Li, and 6 co-authors, reported in 2020 that hematite (Fe2O3) is a common oxidized mineral on Earth, Mars, and some asteroids. Although oxidizing processes have been speculated to operate on the lunar surface and form ferric iron–bearing minerals, unambiguous detections of ferric minerals forming under highly reducing conditions on the Moon have remained elusive. Our analyses of the Moon Mineralogy Mapper data show that hematite, a ferric mineral, is present at high latitudes on the Moon, mostly associated with east- and equator-facing sides of topographic highs, and is more prevalent on the nearside than the farside.
Oxygen delivered from Earth’s upper atmosphere could be the major oxidant that forms lunar hematite. Hematite at craters of different ages may have preserved the oxygen isotopes of Earth’s atmosphere in the past billions of years. Future oxygen isotope measurements can test our hypothesis and may help reveal the evolution of Earth’s atmosphere. Furthermore, geoscientists have proposed that water ice could be mixed in with the lunar regolith, and excavated and melted during impact events. The hematite is also mostly found on the side of the Moon that is always facing Earth. That, according to the researchers, is really interesting. "More hematite on the lunar nearside suggested that it may be related to Earth," Li said.
"This reminded me of a discovery by the Japanese Kaguya mission that oxygen from Earth's upper atmosphere can be blown to the lunar surface by solar wind when the Moon is in Earth's magnetotail. So, Earth's atmospheric oxygen could be the major oxidant to produce hematite."During the full Moon, our satellite is in Earth's magnetotail, the trailing region of the magnetosphere away from the Sun. At these times, over 99 percent of the solar wind is blocked from reaching the Moon, which means that pesky hydrogen reducing agent isn't getting all up in the oxidation process. Combine these three ingredients - minute amounts of molecular water, minute amounts of oxygen, and a brief window of time each month in which rust can form freely - and, over a few billion years, you can get hematite on the Moon. That doesn't mean the mystery is completely solved, however.
"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 said. "The tiny amount of water 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."
Getting our hands on the mineral itself would be really interesting. It's possible that hematite deposits over a range of ages could still retain oxygen isotopes from different ages in Earth's history, dating back billions of years. This could be really useful for understanding our planet's atmospheric evolution. And, of course, it would be deeply enlightening for understanding the Moon's history, too.
"This discovery will reshape our knowledge about the Moon's polar regions," Li said. "Earth may have played an important role on the evolution of the Moon's surface."
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