The collision with Theia dramatically altered Earth's size, composition, and orbit, marking the birth of the Moon. Although Theia was destroyed in the event, its chemical and isotopic traces remain in the present-day compositions of Earth and Moon. Isotope ratios for iron, chromium, calcium, titanium, and zirconium-measured in terrestrial rocks as well as Apollo-returned lunar samples-are key to understanding planetary origins.
Iron isotopic ratios in both Earth and Moon rocks proved indistinguishable. This supports previous data for other elements, yet leaves open multiple collision models: the Moon may be composed mainly of Theia's material, of proto-Earth mantle, or a blend of both. The researchers reconstructed likely scenarios in planetary "reverse engineering," simulating which sizes and compositions for Theia and early Earth would produce the observed isotope matches.
Their work extended beyond iron to other metallic isotopes. Elements like iron and molybdenum collect in a planet's core during formation, while others such as zirconium remain in the rocky mantle, documenting a broader formation history. Calculations using these isotopic ratios, compared against known meteorite classes, illustrated that most of Earth's constituents fit meteorite standards.
Theia's composition, however, does not match any current meteorite class, indicating that some of its building material originated even closer to the Sun than Earth, possibly from unknown Solar System reservoirs. "The composition of a body archives its entire history of formation, including its place of origin," said Thorsten Kleine, Director at MPS.
"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 neighbors," lead author Timo Hopp stated. Meteorites were used as reference material, but Theia's signature could only be explained by the presence of material from a region nearer to the Sun than our own planet.
These findings suggest that both Earth and Theia shared close formation locales, with the lunar impactor's unique isotopic recipe corroborating its birth in the inner Solar System rather than the outer reaches. The scenario not only clarifies Moon formation but also offers a window into the Solar System's dynamic early environment.
Research Report:The Moon-forming impactor Theia originated from the inner Solar System
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Max Planck Institute for Solar System Research
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