Artemis to shed further light on Moon mystery
"I think these results indicate that there are more complex chemical processes happening in our solar system than have been previously recognized," Sun said. "We can understand them better by sending future missions to the Moon to test these hypotheses."
With NASA's Project Artemis aiming to send human missions to the Moon as early as 2024, you can bet this isn't the last we'll hear of this puzzling mystery, which might also help us to understand why airless bodies like asteroids have been found to contain hematite.
It's one of the many lines of investigation that will open up as humans expand their reach into the cosmos.