The inside of the Earth is full of blobs from another planet that crashed into our world during its early life. In the 1980s, geophysicists uncovered a remarkable revelation: Two massive blobs of unusual material lurking deep within the Earth's core, each of a continental scale, one positioned beneath the African continent and the other beneath the Pacific Ocean. These mysterious masses, each double the size of the Moon, have captivated the scientific community for the past decade due to their distinct elemental composition when compared to the surrounding mantle. The question that has perplexed experts is, where did these enigmatic entities, formally known as large low-velocity provinces (LLVPs), originate? However, a new study suggests that they are remnants of an ancient planet that collided violently with Earth billions of years ago, a colossal impact event that was instrumental in the formation of our Moon. A collaborative team of scientists, featuring researchers from the Arizona State University (ASU) School of Earth and Space Exploration, led by ASU alumnus Dr. Qian Yuan, who presently serves as an O.K. Earl Postdoctoral Fellow at Caltech, has shed light on this longstanding planetary mystery. Their findings, published in the prestigious journal Nature on November 1, 2023, also provide a solution to another long-standing enigma in planetary science. For over two decades, scientists have been aware of the existence of these colossal blobs within Earth's interior, marked by their peculiar composition and an elusive origin. These structures are so substantial that, if placed on the Earth's surface, they would form a layer approximately 60 miles (100 km) thick, enveloping the entire planet. Furthermore, it has been postulated that the Moon's creation resulted from a titanic collision between Earth and a smaller celestial body known as Theia. Yet, no trace of Theia has ever been identified in the asteroid belt or meteorites. The recent study proposes that a substantial portion of Theia became integrated into the young Earth, forming the LLVP blobs, while residual debris from the collision coalesced to give birth to our Moon. "The Moon seems to bear materials that are representative of both the Earth prior to the impact and Theia. It was previously believed that any remnants of Theia within Earth would have been obliterated and homogenized by the Earth's dynamic processes over billions of years, such as mantle convection," explained Steven Desch, a professor at ASU's School of Earth and Space Exploration. "This study is the first to argue that distinct 'pieces' of Theia still exist within the Earth, at its core-mantle boundary." The initial discovery of the LLVPs was made by monitoring seismic waves traversing the Earth. These waves travel at varying speeds through different materials, and in the 1980s, scientists discerned indications of extensive three-dimensional anomalies deep within the Earth's structure. In the deepest mantle, the seismic wave patterns exhibited the imprints of two enormous structures, on the scale of continents, situated near the Earth's core. These areas were interpreted as regions with an unusually elevated iron content, which, owing to their heightened density and temperature, decelerated the passage of seismic waves. This is the basis for the moniker "large low-velocity provinces." "But now, we have a plausible explanation for them. It seems that these Earthly blobs are remnants of a celestial collision that gave birth to our Moon," elucidated Ed Garnero, a professor at ASU's School of Earth and Space Exploration. "In other words, the colossal blobs within Earth, hidden deep beneath our feet, are of extraterrestrial origin. Earth doesn't just house 'blobs'; Earth possesses extraterrestrial blobs!" The concept took root when Yuan, originally a geophysicist, attended a seminar on planet formation conducted by Professor Mikhail Zolotov at ASU. Zolotov introduced the hypothesis of a colossal impact, and Yuan observed that the Moon is relatively rich in iron. Zolotov noted that no signs of the impactor responsible for colliding with Earth had ever been located. "Right after Mikhail mentioned that no one knows the whereabouts of the impactor, I experienced a 'eureka moment' and recognized that the iron-rich impactor could have transformed into these mantle blobs," recalled Yuan. The multidisciplinary team of collaborators formulated various scenarios regarding Theia's chemical composition and its collision with Earth. Their simulations confirmed that the physics of the collision could account for the formation of the LLVPs and the Moon. A proportion of Theia's mantle may have remained within Earth's mantle, eventually coalescing and crystallizing to form the two distinct blobs that are currently discernible at the Earth's core-mantle boundary, while other remnants from the collision fused to give rise to the Moon. "This research demonstrates that the large blobs, known as large low-velocity provinces (LLVPs), in the Earth's deep mantle may comprise materials from a celestial body that collided with the young Earth and subsequently formed the Moon," remarked Mingming Li, a professor at ASU's School of Earth and Space Exploration. "Hence, the Moon and the LLVPs share the same origin." In light of such a colossal impact, why did Theia's material congeal into these two distinct blobs instead of mingling with the rest of the nascent planet? The simulations conducted by the researchers revealed that a substantial portion of the energy imparted by Theia's impact remained in the upper section of the mantle, thereby cooling the Earth's lower mantle more than previous, lower-resolution impact models had suggested. Because the impact failed to fully liquefy the lower mantle, the iron-rich materials from Theia remained predominantly intact. "Through mantle convection simulations, we discovered that the dense, iron-rich materials from Theia could descend and accumulate at the base of Earth's mantle. These materials have likely persisted throughout Earth's 4.5 billion-year history," as Mingming Li elucidated. "By focusing our attention on Earth's interior, rather than on the Moon, we have unearthed another piece of evidence relating to the monumental cosmic collision that resulted in the formation of the Moon," stated co-author Travis Gabriel of the U.S. Geological Survey. "This study places Earth within the context of the formation of the inner solar system," added co-author Hongping Deng of the Chinese Academy of Sciences. "Imagine that if we could detect traces of Theia deep within the mantle, we could gain a more profound understanding of the architecture and composition of the nascent solar system, without relying on contemporary meteorites with distorted signatures." The list of additional authors on this research includes ASU alumnus Byeongkwan Ko, a postdoctoral researcher at Michigan State University, as well as Paul Asimow and Yoshinori Miyazaki at Caltech, Jacob Kegerreis from NASA Ames Research Center, and Vincent Eke from Durham University. The study received financial support from the National Science Foundation (NSF), 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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