The Earth and Moon have distinct oxygen isotope compositions that are not identical, reports a study in Nature Geoscience. This finding may challenge the current understanding of the formation of the Moon.
The Giant Impact Hypothesis suggests that the Moon was formed from debris following a giant collision between early Earth and a proto-planet named Theia. The Earth and Moon are geochemically similar, and samples returned from the Moon from the Apollo mission showed a near-identical composition in oxygen isotopes. Although the Giant Impact Hypothesis can explain many of the geochemical similarities between the Earth and Moon, the extreme similarity in oxygen isotopes has been difficult to reconcile with this scenario. Either the two bodies were compositionally identical in oxygen isotopes to start with, which is unlikely, or their oxygen isotopes were fully mixed in the aftermath of the impact, which has been difficult to model in simulations.
Erick Cano and colleagues conducted high-precision measurements of the oxygen isotopic composition of a range of lunar samples. They found that the oxygen isotopic composition varied depending on the type of rock tested. This may be due to the degree of mixing between the molten Moon and vapour atmosphere following the impact. Oxygen isotopes from samples taken from the deep lunar mantle were the most different to oxygen isotopes from Earth. The authors suggest that the deep lunar mantle may have experienced the least mixing and is most representative of the impactor Theia.
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