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Scientists may have solved a longstanding puzzle about the origins of a mysterious element on the Moon with the help of lunar samples that were returned to Earth by a Chinese mission in 2020, reports a new study.
China’s Chang’e-5 mission wowed the world two years ago when it became the first spacecraft in decades to land on the Moon, collect samples, and safely deliver them back to Earth. The probe was able to scoop up about 1,700 grams of lunar dirt from an unexplored region that was volcanically active within the past two billion years, making it one of the youngest parts of the Moon’s surface.
The pristine extraterrestrial pebbles contain globules of ferric iron (Fe3+), a version of iron that contains oxygen, which was previously assumed to be rare on the Moon because the lunar surface is chemically “reductive,” meaning that it is not easy for oxidized compounds to form.
Now, scientists led by Haiyang Xian, a geochemist at the Chinese Academy of Sciences, Guangzhou, demonstrate that this compound can be forged by tiny rocks, called micrometeorites, that constantly pelt the lunar terrain. The discovery reveals “a dominant pathway” for the formation of Fe3+ on the lunar surface, and “suggests that much more [ferric iron] could be present on the Moon than previously thought, and that its abundance is progressively increasing with micrometeoroid impacts,” according to a study published on Monday in Nature Astronomy.
“The occurrence of Fe3+ in lunar materials has been discussed for decades,” Xian and his colleagues said in the study. “How Fe3+ forms, accumulates and evolves in the reductive environment of the Moon surface is still under debate.”
“Here we present robust evidence from a lunar sample”—labeled CE5C0400YJFM00408—”for the formation of a large amount of lunar Fe3+” made by chemical reactions “during micrometeoroid impacts on the airless lunar surface,” the team added.
The mystery of the Moon’s Fe3+ dates back to the Apollo era, when NASA astronauts hauled enormous loads of rocks back from the lunar surface. At that time, Fe3+ was only faintly detected in these rocks, but more recent analyses with better instruments have revealed higher-than-expected concentrations of the oxidized compound in the Apollo samples.
Moreover, a recent orbital study of the Moon found high concentrations of hematite, a rusty iron oxide, at the poles, a finding that has also perplexed scientists. It’s possible that these iron compounds are getting their oxygen from Earth, as our planet does leak this atmospheric gas into space, or from lunar ice deposits. However, these explanations don’t fully account for new findings about the Moon’s Fe3+ content.
Xian and his colleagues think they may have finally solved this riddle by meticulously examining Fe3+ particles that are suspended in glass shards, called agglutinates, from Chang’e-5 samples. The team suggests that the ferric iron in the samples was created by a so-called “charge disproportionation reaction” that occurs during micrometeorite impacts.
Unlike Earth, the Moon has no atmosphere to protect it from micrometeorite impacts, leaving its whole surface exposed to a ceaseless rain of space pebbles. These impacts may help explain how oxidized compounds form on the Moon, and could also suggest that Fe3+ is far more abundant on the lunar surface—and perhaps, similar worlds—than once imagined.
“Importantly, the large amount of Fe3+ in the agglutinate glass produced by charge disproportionation may enhance our knowledge of the evolution and distribution” of oxidized iron compounds on the Moon, the researchers said. “The widespread agglutinate fragments in the lunar regolith and ongoing micrometeoroid impacts…suggest that Fe3+ could be a universal component with increasing abundance on the Moon.”