Yuan, a geophysicist by training, was one day attending a seminar about planet formation given by Mikhail Zolotov, a professor at ASU. Zolotov presented the giant-impact hypothesis, while Qian noted that the moon is relatively rich in iron. Zolotov added that no trace had been found of the impactor that must have collided with the Earth. 

"Right after Mikhail had said that no one knows where the impactor is now, I had a 'eureka moment' and realized that the iron-rich impactor could have transformed into mantle blobs," Yuan said..

The multidisciplinary team of collaborators modeled different scenarios for Theia's chemical composition and its impact with Earth. The simulations confirmed that the physics of the collision could have led to both the LLVPs' formation and the moon's formation. 

"This work showed that the large blobs (the LLVPs) in Earth's deep mantle may be made of materials from a planetary body that impacted the proto-Earth and formed the moon," said Mingming Li, professor at ASU’s School of Earth and Space Exploration. "Therefore, the moon and the blobs have the same origin."

Given such a violent impact, why did Theia's material clump into the two distinct blobs instead of mixing together with the rest of the forming planet? The researchers' simulations showed that much of the energy delivered by Theia's impact remained in the upper half of the mantle, leaving the lower mantle of the Earth cooler than estimated by earlier, lower-resolution impact models. Because the impact did not totally melt the lower mantle, the blobs of iron-rich material from Theia stayed largely intact. 

"Through mantle convection simulations, we found that the dense, iron-rich materials from Theia could sink to and accumulate at the base of Earth's mantle. These materials could stay there throughout Earth's history of about 4.5 billion years," Li said.

"By looking inward, towards Earth's interior, instead of outward, towards the moon, we have found yet another piece of evidence of the cosmic catastrophe that is the moon-forming giant impact," said co-author Travis Gabriel, of the U.S. Geological Survey. 

"The study really places Earth into the context of the inner solar system formation," said co-author Hongping Deng of the Chinese Academy of Sciences. "Imagine that if we managed to recover some Theia signals from the deepest mantle, we could better understand the architecture and composition of the infant solar system without turning to present-day meteorites with messed-up signals.”

Additional authors on this study include ASU alumni Byeongkwan Ko, a postdoc at Michigan State University; Paul Asimow and Yoshinori Miyazaki at Caltech; Jacob Kegerreis of NASA Ames Research Center; and Vincent Eke of Durham University. 

This work was supported 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.

This press release was written by Caltech with contributions from Kim Baptista from ASU's School of Earth and Space Exploration.

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