Rocks from the Moon-Forming Impact Still Hidden in Earth's Mantle

Rocks from the Moon-Forming Impact Still Hidden in Earth's Mantle

Around 4.5 billion years ago, a celestial catastrophe of epic proportions unfolded, shaping the destiny of our planet and setting the stage for the formation of the moon. This mind-boggling collision, in which an object roughly the size of Mars known as Theia struck the young Earth, left a lasting mark on our celestial history. Recent research, published on November 1 in the journal Nature, sheds new light on the intriguing aftermath of this cosmic event.

The prevailing hypothesis suggests that the collision between Earth and Theia unleashed a torrent of debris, much of which eventually coalesced to form our beloved lunar companion. However, as scientists delve deeper into the mysteries of the Earth's composition, they've uncovered continent-sized regions within the Earth's mantle that defy conventional wisdom. These enigmatic zones exhibit abnormally slow seismic wave travel, indicating that the rock there is denser than the surrounding mantle rock. One of these regions, named "large low-velocity provinces," lies beneath Africa, while its counterpart resides beneath the vast expanse of the Pacific Ocean, according to Qian Yuan, a planetary geodynamic researcher at Caltech.

For years, researchers contemplated whether these massive regions originated from tectonic plates that descended into the Earth's mantle. However, Yuan and his team propose a different and thought-provoking narrative. To address the enigma of the missing Theia, they turned to supercomputer simulations to recreate the cataclysmic collision between a young Earth and an object approximately 10% of its size.

In these intricate simulations, both the Earth and Theia were equipped with dense iron cores encased in lighter rock mantles. Each object was digitally represented as particles about six miles across, allowing the researchers to track the post-collision fragments. Impressively, they analyzed approximately 100 million particles in total.

The simulations suggest that a significant portion of Theia's core, approximately 3% of Earth's mass today, remained on our planet. Soon after the collision, this dense material would have submerged, merging with the Earth's core. Meanwhile, a substantial portion of Theia's mantle, equivalent to around 5% of Earth's mass, integrated into Earth's mantle, which extends over 900 miles.

The remnants of Theia in Earth's mantle contain higher concentrations of iron oxide minerals, signifying that it was denser than Earth's mantle. Over tens of millions of years following the collision, this denser material descended, accumulating to form the large low-velocity provinces we observe today.

The notion that these low-velocity provinces may have originated from Theia's remains is a relatively new concept, and it brings a fresh perspective to the debate. While some scientists have suggested alternative origins for these regions, the enduring mystery surrounding the moon's formation continues to captivate the scientific community.

Whether or not these provinces are indeed remnants of Theia's impact, one thing is clear: they have endured nearly 4.5 billion years of Earth's tumultuous history. These regions may be so dense that they remain distinct from the surrounding mantle, surviving the relentless passage of geological time.

The "giant impact hypothesis," which posits a colossal collision between Earth and a protoplanet as the moon's origin story, remains the leading theory. This theory has not only helped explain the moon's formation but also contributed to understanding the subtle chemical distinctions between moon rocks and Earth rocks. The ongoing exploration of this extraordinary event and its consequences continually unveils the remarkable tales hidden within our planet's geological history, shedding new light on the celestial dance of the cosmos.


Q. Yuan et al. Moon-forming impactor as a source of Earth’s basal mantle anomalies. Nature. Published online November 1, 2023. doi:10.1038/s41585-023-06589-1.

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