Study Traces Moon-Forming Impact to an Inner Solar System Neighbour Named Theia

The Moon was formed as a result of a massive collision between Earth and Theia, a body the size of Mars, some 4.5 billion years ago. In order to determine the origin of Theia, scientists led by Timo Hopp of the Max Planck Institute for Solar System Research and associates at the University of Chicago examined isotopic "fingerprints" in samples of Earth, the moon, and meteorites in a recent study published in Science. They come to the conclusion that Theia and Earth were initially neighbours in the inner Solar System.

Isotopic clues to Theia's origin

According to the study, the team measured heavy-element isotope ratios (iron, chromium, zirconium) in Apollo lunar rocks and Earth samples. Earth and Moon proved indistinguishable in these ratios, matching non-carbonaceous meteorites from the inner Solar System. Such isotopic fingerprints encode a world's formation region in the solar system. The shared signature implies that Theia, too, formed in the same part of the solar nebula.

Theia's inner Solar System birthplace

Using these data, researchers ran “mass-balance” simulations of Earth–Theia scenarios. The best solutions require both bodies to have accreted from inner-disk material. Lead author Timo Hopp says, “The most convincing scenario is that most of the building blocks of Earth and Theia originated in the inner Solar System. Earth and Theia are likely to have been neighbours”.

Comparing Theia's inferred isotopic signature with meteorite data, the team infers Theia “must have originated from the inner part of the early Solar System, closer to the Sun than Earth's current orbit”, reinforcing a shared birthplace in the inner Solar System.