Samples from the Moon's surface returned by China's Chang'e-5 mission, revealed the presence of abundant iron content in the +3 oxidation state.
Scientists who studied the samples believe that micrometeorites are changing the chemistry of the Moon's surface, turning Fe 2+ into a mixture of uncharged metals and Fe 3+.
Iron is famous for its wide range of oxidation states, from -2 to +7. But on Earth, the most common are +2, and +3, known as ferrous and ferric, respectively. However, the samples returned by the Apollo missions mostly contain iron or metallic iron (Fe 0 ).
This leads to the conclusion that the surface of the Moon, and possibly its interior, is highly reduced (causing other substances to gain electrons), with important implications for our understanding of chemistry on the Moon.
The Chang'e-5 mission did something very similar 50 years after the Apollo missions. In Nature Astronomy, a study of samples returned by the Chang'e-5 mission reveals a rich iron content that Apollo did not find.
Chang'e-5 was sent to one of the youngest parts of the Moon's surface, precisely an area that was volcanically active less than two billion years ago.
There, it collects agglutinate molten particles (clumps of material that have stuck together) about a tenth of a millimeter in diameter, which reportedly contain ferric iron in abundance, more than 40% of the ionized ions being iron.
This then raises the question of where Fe 3+ comes from. Several attempts to explain the small amount of ferric iron in the Apollo samples have accounted for hydrogen or carbon monoxide.
Both can react with iron to produce Fe 3+. Other information obtained from this sample refers to the effect of oxygen atoms that are stripped from the Earth's atmosphere.
The lead author of the study, Professor Xu Yigang from the Guangzhou Institute of Geochemistry said there is one clue that helps explain the iron content found here, and possibly the much smaller amounts detected previously.
"As an airless object, the Moon has undergone extraterrestrial weathering due to solar wind radiation and micrometeoroid impacts," the authors said as quoted from IFL Science.
The melt shows signs of being hit by a micrometeoroid, and the authors speculate that this caused charge redistribution, with Fe 2+ being converted into a mixture of Fe 0 and Fe 3+, possibly with the addition of some electrons from elsewhere.
Even small meteorites can generate a lot of heat if there is no atmospheric friction to slow them down. The accumulation of iron metal particles shows that the energy of the meteorite impact raised the temperature in the glass above 1,524 degrees Celsius. The researchers are not sure if the charge is 'rearranged' when the material is melted, or during post-shock cooling.