The Moon was created through a massive collision between the proto-Earth and the ancient protoplanet Theia. A recent study by a collaborative team of scientists from the United States, Germany, France, and China analyzed iron isotopes in lunar samples, Earth rocks, and meteorites believed to represent the isotope reservoir from which both Theia and early Earth may have formed. Their findings indicate that Theia and most of Earth’s constituent materials originated from the inner solar system, suggesting that Theia formed closer to the sun than Earth.
Artist’s impression of the collision between proto-Earth and Theia. Image credit: MPS/Mark A. Garlick.
“The composition of the body reflects its entire formation history, including its origin,” said Dr. Torsten Kleine, lead author of the study from the Sonnensystemforschung Institute at the Max Planck Institute.
“The ratio of specific metal isotopes within the body is particularly insightful.”
“Isotopes are different versions of the same element, varying only in neutron count in the atomic nucleus, which affects their weight.”
“In the early solar system, the distribution of isotopes was likely not uniform. For instance, at the solar system’s outer edges, isotopes existed in proportions that differed from those found near the Sun.”
“Thus, the isotopic makeup of a body holds clues about the origins of its components.”
The authors measured iron isotopes in Earth and Moon rocks with exceptional accuracy in this study.
The research involved 15 terrestrial rocks and six lunar samples collected by Apollo astronauts.
This outcome aligns with earlier findings, indicating that the Earth and the Moon are indistinguishable in terms of isotope ratios for chromium, calcium, titanium, and zirconium.
However, direct conclusions about Theia are elusive due to their similarities.
The multiplicity of potential collision scenarios also complicates matters.
While most models suggest that the Moon is largely composed of Theia material, it’s also plausible that it consists primarily of early Earth’s mantle material, or a mix of both Earth and Theia rocks.
To explore Theia’s characteristics, researchers employed a method akin to reverse engineering.
They analyzed the isotope ratios of contemporary Earth and Moon rocks to infer the size and composition of Theia, as well as the early Earth composition that resulted in the current state.
The study examined not only iron isotopes but also those of chromium, molybdenum, and zirconium.
Different elements provide insights into various phases of planetary formation.
Before the catastrophic collision with Theia, a sorting process was occurring within the early Earth.
As the iron core formed, elements like iron and molybdenum were sequestered there, almost completely removing them from the rocky mantle.
Thus, the iron found in Earth’s mantle today may have arrived post-core formation, potentially aboard Theia.
Other elements, like zirconium, which did not sink into the core, encapsulate the entire history of Earth’s formation.
Some mathematically feasible compositions of Theia and early Earth can be dismissed as unlikely.
“The most credible scenario suggests that the majority of components in Earth and Theia originated from the inner solar system,” stated Dr. Timo Hopp, a researcher at the University of Chicago and the Max Planck Institute.
“Earth and Theia were likely neighbors.”
“While the early Earth’s composition can be explained primarily through known meteorite mixtures, the same does not hold for Theia.”
“Distinct classes of meteorites formed in various regions of the outer solar system.”
“These provide a reference for the materials accessible during the early formation of Earth and Theia.”
“However, Theia’s composition may also include previously unidentified substances.”
“We hypothesize that this material originated closer to the Sun than to Earth.”
“Thus, our calculations imply that Theia was formed nearer to the sun compared to our planet.”
of result Published in this week’s Science magazine.
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Timo Hopp et al.. 2025. Theia, the impactor that formed the Moon, originated from within the solar system. Science 390 (6775): 819-823;doi: 10.1126/science.ado0623
Source: www.sci.news
