The giant-impact hypothesis, the leading explanation for the Moon’s formation, proposes that about 4.4 to 4.5 billion years ago, a Mars-sized protoplanet named Theia collided with the early Earth. This colossal impact vaporized and melted much of both bodies, ejecting debris that eventually coalesced into the Moon. But what happened to Theia itself? Did it vanish entirely, or could remnants of this ancient planet still lurk deep within Earth today?
Recent research suggests that substantial portions of Theia’s mantle may have survived the violent collision and now reside deep within Earth’s lower mantle. Seismic studies have identified two massive, dense regions known as large low-shear-velocity provinces (LLSVPs) near the core-mantle boundary. These “blobs” are roughly twice the mass of the Moon each and exhibit unusual seismic signatures that differ from the surrounding mantle. Computer simulations of the giant impact indicate that while Theia’s iron core likely sank and merged with Earth’s core, parts of its mantle-being denser and compositionally distinct-could have resisted complete mixing and settled into these deep mantle reservoirs. This scenario is akin to wax blobs in a lava lamp, where cooler, denser material remains separate and stable over billions of years.
The idea that Theia’s remnants persist inside Earth helps explain some puzzling geochemical signatures found in ancient mantle-derived rocks, which show isotopic compositions hinting at material originating from the outer Solar System-consistent with Theia’s hypothesized origin beyond Earth’s orbit. It also sheds light on Earth’s early geodynamics, possibly influencing mantle convection, plate tectonics, and the formation of Earth’s earliest continents.
While direct sampling of these deep mantle regions is currently impossible, the combination of seismic imaging, geochemical analysis, and advanced impact simulations provides compelling indirect evidence that Theia’s legacy endures beneath our feet. Far from being completely obliterated, Theia’s core merged with Earth’s, and its mantle left a lasting imprint, making Earth a hybrid planet forged from two ancient worlds.
In summary, yes-part of Theia likely still exists inside Earth today, hidden deep in the lower mantle, quietly shaping our planet’s interior and offering a fascinating glimpse into the chaotic origins of the Earth–Moon system.
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[1] https://en.wikipedia.org/wiki/Theia_(hypothetical_planet)
[2] https://en.wikipedia.org/wiki/Giant-impact_hypothesis
[3] https://www.astronomy.com/science/theia-slammed-into-earth-left-marks-and-then-formed-the-moon-study-suggests/
[4] https://www.nasa.gov/solar-system/collision-may-have-formed-the-moon-in-mere-hours-simulations-reveal/
[5] https://thedebrief.org/researchers-find-evidence-of-a-lost-planet-theia-deep-within-earths-core-shedding-light-on-the-moons-mysterious-origins/
[6] https://payloadspace.com/researchers-find-possible-traces-of-theia-under-the-earths-surface/
[7] https://www.oxfordstudent.com/2023/11/21/earths-past-collision-with-theia/
[8] https://www.reddit.com/r/askscience/comments/17lz6cy/i_was_just_reading_up_on_the_ancient_theia_planet/