Samples from the lunar surface returned by China's Chang'e-5 mission revealed the presence of abundant iron in the +3 oxidation state.
Scientists studying the samples believe that the micrometeorites are changing the chemistry of the Moon's surface, converting Fe 2+ into an uncharged mixture of metals and Fe 3+.
Iron is notable for its wide oxidation state, 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 contained mostly ferrous or metallic iron (Fe 0 ).
This led to the conclusion that the Moon's surface, and possibly its interior, is highly reduced (causing other substances to gain electrons), with important implications for our understanding of Moon chemistry.
The Chang'e-5 mission did something very similar 50 years after the Apollo mission. In Natural Astronomy, studies of samples returned by the Chang'e-5 mission reveal much iron content that was not found by Apollo.
Chang'e-5 was delivered to one of the youngest parts of the Moon's surface, to be precise an area that was volcanically active less than two billion years ago.
There, it collects molten agglutinate particles (clumps of material that has adhered) about one-tenth of a millimeter, which reportedly contain abundant amounts of ferric iron, over 40% of the ionized ion is ferrous.
This then raises the question of where does Fe 3+ come from. Several attempts to account for the small amount of ferric iron in the Apollo samples have suggested hydrogen or carbon monoxide.
Both can react with iron to produce Fe 3+. Other information obtained from these samples refers to the effect of oxygen atoms being stripped from the Earth's atmosphere.
The study's lead author, Professor Xu Yigang of 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 earlier.
"As an airless body, the Moon is subject to space weathering due to solar wind radiation and micrometeoroid impacts," the authors said as quoted by IFL Science.
The melt showed signs of being hit by micrometeoroids, and the authors suggest that this caused a redistribution of charge, with the Fe 2+ being converted to a mixture of Fe 0 and Fe 3+, possibly by adding some electrons from elsewhere.
Even a small meteorite can generate a lot of heat if there is no atmospheric friction to slow it down. The collection of ferrous metal particles indicates the meteorite's impact energy raised the temperature in the glass above 1,524 degrees Celsius. The researchers aren't sure whether the charge is 'reset' when the material is thawed, or during post-shock cooling.