Theia Impact: Origin of the Moon – Explained

Around 4.5 billion years ago, the history of our young planet changed. When the Solar System was only a few tens of millions of years old, a massive object, named Theia, struck proto-Earth. And the Moon was born from the debris of the collision. Recently, Timo Hopp, a geochemist at the Max Planck Institute for Solar System Research in Germany, and his collaborators believe they have identified the region of the Solar System where Theia would have come into existence.

In its early days, the Solar System was just a vast disk of gas and dust. Around the emerging star that would become the Sun, planetary embryos evolved in a chaotic environment, the site of numerous collisions. It was in this violent setting that the Moon emerged. When Theia hit proto-Earth head-on, the impact, of considerable energy, led to their fusion and the ejection of a large quantity of matter from these two bodies. Over a few dozen to a few hundred years, this orbiting material cooled and then coalesced to give birth to our natural satellite.

If in broad terms the formation scenario of the Moon seems clear, many uncertainties still surround Theia. Since the object was destroyed in the collision, it is difficult to say what its size, composition or place of origin was. On this last point, different hypotheses were considered: that of an asteroid coming from the belt located between the orbit of Mars and that of Jupiter, that of a much more distant object formed in the icy regions beyond the orbit of Neptune or that of a region much closer to the Earth, or even in an even tighter orbit around the Sun. It was with the aim of deciding between these hypotheses that Timo Hopp and his colleagues studied terrestrial and lunar rocks as well as meteorites that fell on Earth, to determine their iron isotopic composition, which provides information on the place of formation of the samples.

Indeed, the composition of bodies in the Solar System varies according to their distance from the Sun. Due to the temperature gradient in the protoplanetary disk, the most volatile and lightest elements (water, hydrogen, etc.) can only condense at a great distance from the star. Bodies formed far from the Sun thus accumulate more ice and light materials. This phenomenon also has an influence on the isotopes of the same element (which are distinguished by their number of neutrons): depending on the temperature and irradiation conditions, certain isotopes are slightly favored over others during the formation of materials. The isotopic composition of celestial objects thus preserves traces of the environment in which they were formed.

In this new study, the researchers began by characterizing the iron isotopic composition of 15 terrestrial rocks, six lunar rocks brought back by the missions Apollo and enstatite chondrites, meteorites whose composition is considered representative of materials formed in the inner region of the Solar System, such as Earth. “This is the first time that iron isotopic ratios have been measured so precisely,” underlines Maud Boyet, geochemist at the Magmas and Volcanoes Laboratory at Clermont-Auvergne University and co-author of the study. By combining these data with the isotopic signatures of chromium, calcium, titanium, molybdenum and zirconium previously obtained, “we arrived at a complete model where each element tells us about the conditions of formation. »

Once the analyzes were done, the team performed calculations to link the isotope ratios to the possible origin of Theia. Several collision models exist. At both ends, we find a scenario where the impactor is relatively large compared to the Earth and another where our planet is much more imposing. “Based on these two scenarios, we put forward several hypotheses on the composition of the Earth and Theia, and we looked at which ones allowed us to find the measured isotopic ratios,” describes Maud Boyet. And the conclusion is undoubtedly that Theia would have formed a little closer to the Sun than the Earth. »