The presence of hematite at high latitudes of the Moon

Hematite is commonly found on Earth, Mars, and asteroids as a product of aqueous alteration. However, its presence not associated with aqueous alteration is enigmatic. The lunar surface and interior are highly reducing. Metallic iron is found as a primary igneous phase in lunar basalts, and is also commonly seen in rims of lunar regolith grains. Although it has been speculated that oxidizing processes could operate on the lunar surface and form ferric iron bearing minerals, detection of those minerals has remained elusive. Our analyses of the Moon Mineralogy Mapper (M³) data show that hematite (Fe₂O₃) is present at high latitudes (> ~60^o) of the Moon and is dominantly associated with the east side of topography highs. Hematite is more prevalent on the nearside than the far side of the Moon. Previous studies provided possible pathways for hematite formation on the lunar surface. However, the distribution of hematite on the global lunar surface suggests unique formation mechanisms. We hypothesize that lunar surface hematite might be formed through oxidizing primary ferrous minerals by oxygen delivered to the lunar surface from Earth's atmosphere during the passage of Earth's magnetotail. The Earth's magnetotail provides a more oxidizing environment for the Moon by shielding most of the solar wind flux. However, only the tidally-locked nearside of the Moon can receive a substantial amount of Earth's oxygen in the magnetotail, which explains the dichotomy distribution of hematite between the near and far side of the Moon. The water-rich polar regions provide a desirable environment for the oxidization process. This might explain that hematite is only detected at high latitudes. Interplanetary dust impacts may provide the heat source for fertilizing the oxidization reaction, which may have resulted in more hematite formation at the east side of topography highs. Our hypothesis can be tested with oxygen isotope measurements by future missions. If that is the case, hematite at craters of different ages may have preserved the oxygen isotopes of Earth's atmosphere in the past billions of years, through which we can reveal the evolution of Earth's atmosphere and possibly the origin of life on Earth. Lunar hematite can be important in situ resources for oxygen and iron.