Research on the origin of the Moon agree more and more on the giant impact hypothesis. According to this theory, about 150 million years after the birth of the solar system, the early Earth was hit by a planet the size of Mars. This caused the expulsion of a huge amount of debris that formed the Moon. Doubts still exist concerning the composition of the Earth and the Moon but two research just published in the journal “Nature” add new information about the giant impact hypothesis.
The major problem with this theory on the birth of the Moon is that it has an isotopic composition very similar to the Earth’s. In different areas of the solar system, the same chemical elements are present with a different distribution of their isotopes. The consequence is that the various planets have a kind of chemical fingerprint.
The research on the giant impact investigated the isotopic composition of the Earth and the Moon because the idea was that the early Earth and Theia, the planet that hit it, had a different composition. The Earth and the Moon are a mixture of the materials that formed the early Earth and Theia mixed in different quantities so they should have a different isotopic distribution. Instead, all the analyzes shows that they are extremely similar.
According to a research conducted by a team led by Hagai Perets, an astrophysicist at the Israel Institute of Technology in Haifa, the explanation is that Theia had a composition similar to the Earth’s. A series of simulations of the formation of the solar system showed that the possibility of collisions between primordial planets that were very similar is between 20% and 40%.
It’s possible that the Earth and Theia were born in the same area of the solar system and for this reason they had a very similar isotopic composition. If that’s the case, the consequence is that inevitably the Earth and the Moon have also a very similar isotopic composition.
Another research focused on tungsten isotopes on the Earth and the Moon. It’s assumed that both of them gathered additional material after the giant impact. Among the elements there was a high amount of tungsten and a small part was formed by the tungsten-182 isotope. A team of scientists at the University of Maryland measured its presence.
The result is that the Moon rocks contain a bit more tungsten-182 compared to those on Earth. This corresponds to the amount of different materials that are expected to have been gathered by the Earth and the Moon after the giant impact. This suggests that the materials that mixed after that event did it before the Moon coalesced and cooled.
In essence, the Moon initially had the same isotopic composition of the Earth’s mantle. Subsequently, the lighter tungsten-182 isotope gathered a bit more on the Moon causing a slight difference in their isotopic composition. Such a homogeneous mixing of the primordial Earth and Theia means that they could have had a different isotopic composition but the Earth and Moon can be chemically very similar.
In the end, the two studies provide conclusions that can be discordant. One problem is that the in-depth analysis of lunar rocks are limited because we have few of them available and remote analyzes are not enough. The consequence is that the question is still open.