The Moon, a very down-to-earth composition

Using magmatic lunar rocks with a high concentration of magnesium collected during the Apollo 15 and Apollo 17 missions, they show in a study published in Earth and Planetary Science Letters on August 1^st 2017 that the compositions of the Earth and Moon are even more similar than previously suspected.

Lunar volcanic samples brought back to Earth from the Apollo missions in the 1970s stood out for their chemical similarities to terrestrial volcanic rocks, albeit very depleted in volatile compounds. This observation has given rise to a number of theories as to how the Moon could have formed, such as the clumping together of a large ‘chunk’ of rock ejected from the Earth, the fission of a young Earth, or a giant impact. It is this last theory that has been favoured in recent years, as it has been accepted that collisions between planetary embryos in the growth phase are fundamental factors in the formation of planets, and can therefore explain the depletion of volatile elements. In these different impact scenarios, numerical models predict that the material that forms the Moon should come from the impactor (and not from Earth), producing lunar rocks with compositions that differ from terrestrial rocks.

Iron, the only element that is more abundant in lunar rocks than in terrestrial magmas, is an exception. In fact, if this iron enrichment came from the impactor, an isotopic signature of this impacting material should be present in the lunar rocks. Since isotopes are types of atoms of the same chemical element with slightly different masses, they can be separated from each other during the various processes of planetary formation, such as core formation or evaporation. Initial studies of the isotopic composition of lunar volcanic rocks have shown a slight enrichment in heavy iron isotopes compared with the Earth, suggesting that the iron on the Moon does indeed come from a source other than the Earth. This is based on the assumption that lunar volcanic rocks are representative of the composition of the Moon as a whole, a questionable hypothesis due to a lack of internal heat, a necessary condition for the homogenisation of the lunar mantle following its solidification, unlike on Earth.

Sossi and Moynier assessed the iron isotopic composition of the Moon by analysing for the first time the iron isotope ratios in plutonic rocks (crystallised at depth), known as the “Magnesian Suites”, which are the oldest lunar magmatic samples derived from the fusion of the first crystals formed as the Moon cooled. It turns out that these samples have Fe/Mg ratios identical to the rocks in the Earth’s mantle (which makes up most of the planet), unlike the lunar volcanic rocks, which have a higher Fe/Mg content and an iron isotopic signature similar to that on Earth. The two researchers thus demonstrate that the ‘Magnesian Suites’ provide a better analogue for the Moon’s composition than do the volcanic rocks, ruling out any loss or gain of Fe during the giant impact. It is therefore reasonable to assume that the Moon is composed almost entirely of material from the Earth’s mantle.

Ref: P. Sossi, F. Moynier – Chemical and isotopic kinship of iron in the Earth and Moon deduced from the lunar Mg-Suite: https: //doi.org/10.1016/j.epsl.2017.04.029