Theiathe planetoid about the size of Mars that impacted the Earth about 4.5 billion years ago, giving rise to Moonwould not come from remote areas of the Solar System but from the internal Solar System: in short, it was a “neighbor” of the Earth. This is the conclusion suggested by a new study published in Science and led by Max Planck Institute for Solar System Research (MPS) and from University of Chicagomade possible through very high precision measurement of isotopic proportions (iron, chromium, molybdenum, zirconium) of terrestrial and lunar rocks reported by the Apollo missions, thanks to which scientists discovered that these are compatible with a scenario in which most of the building blocks of Earth and Theia had originates in the inner Solar System. It is therefore likely that Earth and Theia were in the past “close” to each other.
Details of the study on the planet Theia
In the study the team of researchers analyzed 15 samples of terrestrial rocks and 6 samples of lunar rocksthe latter collected during the Apollo missions to the Moon. Thanks to these rocks, scientists were able to measure the proportion of isotopes (an isotope is an atomic nucleus with the same number of protons, but a different number of neutrons) of iron, as well as those of chromium, molybdenum and zirconium. The idea behind the study is that the isotopic composition of a planetary body represents a sort of “memory book” of the object which can reveal its origin, the type of material from which it was assembled, its distance from the Sun in the primordial disk, etc.
The most significant conclusions come from the analysis of iron and molybdenum isotopes. Since before the impact the Earth had already undergone a segregation which led her to create a iron corethe heavy elements in the early Earth were all there in the center, which implies that the Earth’s mantle could not have contained all the iron we see today. Rather, a significant part of the iron in the mantle must have arrived later, for example through a impact with a body that was also made up of large quantities of iron and molybdenum, Theia precisely.
The scientists compared the measured isotopic values with those of different classes of meteorites, since they represent the remnants of the formation of the Solar System and therefore are the reference for the material available during the formation of the early Earth and Theia. From the comparison, scientists concluded that the composition of Theia it does not correspond to that of meteorites coming from the outer part of the Solar System (richer in light elements), but its signature is compatible with materials from the internal regionthat is, closer to the Sun than the Earth’s orbit.
The method used is different from the usual one which consists in producing simulations of the impact of the two bodies. The scientists called it a sort of reverse engineering process: rather than simulating all possible impact scenarios, the team starts from the current isotopic signatures of the Earth and the Moon, and works back to the combinations of masses/compositions of Theia and the pre-impact Earth that would have generated the observed result. The result is that the most convincing scenario is the one in which Theia and Terra formed next to each other in the inner solar disk and therefore the impact occurred between “neighbors” rather than with a large body coming from a remote area of the Solar System.
The great impact between Earth and Theia as the origin of the Moon
According to the most accredited hypotheses so far, based on computer simulations of the collision between planetoids, Theia collided with young Earth in a catastrophic impact approximately 4.5 billion years ago. From the impact, enormous quantities of material from the Earth’s mantle and Theia were thrown into orbit around the Earth, aggregating to form the Moon. In this new study, the discovery that Theia had a similar, but not identical, composition to that of Earth implies that a significant portion of the impact material entered the mix that generated the Moon along with that of the Earth. In practice, the Luna is the daughter of a fusion of two similar bodies. The result therefore reinforces the idea that the Moon is not simply a fragment of the Earth, or alternatively a fragment only of Theia, but rather the product of an impact between two bodies of similar mass formed in the same region of the Solar System. This picture helps explain the strong isotopic similarity between the Earth and the Moon. Placing Theia closer to the Sun changes how we think about the population of internal protoplanetary bodies and how the Earth acquired mantle materials such as iron, i.e. through collision with a planetary “housemate”.
