A new study suggests that the Moon formed when Earth collided with a nearby planet, drastically reshaping our early world.
About 4.5 billion years ago, a colossal impact occurred on the young Earth. A massive body struck with such force that fragments were hurled into space, eventually gathering to become the Moon that accompanies us today.
Many details remain uncertain—how big Theia was, what it was made of, and where it originated. The main reason those questions linger is that Theia was obliterated in the collision, leaving behind only the Moon and Earth as witnesses to the event. Yet by examining Moon rocks returned by the Apollo missions, researchers are starting to reconstruct Theia’s composition and possible birthplace.
The latest results come from a collaboration among scientists at the University of Chicago, the Max Planck Institute for Solar System Research, and the University of Hong Kong, and are published in Science (https://doi.org/10.1126/science.ado0623).
According to lead author Timo Hopp, a former UChicago postdoc now with the Max Planck Institute, “the most persuasive scenario is that Earth and Theia shared most of their building blocks, and thus originated in the inner solar system.” He adds, “Earth and Theia were likely neighbors.”
Every celestial body—Earth, the Moon, meteors, and stars—bears a record of its formation, written in isotopes: tiny variations in elemental weights that reveal their origins. These isotopes are forged in stars, but after such material is ejected, it rarely mixes uniformly across the solar system. As Nicolas Dauphas, now at the University of Hong Kong, explains, “Different regions inherited distinct isotopic signatures, which now act as fingerprints to trace the sources of meteorites and other bodies.”
For years scientists have debated the specifics of the Theia–Earth collision. Was the Moon formed solely from Theia’s material, or mostly from material torn from Earth’s mantle, or did the rocks truly mix during the impact? Extracting precise isotope information to answer these questions is incredibly challenging. Isotopes differ by only a few neutrons, and the Moon’s samples are scarce and precious, demanding extraordinary measurement accuracy.
Dauphas’ laboratory specializes in developing methods to measure isotope ratios with high precision. The team compared terrestrial rocks, six lunar samples, and meteorites believed to originate from areas of the solar system where Theia might have formed. They conducted exacting iron measurements and integrated them with existing isotope data for chromium, calcium, titanium, molybdenum, and zirconium. By understanding how these elements behaved during planetary formation, they inferred that a large portion of iron found on Earth today probably came from Theia’s contribution, especially in forming Earth’s core prior to the collision.
These inferences, supported by simulations, helped narrow Theia’s possible origins. The results point away from Theia being a distant interstellar visitor and toward a locale closer to the Sun than Earth—the meteorite signatures most akin to Theia’s ratios come from regions near the Sun.
As Dauphas puts it, “In the early solar system’s cosmic billiards, Earth was struck by a neighbor.” That fortunate strike may have granted Earth a Moon that steadied its tilt, enabling a climate conducive to the development of complex life.
The research was funded by NASA, the National Science Foundation, the US Department of Energy, the Deutsche Forschungsgemeinschaft, and the European Research Council.
Source: University of Chicago (https://news.uchicago.edu/story/moon-may-have-formed-through-earths-collision-close-neighbor-study-finds)
