It takes oxygen to make iron rust. So when scientists discovered hematite spread widely through lunar high latitudes, they were surprised. How did that happen?
A new study suggests that oxygen from Earth could be playing a role in rusting the Moon.
Hematite, also spelled haematite, is an iron oxide with the formula Fe2O3. It’s common on Earth, and is the main iron ore extracted by mining. It only forms in the presence of oxygen.
The formation of hematite on the Moon is an unlikely occurrence. There’s no oxygen to speak of there, so no way for the iron to oxidize and form hematite. Not only that, but the lunar surface is fully exposed to hydrogen in the solar wind, which actively opposes oxidization. So what’s going on?
A new study suggests that the source of oxygen for lunar hematite is Earth’s atmosphere. The paper is titled “Widespread hematite at high latitudes of the Moon.” The lead author is Shuai Li, assistant researcher at the Hawai‘i Institute of Geophysics and Planetology (HIGP) in the University of Hawai’i at M’noa School of Ocean and Earth Science and Technology (SOEST). The paper is published in the journal Science Advances.
“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,” Li said in a press release.
These results stem from a collaboration between NASA and ISRO, the Indian Space Research Organization. In 2008, ISRO launched their Chandrayaan-1 lunar orbiter. The mission only lasted 10 months out of an expected two years, but it still gathered a significant amount of scientific data. Along with all of its Indian scientific instruments, the orbiter also carried six other instruments from other organizations and countries.
NASA contributed the Moon Mineralogy Mapper, or M3. It was an imaging spectrometer designed to map the mineralogy of the lunar surface. It was the first instrument to provide high-resolution imagery of the minerals on the surface of the Moon. This new research is largely based on that data.
“When I examined the M3 data at the polar regions, I found some spectral features and patterns are different from those we see at the lower latitudes or the Apollo samples,” said Li. “I was curious whether it is possible that there are water-rock reactions on the Moon. After months of investigation, I figured out I was seeing the signature of hematite.”
Li’s previous research into the Moon also fed into this discovery. Back in 2018, Li was lead author on a paper announcing the discovery of water ice on the Moon. That research was based on M3 data, and on data from the Lunar Reconnaissance Orbiter LOLA, the Lunar Orbiter Laser Altimeter.
In this new research, Li and his colleagues found that the hematite concentrations are strongly correlated with the water ice deposits. The hematite is also more heavily concentrated on the Moon’s near side, which always faces the Earth.
“This discovery will reshape our knowledge about the Moon’s polar regions,” said Li. “Earth may have played an important role on the evolution of the Moon’s surface.”
This is where some other previous research plays a role. The Japanese (JAXA) Kaguya mission, also known as SELENE (Selenological and Engineering Explorer) was a lunar orbiter launched in 2007, and orbited the Moon for one year and eight months.
Kaguya’s mission was to to study the geology of the Moon, its origins, its surface environment, and its gravity field. But it also found evidence of oxygen from Earth’s atmosphere being transported to the Moon. For five days in each of the Moon’s orbits, it is protected from the solar wind by Earth’s magnetosphere. During that time, oxygen is able to move from Earth’s atmosphere to the lunar surface.
“More hematite on the lunar nearside suggested that it may be related to Earth,” said Li. “This reminded me of a discovery by the Japanese Kaguya mission that oxygen from the Earth’s upper atmosphere can be blown to the lunar surface by solar wind when the Moon is in the Earth’s magnetotail. So, Earth’s atmospheric oxygen could be the major oxidant to produce hematite. Water and interplanetary dust impact may also have played critical roles.”
Though hematite concentrations were much more prevalent on the near side of the Moon, there was still some on the far side. The reasons for that are still unclear, though Li says it may shed some light on hematite formation on asteroids.
“Interestingly, hematite is not absolutely absent from the far-side of the Moon where Earth’s oxygen may have never reached, although much fewer exposures were seen,” Li added. “The tiny amount of water (< ~0.1 wt. percent) observed at lunar high latitudes may have been substantially involved in the hematite formation process on the lunar far-side, which has important implications for interpreting the observed hematite on some water poor S-type asteroids.”
If this research is correct, it means that hematite deposits may hold vital clues to Earth’s history. If the hematite is preserved in impact craters of different ages, then those deposits will contain different isotopes of oxygen from different time periods in the Earth’s geologic past. If future missions like Artemis can gather samples from those craters, scientists could learn a lot. Not only could it prove Li’s hypothesis correct, it could shed new light on Earth’s history.
As the authors write in their paper, “Hematite formed at craters of different ages on the lunar nearside may record the oxygen signatures of Earth’s atmosphere in the past ~2.4 billion years. Isotope measurements at these hematite exposures can reveal the evolution of Earth’s atmosphere in the past billions of years.”
Li and his colleagues also say that these hematite deposits could be an important in-situ resource in the future. “It is suggested that the lunar polar regions are relatively iron poor. Thus, our mapped hematite can be important in situ resources for iron metal.”