Planetary scientists didn’t find a lot of highly ionized iron in the samples returned by the Apollo missions, but the Chang’e 5 samples changed the picture.
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At the landing site of the Chang’e-5, a large amount of ferric iron was found, which is largely absent in the samples delivered by the Apollo mission.
In samples of lunar soil returned by the Chang’e-5 mission, an abundance of iron in the +3 oxidation state was found. Scientists who have studied the samples believe that micrometeorites change the chemical composition of the lunar surface, turning Fe 2 + into a mixture of uncharged metal and Fe 3 +.
Iron is known for its wide range of oxidation states, from -2 to +7, but on Earth the most common are +2 and +3, respectively. However, the samples returned by the Apollo missions contained mostly iron (Fe 0). This led to the conclusion that the lunar surface, and possibly the interior, is being strongly reduced (causing other substances to gain electrons), with important implications for our understanding of lunar chemistry.
An examination of the samples returned by the Chang’e 5 mission shows that Apollo did not find much ferric iron.
Chang’e 5 was sent to one of the youngest parts of the lunar surface, an area that was volcanically active less than 2 billion years ago. There he collected particles of an agglutinate melt (clumps of material stuck together) with a diameter of about a tenth of a millimeter, which, according to the document, contain ferric iron in abundance: more than 40 percent of the ionized ion is ferric iron.
Then the question arises, where does Fe 3 + come from?
Some attempts to explain the small amount of ferric iron in the Apollo samples have suggested that hydrogen or carbon monoxide, either of which can react with iron to form Fe3+, occasionally escaped from the lunar surface. According to another version, this could be the result of the exfoliation of oxygen atoms from the Earth’s atmosphere. However, since there was nothing to explain, the question was not a priority.
The higher numbers reported by Professor Xu Yigang of the Guangzhou Institute of Geochemistry and co-authors are changing the situation. One clue helps explain the ferric iron found here, and possibly the much smaller amounts seen earlier.
“As an airless body, the Moon suffers from cosmic weathering due to solar wind irradiation and micrometeoroid impacts,” the study authors write.
The melts show signs of micrometeoroid impact, and the authors suggest that this caused a charge redistribution, with Fe 2 + converted to a mixture of Fe 0 and Fe 3 +, possibly with the addition of some electrons from other sources.
Even a tiny meteorite can give off a lot of heat when it doesn’t have atmospheric friction to slow it down. Combining metallic iron particles suggests that the energy of the meteorite impact raised the temperature in the glass above 1524 °C. The authors are not sure if the charges were rearranged at this point during the liquefaction of the material or during post-shock cooling.
Ironically, the Apollo missions did detect at least one higher concentration of ferric iron. Up to a quarter of the iron in some of the glass beads returned by the four Apollo missions is iron, but this has only been noticed in the last few years, by which time the impression of a heavily restored surface has already begun.
The article is in the public domain in Nature Astronomy.