Protoplanet remnant may hide inside Earth
Aprotoplanet slammed into Earth about 4.5 billion years ago, knocking loose a chunk of rock that would later become the Moon. Now scientists say remnants can be found lodged deep inside Earth. If remains of the protoplanet, Theia, did stick around after the impact, that may explain why two continent-size blobs of hot rock now lie in Earth’s mantle, one beneath Africa and the other under the Pacific Ocean. These massive blobs would stand about 100 times taller than Mount Everest were they ever hauled up to Earth’s surface.
Theia’s impact both formed the Moon and transformed Earth’s surface into a roiling magma ocean, and some scientists theorise that the blobs formed as that ocean cooled and crystallised. Others think the blobs contain Earth rocks that somehow escaped the effects of the collision and nestled, undisturbed for millions of years, near the planet’s centre.
At the Lunar and Planetary Science Conference, Qian Yuan, a doctoral student in geodynamics at Arizona State University, presented an alternate hypothesis. He proposed that after the Moon-forming impact, dense material from Theia’s mantle descended deep beneath Earth’s surface, accumulating into what we now know as ‘the blobs’.
According to Yuan’s models, rocks that are 1.5 to 3.5 per cent denser than Earth’s mantle would not mix into the surrounding rock. They would sink to the bottom of the mantle, near the inner core. “This crazy idea is at least possible,” said Yuan.
A 2019 study supports the idea that Theia’s mantle was denser than Earth’s by around two to 3.5 per cent. Conclusions of this study were drawn about Theia’s size and chemical composition based on an analysis of Apollo Moon rocks, which contained a far higher ratio of light hydrogen to heavy hydrogen than
Earth rocks.
To supply the Moon with so much light hydrogen, Theia must have been very large, nearly the size of Earth at the time of impact, and very dry, since water formed in interstellar space would contain a heavy form of hydrogen called deuterium, which Theia lacked. Meanwhile, the interior of the hulking protoplanet would have held a dense, iron-rich mantle.
Per Yuan’s theory, while the lighter rocks hurtled into space to form the Moon, chunks of the iron-rich mantle would have barrelled down towards the Earth’s core in the wake of Theia’s impact, where they settled and formed the enigmatic blobs.