Earth’s unusually large moon likely formed from a violent collision with a Mars-sized planet named Theia about 4.5 billion years ago. New geochemical analyses of Apollo lunar samples and terrestrial rocks indicate that Theia and early Earth originated from a similar region of the inner solar system.
For decades, astronomers debated how Earth acquired a satellite measuring a little over a quarter of its own diameter. Most rocky planets in the solar system have tiny moons or none at all. Mercury and Venus possess no moons, while Mars holds two small, lumpy bodies widely believed to be captured asteroids. Earth’s moon stands apart as the largest relative to its host planet anywhere in the system.
Disproving Competing Theories with Apollo Samples
Early attempts to explain the anomaly relied on three distinct scenarios. One theory proposed that the moon formed elsewhere and wandered into Earth’s gravitational grasp. Another suggested a rapidly spinning primordial Earth flung off a piece of itself, while a third argued both bodies formed side by side from the same cloud of dust.
Those ideas collapsed when scientists compared the chemistry of rocks brought back by Apollo astronauts to samples retrieved from Earth’s mantle. The compositional similarities were too specific to support an interloper captured from afar or a coincidental side-by-side birth.
The Giant Impact Hypothesis and Theia
The surviving model is the giant-impact hypothesis, which posits that a Mars-sized world dubbed Theia struck the early Earth at a glancing angle. That cataclysm vaporized rock and blasted a ring of debris into orbit, which coalesced into the moon. Simulations run at NASA’s Ames Research Center suggest the process may have unfolded rapidly, with debris clumping into a moon-sized body within hours.
“This opens up a whole new range of possible starting places for the Moon’s evolution.”
Jacob Kegerreis, postdoctoral researcher at NASA’s Ames Research Center
A new study published Nov. 20 in the journal Science adds detail to that origin story by examining where Theia itself formed. Researchers analyzed six lunar samples from the Apollo 12 and 17 missions alongside 15 terrestrial rocks, including specimens from the Kīlauea volcano in Hawaii and Antarctic meteorites.
Tracing Iron and Molybdenum Signatures
Led by geoscientist Timo Hopp at the Max Planck Institute for Solar System Research in Germany, the research team measured subtle differences in iron isotopes, molybdenum, and zirconium. Elements like iron and molybdenum normally sink into a planet’s core during formation; their presence in modern mantle rocks implies they arrived later, carried by Theia during the collision.
“Theia and proto-Earth come from a similar region of the inner solar system.”
Timo Hopp, geoscientist at the Max Planck Institute for Solar System Research
According to the study’s models, Theia was a rocky, metal-cored world containing roughly 5 to 10% of Earth’s mass. The findings align with classical theories of terrestrial planet assembly, which picture the early inner solar system crowded with dozens to hundreds of planetary embryos colliding and merging under the gravitational influence of Jupiter and neighboring bodies.
Tracking the Moon’s Retreat from Earth
Since its formation, the moon has exerted a steady gravitational pull on Earth’s oceans and crust, creating tidal bulges. Because Earth rotates faster than the moon orbits, those bulges sit slightly ahead of the moon, transferring rotational energy and slowing Earth’s rotation by about 1.5 milliseconds per century while pushing the moon outward by roughly 3.8 centimeters per year.
“When the moon first formed some 4.5 billion years ago, the day was less than 10 hours long. But since then, the moon’s gravitational pull on the Earth has been slowing our planet’s rotation, resulting in an increasingly longer day.”
Norman Murray, theoretical astrophysicist at the University of Toronto’s Canadian Institute for Theoretical Astrophysics
Geologists studying ancient sediment layers and tidal rhythmite bands in South Australia have confirmed that a day lasted 21.9 hours roughly 620 million years ago. Meanwhile, space agencies have tracked the moon’s ongoing retreat using laser reflectors left on the lunar surface by Apollo astronauts.
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