Does Remnants of the Celestial Body that Impacted Earth and Gave Rise to the Moon Still Exist within Our Planet?

Recent studies propose that the remnants of the substantial object that crashed into the early Earth, resulting in the moon’s formation, can still be discerned in the Earth’s interior as two sizable masses. These masses constitute approximately 8% of the Earth’s mantle volume, the rocky region situated between the planet’s iron core and its outer crust.

Conducted by Qian Yuan from Arizona State University and Caltech, the recent research contends that the heat produced by this collision lacked the intensity to entirely liquefy the Earth’s mantle. As a result, the innermost mantle retained its solid state.

As a result, the scientists posit that the molten mantle of Theia did not undergo total assimilation with Earth’s mantle. Such amalgamation would have rendered the remnants of Theia indiscernible from the entirety of Earth’s mantle. Instead, a substantial portion of Theia’s mantle consolidated into two masses of continental scale, presently situated atop the boundary between Earth’s core and mantle.

Huge low-speed zones at the bottom of Earth’s interior

Yuan contends that these masses align with and offer an explanation for the presence of the two extensive low-velocity provinces (LLVPs) identified decades ago—specifically, one beneath the Pacific region and another beneath Africa and the eastern Atlantic.

The identification of this phenomenon resulted from noticing that seismic waves, the vibrations generated by earthquakes, exhibit a slightly slower passage through these areas compared to the “Normal” lower mantle.

Earlier interpretations of the LLVPs posited that they might be extensive accumulations of subducted oceanic plates, drawn beneath continents by plate tectonics. Another hypothesis suggested these regions could be where abnormally hot lower mantle initiates an ascent, resembling a “Superplume” characterized by substantial columns of partially molten rock.

Yet, neither of these models adequately explains an unusual concentration of volatile elements, such as helium and xenon, observed in lava from volcanic eruptions on oceanic islands situated above LLVPs. Yuan asserts that these anomalies serve as “Fingerprints” indicative of Theia’s formation within the gas and dust surrounding the early sun before its collision with Earth.

To melt or not to melt: Contemplating the fate of the entire mantle

The computer simulations conducted by Yuan’s research team propose that the colossal impact responsible for moon formation didn’t supply sufficient energy to entirely liquefy Earth’s mantle. Instead, the molten remnants of Theia’s mantle, slightly higher in iron content and denser than Earth’s mantle, accumulated at the bottom of the transient magma ocean formed by the collision.

Subsequently, following the solidification of the magma ocean, material from Theia was pulled into the lower section of Earth’s mantle through convection currents. These currents, persisting even within the solid mantle, move at rates on the order of centimeters per year.

The accumulation of Theia material into the present-day LLVPs may have transpired over billions of years through convection currents. However, it’s crucial to perceive them not as substantial remnants of Theia’s mantle persisting from the impact but as formations composed of Theia mantle material initially scattered and subsequently reassembled.

Can this be confirmed?

Many scientists are likely to approach this theory with skepticism. Yuan anticipates that if his hypothesis holds true, future missions collecting samples of the moon’s mantle will exhibit geochemical signatures consistent with those found in volcanic rock from the LLVPs. However, concrete proof is expected to take a considerable amount of time.

Additionally, it’s worth mentioning that Yuan’s modeling does not explicitly address the destiny of Theia’s core. Typically, scientists posit that Theia’s core amalgamated with Earth’s core in the hours following the collision.

The mechanism by which this amalgamation occurred is uncertain, especially if the lower segment of Earth’s mantle retained its solidity. However, considering that Theia’s impact took place shortly after the formation of Earth, potentially following a sequence of distinct collisions, the interior of the Earth might still have been sufficiently hot and molten in the aftermath of those events.

The ramifications of Yuan’s model prompt contemplation. One consideration is whether the gradual accumulation of Theia mantle material into the LLVPs could have influenced the configuration of plate tectonics far above. It raises the possibility that, without the collision of Theia with the proto-Earth four and a half billion years ago, the present-day Atlantic Ocean might not have existed.

Resources

1. ^NEWSPAPER Rothery, D. & The Conversation. (2023, November 12). Is some of the body that collided with Earth to form the moon still recognizable inside our planet? Phys.org. [Phys.org]
2. ^JOURNAL Yuan, Q., Li, M., Desch, S. J., Ko, B., Deng, H., Garnero, E. J., Gabriel, T. S. J., Kegerreis, J., Miyazaki, Y., Eke, V. R., & Asimow, P. D. (2023). Moon-forming impactor as a source of Earth’s basal mantle anomalies. Nature, 623(7985), 95–99. [Nature]