A new study published in Science Advances reveals that researchers have discovered the presence of hematite, an iron oxide similar to rust, on the Moon. This discovery is notable because iron corrodes when exposed to water and oxygen—and neither element exists in large quantities on the Moon’s lunar surface.
“It’s very puzzling,” says Shauai Li, lead author of the study and an assistant researcher from the University of Hawai’i (Honolulu, Hawaii, USA). “The Moon is a terrible environment for hematite to form in.”
In order to unlock this mystery, Li teamed up with fellow Hawai’i professor Paul Lucey and two scientists from NASA Jet Propulsion Laboratory (JPL) (Pasadena, California, USA), Abigail Fraeman and Vivian Sun. The research team reviewed data from the Indian Space Research Organization's Chandrayaan-1 orbiter, which discovered water ice while surveying the Moon’s surface in 2008.
Li extensively studied this water ice in data from Chandrayaan-1's Moon Mineralogy Mapper (M3) instrument, which was built by JPL. Upon inspecting the M3 data, he detected spectra—or light reflected off surfaces—around the Moon’s poles that had a different composition than the rest of the surface. This spectral signature, he later concluded, had a close match with the signature of hematite.
Fraeman and Sun were called in to confirm Li’s discovery based on the M3 data. “At first, I totally didn't believe it. It shouldn't exist based on the conditions present on the Moon,” says Fraeman. “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.”
“In the end, the spectra were convincingly hematite-bearing, and there needed to be an explanation for why it's on the Moon,” adds Sun.
In their paper, the team points to three factors that may have resulted in hematite formation on the Moon. First, while the Moon does lack an atmosphere, it has trace amounts of oxygen due to Earth’s magnetic field. Second, solar winds that bombard the Moon with hydrogen is mediated by Earth’s magnetotail, which blocks nearly all of the solar wind during certain phases in the Moon and thereby allowing occasional windows in which rust can form. Third, water molecules on the lunar surface could have mixed with iron in the soil to facilitate a rust-inducing chemical reaction.
According to the researchers, more data is needed to determine how water molecules on the Moon’s surface interact with lunar soil and rock. Answering this question could, in turn, help researchers understand why hematite forms on the far side of the Moon where Earth’s oxygen is unable to reach it. “It could be that little bits of water and the impact of the dust particles are allowing iron in these bodies to rust,” Fraeman speculates.
JPL is developing a new version of M3 that would map water ice in permanently shadowed craters on the Moon and possibly reveal new details about hematite as well. “I think these results indicate that there are more complex chemical processes happening in our solar system than have been previously recognized,” says Sun. “We can understand them better by sending future missions to the Moon to test these hypotheses.”