New research analyzing metallic deposits deep within the lunar surface has provided compelling support for the theory that the moon formed from debris created when a Mars-sized body collided with early Earth.
This ancient cosmic impact, believed to have taken place about 4.5 billion years ago, shattered a protoplanet named ‘Theia,’ launching molten rock fragments into space.
Tracing Theia’s Impact: Lunar Metal Deposits Link Moon to a Lost Planet
While some fragments from Theia are thought to remain embedded beneath Earth's crust, particularly under Africa and the Pacific tectonic plates, the fate of much of its material has been uncertain.
However, recent findings from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission reveal vast reserves of titanium-iron ore locked deep under the moon’s surface, suggesting these are remains of Theia that fused to form the moon.
Planetary geophysicist Adrien Broquet from the German Aerospace Center in Berlin described the GRAIL data as truly captivating.
Published in Nature Geoscience this April, Broquet’s team focused on studying anomalies in the moon’s gravitational field—dense, heavy pockets detected by GRAIL’s instruments.
“Examining these gravity variations gives us a window beneath the lunar surface to uncover hidden structures,” Broquet explained.
The GRAIL mission pinpointed two unusually dense regions within the moon’s mantle, matching titanium-rich ilmenite deposits expected if the moon contains Theia’s remnants.
Following Theia’s collision and accretion, molten titanium and iron accumulated below the lunar crust before sinking into its core, displacing lighter rock upward.
“The moon essentially flipped its internal layers,” said Jeff Andrews-Hanna, a geophysicist at the University of Arizona and co-author of the study.
Computer simulations by Nan Zhang from Peking University initially predicted the presence of titanium-enriched material deep inside the moon as a consequence of Theia’s debris merging.
“The models illuminated a distinctive pattern,” Andrews-Hanna noted. “Seeing that pattern reflected in the moon’s gravity was a major breakthrough.”
Insights into Earth's Formative Years and Lunar Genesis
On Earth, two dense structures near the mantle’s base, known as Large Low Velocity Provinces (LLVPs), reinforce the hypothesis that the moon originated from Theia’s collision.
One LLVP lies beneath the African plate and another beneath the Pacific plate, identified through seismic wave studies that detect variations in earthquake propagation.
These zones significantly slow seismic waves at roughly 1,800 miles (2,900 km) depth, indicating materials denser than the surrounding mantle.
Scientists estimate LLVP materials to be about 2–3.5% denser than the Earth’s adjacent core regions.
Researchers from Caltech recently proposed that these LLVPs are remnants of Theia’s material that merged into Earth’s lower mantle during the ancient collision.
To test this, Professor Hongping Deng of the Shanghai Astronomical Observatory employed advanced computational fluid dynamics simulations.
His models demonstrated that roughly 2% of Earth’s mass, comprised of Theian debris, likely penetrated the deep mantle following the moon-forming impact.
“By combining detailed rock analyses with improved impact and evolutionary models, we can better understand the composition and dynamics of the primordial Earth, ‘Gaia,’ and Theia,” explained Qian Yuan, a Caltech geophysicist and co-researcher.
The pair’s findings were published in Nature in late 2023.
Broquet expressed optimism that upcoming lunar missions, such as NASA's Artemis program, will help gather novel seismic data to further validate the Theia impact scenario.
“Deploying a network of seismic instruments on the moon would provide critical insights into these subterranean structures,” he said.
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