Earth’s oxygen rusted the Moon for billions of years

Earth may have played an important role in the evolution of the Moon’s surface.

Hematite (Fe2O3) is a common oxidization product on Earth, Mars, and some asteroids. To the surprise of many planetary scientists, the oxidized iron mineral hematite is also available on the Moon, suggests a new study.

Shuai Li, an assistant researcher at the Hawai’i Institute of Geophysics and Planetology (HIGP) in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST), said, “We analyzed the hyperspectral reflectance data acquired by the Moon Mineralogy Mapper (M3) designed by NASA JPL onboard India’s Chandrayaan-1 mission.”

“We hypothesize 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 the solar wind when the Moon is in Earth’s magnetotail during the past several billion years.”

The discovery was made after analyzing the hyperspectral reflectance data acquired by the Moon Mineralogy Mapper (M3) designed by NASA JPL onboard India’s Chandrayaan-1 mission.

Li said, “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. I was curious whether there may be water-rock reactions on the Moon. After months of investigation, I figured out I was seeing the signature of hematite.”

Map of hematite on the moon—redder color means more hematite
Map of hematite on the moon—redder color means more hematite. (Photo credit: Shuai Li

Scientists discovered locations- where hematite is present- strongly correlated with water content at high latitude.

Li said, “More hematite on the lunar nearside suggested that it may be related to Earth. This reminded me of discovery by the Japanese Kaguya mission that oxygen from the Earth’s upper atmosphere can be blown to the lunar surface by the 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.”

“Interestingly, hematite is not absent from the Moon’s far side where Earth’s oxygen may have never reached, although much fewer exposures were seen. The tiny amount of water (< ~0.1 wt.%) observed at high lunar 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.”

“This discovery will reshape our knowledge about the Moon’s polar regions. Earth may have played an important role in the evolution of the Moon’s surface.”

Scientists are now hoping that NASA’s ARTEMIS mission will return hematite samples from the polar regions. Those samples’ chemical signatures can confirm their hypothesis whether the lunar hematite is oxidized by Earth’s oxygen and may help reveal the evolution of the Earth’s atmosphere in the past billions of years.

Journal Reference:
  1. Widespread hematite at high latitudes of the Moon” Science Advances (2020). DOI: 10.1126/sciadv.aba1940

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