The oxidized iron mineral hematite has been discovered at high latitudes on the Moon, according to a study published in Science Advances led by Shuai Li, a junior research fellow at the Hawaiian Institute of Geophysics and Planetology (HIGP) at UH Mānoa’s School of Oceanic and Geological Sciences and Technology (SOEST). This greatly surprised many planetary scientists.

Iron reacts very actively with oxygen to form the reddish rust commonly found on Earth. However, the surface and interior of the moon are practically devoid of oxygen, so pure metallic iron predominates there. Strongly oxidized iron has not been confirmed in samples obtained from the Apollo missions. In addition, hydrogen in the solar wind erodes the surface of the moon, which inhibits oxidation. So, the presence on the Moon of highly oxidized iron-bearing minerals such as hematite is an unexpected discovery.

Our hypothesis is that lunar hematite is formed by the oxidation of iron on the lunar surface with oxygen from the Earth’s upper atmosphere, which has been continuously blown out to the lunar surface by the solar wind when the moon has been in the tail of the earth’s magnetosphere for the past several billion years.

Shuai Li, Associate Research Fellow, Hawaii Institute of Geophysics and Planetology (HIGP)

To make this discovery, Lee, HIGP Professor Paul Lucy, and co-authors at NASA’s Jet Propulsion Laboratory (JPL) analyzed hyperspectral reflection data from NASA’s JPL Moon Mineralogy Mapper (M3) program aboard India’s Chandrayaan-1 mission.

This new study was inspired by Lee’s previous discovery of water ice in the moon’s polar regions in 2018.

“When I examined the M3 data in the polar regions, I found that some spectral features and patterns were different from those we see at lower latitudes or in the Apollo samples”, Lee said. – I was curious if it was possible that there is a reaction of water and rock on the moon. After several months of research, I realized that I was seeing a trace of hematite”.

The team found that places where hematite is present correlate strongly with water content at high latitudes and are more concentrated on the near side, which is always facing Earth.

“More hematite on this side suggests it may be related to Earth,” Lee said. “It reminded me of the Japanese Kaguya Mission’s discovery that oxygen from the Earth’s upper atmosphere can be brought to the surface of the Moon by the solar wind when the Moon is in the tail of the Earth’s magnetosphere. Thus, Earth’s atmospheric oxygen can be the main oxidant for the production of hematite. The impact of water and interplanetary dust could also play a decisive role.

“Interestingly, hematite is not entirely absent on the far side of the moon, where Earth’s oxygen may never have reached”, Lee said. “The tiny amount of water (<~ 0.1 wt%) observed at high lunar latitudes may have been heavily involved in the formation of hematite on the far side of the moon, which has important implications for the interpretation of observed hematite on some water-poor asteroids class S".Spectral class S - a class of asteroids, which includes objects that have a silicon (rock) composition. Therefore, asteroids of this class are also called stone. They make up 17% of all known asteroids, thus forming the second most common class of asteroids, after carbon ones.“This discovery will change the way we think about the polar regions of the moon,” Lee concludes. "The earth could have played an important role in the evolution of the lunar surface."The research team hopes NASA's ARTEMIS missions will be able to recover hematite samples from the polar regions. The chemical signatures of these samples could support their hypothesis about whether lunar hematite is oxidized by Earth's oxygen, and could help reveal the evolution of Earth's atmosphere over the past billions of years.