These structures, known as large low-velocity provinces (LLVPs), are massive regions of dense material located near the Earth's core, beneath the African and Pacific plates. Detected initially in the 1980s, LLVPs have challenged scientists due to their unusual properties-most notably their size, which is comparable to twice that of the Moon, and their composition, which seems to include a different proportion of elements than the surrounding mantle.
Qian Yuan, a postdoctoral scholar at Caltech, and a team of scientists conducted simulations to probe the origins of these enigmatic blobs. They have hypothesized that these could be the remains of Theia, an early planet that, according to the giant-impact hypothesis, collided with our nascent Earth, leading to the formation of the Moon.
"For years, the cosmic puzzle of Theia's fate has perplexed us," explains Yuan, who experienced a breakthrough during a seminar on planet formation. It was there that the idea struck-what if Theia's remains were actually within Earth, as opposed to being scattered in the solar system?
The simulations by Yuan and his team, which also involved input from professors Paul Asimow and Michael Gurnis, explored various scenarios of the aftermath of Theia's collision with Earth. Their models supported the notion that the collision's energy distribution resulted in Theia's iron-rich material sinking into Earth's lower mantle, forming the LLVPs, while other debris eventually aggregated to become the Moon.
The seismic properties of LLVPs have always been peculiar, with seismic waves slowing down as they pass through these dense regions. This study provides a plausible explanation for their distinctive characteristics, suggesting that the impact with Theia deposited a large iron content in these regions, distinguishing them from the rest of Earth's mantle.
As to why Theia's remnants formed two distinct blobs rather than mixing uniformly with Earth's mantle, the research points to the distribution of energy during the impact. Most of the energy remained in the upper mantle, keeping the lower mantle cooler and preventing complete mixing. Thus, Theia's material, being iron-rich, clumped together near the core-mantle boundary, like a lava lamp that has been switched off.
This new understanding has opened avenues for further investigations into how the presence of Theia's remnants influenced the early dynamics of Earth's interior. "A logical consequence of the idea that the LLVPs are remnants of Theia is that they are very ancient," notes Asimow. The research team plans to explore how these ancient structures may have impacted the onset of Earth's geophysical processes, including plate tectonics, the formation of continents, and the development of Earth's oldest minerals.
The implications of this research are significant for our understanding of planetary formation and the early history of Earth. If substantiated by further evidence, the existence of Theia within Earth's mantle could revolutionize our perception of terrestrial planet formation and the violent, yet ultimately constructive, events that shape them.
The paper is titled "Moon-forming impactor as a source of Earth's basal mantle anomalies." Qian Yuan is the first author. In addition to Yuan and Asimow, the additional Caltech coauthor is Yoshinori Miyazaki, Stanback Postdoctoral Scholar Research Associate in Comparative Planetary Evolution. Additional coauthors are Mingming Li, Steven Desch, and Edward Garnero (PhD '94) of Arizona State University (ASU); Byeongkwan Ko of ASU and Michigan State University; Hongping Deng of the Chinese Academy of Sciences; Travis Gabriel of the U.S. Geological Survey; Jacob Kegerreis of NASA's Ames Research Center; and Vincent Eke of Durham University. Funding was provided by the National Science Foundation, the O.K. Earl Postdoctoral Fellowship at Caltech, the U.S. Geological Survey, NASA, and the Caltech Center for Comparative Planetary Evolution.
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