A Mars-sized planet that collided with Earth 4.5 billion years ago and gave birth to the moon may have left two mᴀssive chunks of itself deep in Earth’s mantle, according to new research.

Scientists have long agreed on the presence of Theia and its involvement in the formation of the moon. According to one theory, Theia collided with Earth early in its existence and shattered a fragment of rock that became the moon. According to a recent study headed by Qian Yuan, a geodynamics researcher at Arizona State University (ASU), Tempe, Theia’s remnants are still within Earth, most likely in two continent-sized layers of rock under West Africa and the Pacific Ocean.

For decades, seismologists have been examining these two rock layers. They discovered that seismic waves from earthquakes dramatically slow down when they travel through the layers, implying that they are denser and chemically distinct from the surrounding mantle rock. Seismologists refer to them as large low-shear velocity provinces (LLSVPs). They have a combined mᴀss of nearly six times that of the moon.

“They are the largest thing in the Earth’s mantle,” Yuan said when presenting his work last week at the 52nd Lunar and Planetary Science Conference 2021.

Yuan says the LLSVPs represent the remnants of Theia based on isotopic data and modeling. “You could say that these are the biggest and largest meteorites if they are mostly Theia’s mantle. It’s very cool,” he told Vice.

According to Yuan’s research, Theia’s core joined with Earth’s shortly after the impact 4.5 billion years ago. His model then attempts to determine the circumstances under which Theia’s mantle would have sunk to the current location of the two LLSVPs rather than merging in with Earth’s mantle. According to simulations, Theia’s mantle is required to be 1.5 percent to 3.5 percent denser than Earth’s in order to survive mixing and wind up as distinct lumps near Earth’s core.

The conclusion is eerily similar to the results of a 2019 research on Theia’s participation in the formation of the moon done by Yuan’s colleague, ASU Tempe astronomer Steven Desch.

The ages of the LLSVPs in issue are also consistent with the Theia collision idea. According to Science Magazine, geochemists have revealed that lava in Iceland and Samoa have an isotopic record of radioactive elements that originated during the first 100 million years of Earth’s existence, a time during which the moon formed. (The Earth has a maximum age of 4.54 billion years.)

When scientists retrieve undamaged rocks from the moon’s mantle, they will have further proof. These rocks are thought to reside in a mᴀssive impact crater on the moon’s south pole, which NASA and China both want to examine this decade.

Reference(s): Science.org, VICE