Moon formed from a giant collision of proto-earth with an 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, earth stonesand meteorites representing the isotopic reservoirs from which Theia and the proto-Earth may have formed. They discovered that all of Theia and most of Earth's other constituent materials originated from the inner solar system. Their calculations show that Theia could have formed closer to the Sun than the Earth.
“The composition of a body stores the entire history of its formation, including its place of origin,” said senior study author Dr. Torsten Kleine, a researcher at the Max Planck Institute for Sensing Systems Research.
“Particularly revealing are the ratios in which certain metal isotopes are present in the body.”
“Isotopes are variants of the same element that differ only in the number of neutrons in the atomic nucleus—and therefore in their weight.”
“In the early Solar System, isotopes of a given element were likely distributed unevenly: at the outer edge of the Solar System, for example, isotopes occurred in slightly different ratios than near the Sun.”
“In this way, information about the origin of the original building blocks is stored in the isotopic composition of the body.”
In the course of the study, the authors determined with unprecedented accuracy the ratio of various iron isotopes in the rocks of the Earth and the Moon.
They examined 15 Earth rocks and six lunar samples that Apollo astronauts brought back to Earth.
The result is not surprising: as earlier measurements of the isotope ratios of chromium, calcium, titanium and zirconium have already shown, the Earth and the Moon are indistinguishable in this regard.
However, the large similarities prevent any direct conclusions about Theia from being drawn.
There are simply too many possible collision scenarios.
Although most models suggest that the Moon formed almost exclusively from Theia material, it is also possible that it consists mainly of material from the Earth's early mantle, or that Earth and Theia rocks were inextricably mixed.
To learn more about Theia, researchers used a kind of reverse engineering of the planets.
Based on matching isotope ratios in today's Earth and Moon rocks, the team determined what composition and size of Theia and what composition of early Earth might have led to this final state.
During the study, scientists looked at not only isotopes of iron, but also isotopes of chromium, molybdenum and zirconium.
Different elements provide access to different phases of planet formation.
Long before the destructive encounter with Theia, a sorting process of sorts took place within the early Earth.
When the iron core was formed, some elements such as iron and molybdenum accumulated there; subsequently they were practically absent from the rocky mantle.
Thus, the iron found in the Earth's mantle today could only have appeared after the core had formed, such as on board Theia.
Other elements, such as zirconium, that 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 obtained from calculations, some can be excluded as implausible.
“The most compelling scenario is that most of the building blocks of Earth and Theia originated in the inner solar system,” said study first author Dr. Timo Hopp, a postdoctoral fellow at the University of Chicago and the Max Planck Institute for Sensing Systems Research.
“Earth and Theia were probably neighbors.”
“Although the composition of the early Earth can be thought of as predominantly a mixture of known classes of meteorites, this is not the case with Theia.”
“Different classes of meteorites originated in different regions of the outer solar system.”
“They therefore serve as a reference material for the building material that was available during the formation of early Earth and Theia.”
“However, in Theia's case, previously unknown material may have been involved.”
“We believe the origin of this material is closer to the Sun than to Earth.”
“Calculations therefore show that Theia originated closer to the Sun than our planet.”
results were published this week in the magazine Science.
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Tim Hopp etc.. 2025. The Moon-forming impact object Theia emerged from the inner Solar System. Science 390 (6775): 819-823; doi: 10.1126/science.ado0623
