Scientists have figured out the origin of the moon

About 4.5 billion years ago, a large celestial body, protoplanet Teia, collided with the young Earth. This impact changed the size and orbit of our planet and led to the birth of the Moon. But where Teia came from, what its nature was, and how it differed from Earth, has until now remained unclear.

New work by an international team led by the Max Planck Institute for Solar System Research and the University of Chicago offers an answer: Teia appears to have formed in the inner part of the solar system and was a "neighbour" of the Earth in orbit.

The scientists assumed that the composition of a celestial body preserves the history of its formation, including its birth region. A special role is played by isotope ratios - varieties of the same element with a different number of neutrons in the nucleus. In the early Solar System, isotopes were not evenly distributed: closer to the Sun, their proportions differed from those typical of the outer regions.

The team measured with unprecedented precision the ratios of iron isotopes in rocks on Earth and the Moon. To do this, they examined 15 Earth samples and six lunar samples delivered by the astronauts of the Apollo missions. The result confirmed previous data on chromium, calcium, titanium and zirconium: the isotopic composition of the Earth and the Moon are virtually indistinguishable.

However, the similarity alone does not tell us directly what Teia was like: different scenarios are possible, from a Moon almost entirely composed of Teia matter to variants where the mixture of Earth and Teia rocks is closely mixed.

To get around this uncertainty, the researchers went the route of "reverse engineering" the planets. Knowing the end result - matching isotopic proportions in terrestrial and lunar rocks - they calculated what the composition and size of Teia and early Earth might have been to arrive at the observed picture.

They looked not only at iron, but also at isotopes of chromium, molybdenum and zirconium that "record" different stages of planet formation. Even before the collision with Teia, the young Earth was separating into core and mantle: iron and molybdenum went into the metallic core, while, for example, zirconium remained in the silicate mantle and recorded the earlier history of the planet's growth. The iron that is in the Earth's mantle today, according to calculations, must have arrived after the core was formed - including with Teia.

After comparing all this data, the scientists went through many mathematically possible combinations of the composition of the early Earth and Teia - and discarded those that do not agree with observations.

The most convincing scenario, according to lead author Timo Hopp, is this: the bulk of the building material of both Earth and Teia came from the interior of the solar system. In other words, these bodies formed in close orbits - they were neighbours.

The composition of the early Earth is well described by a mixture of known classes of meteorites, which are thought to be fragments of ancient bodies from different zones of the protoplanetary disc. But for Teia, a simple combination of already known meteorite "recipes" is not enough. Modelling shows that in its composition was probably a substance formed even closer to the Sun than the Earth's orbit.

This means that Teia could have been born in the inner part of the system, migrated to the Earth's orbit and eventually collided with it, giving birth to the Moon.