The Moon formed from a giant impact of the proto-Earth with the ancient protoplanet Theia. In a new study, a team of scientists from the United States, Germany, France and China measured iron isotopes in lunar samples, terrestrial rocks, and meteorites representing the isotopic reservoirs from which Theia and proto-Earth might have formed. They found that all of Theia and most of Earth’s other constituent materials originated from the inner Solar System. Their calculations suggest that Theia might have formed closer to the Sun than Earth did.
An artist’s impression of the collision between the proto-Earth and Theia. Image credit: MPS / Mark A. Garlick.
“The composition of a body archives its entire history of formation, including its place of origin,” said study’s senior author Dr. Thorsten Kleine, a researcher at the Max-Planck-Institut für Sonnensystemforschung.
“The ratios in which certain metal isotopes are present in a body are particularly revealing.”
“Isotopes are variants of the same element that differ only in the number of neutrons in their atomic nucleus — and thus in their weight.”
“In the early Solar System, the isotopes of a given element were probably not evenly distributed: at the outer edge of the Solar System, for example, the isotopes occurred in a slightly different ratio than near the Sun.”
“Information about the origin of its original building blocks is thus stored in the isotopic composition of a body.”
In the study, the authors determined the ratio of different iron isotopes in Earth and Moon rocks with unprecedented precision.
They examined 15 terrestrial rocks and six lunar samples that astronauts from the Apollo missions brought back to Earth.
The result is hardly surprising: as earlier measurements of the isotope ratios of chromium, calcium, titanium, and zirconium had already shown, Earth and Moon are indistinguishable in this respect.
However, the great similarity does not allow any direct conclusions about Theia.
There are simply too many possible collision scenarios.
Although most models assume that the Moon was formed almost exclusively from material from Theia, it is also possible that it consists primarily of material from the early Earth’s mantle or that the rocks from Earth and Theia mixed inseparably.
In order to learn more about Theia, the researchers applied a kind of reverse engineering for planets.
Based on the matching isotope ratios in today’s terrestrial and lunar rocks, the team played through which compositions and sizes of Theia and which composition of the early Earth could have led to this final state.
In the study, the scientists looked not only at iron isotopes, but also at those of chromium, molybdenum, and zirconium.
The different elements give access to different phases of planetary formation.
Long before the devastating encounter with Theia, a kind of sorting process had taken place inside the early Earth.
With the formation of the iron core, some elements such as iron and molybdenum accumulated there; they were afterwards largely absent from the rocky mantle.
The iron found in the Earth’s mantle today can therefore only have arrived after the core was formed, for example on board of Theia.
Other elements such as zirconium, which did not sink into the core, document the entire history of our planet’s formation.
Of the mathematically possible compositions of Theia and the early Earth that result from the calculations, some can be ruled out as implausible.
“The most convincing scenario is that most of the building blocks of Earth and Theia originated in the inner Solar System,” said study’s first author Dr. Timo Hopp, a researcher at the University of Chicago and the Max-Planck-Institut für Sonnensystemforschung.
“Earth and Theia are likely to have been neighbors.”
“While the composition of the early Earth can be represented predominantly as a mixture of known meteorite classes, this is not the case with Theia.”
“Different meteorite classes originated in different areas of the outer Solar System.”
“They therefore serve as reference material for the building material that was available during the formation of the early Earth and Theia.”
“In the case of Theia, however, previously unknown material may also have been involved.”
“We believe this material’s origin to be closer to the Sun than Earth.”
“The calculations therefore suggest that Theia originated closer to the Sun than our planet.”
The results were published this week in the journal Science.
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Timo Hopp et al. 2025. The Moon-forming impactor Theia originated from the inner Solar System. Science 390 (6775): 819-823; doi: 10.1126/science.ado0623
