Key Takeaways:

  1. Origin of the Moon: Scientists propose that remnants of the protoplanet Theia, which collided with Earth 4.5 billion years ago, may be deep within Earth’s mantle.
  2. Mysterious Layers: Two massive rock formations, known as Large Low-Shear Velocity Provinces (LLSVPs), below West Africa and the Pacific Ocean, are believed to hold clues about Theia’s remains.
  3. Seismic Clues: Seismic waves passing through the LLSVPs indicate they are denser and chemically distinct, fueling speculation about their extraterrestrial origin.
  4. Isotopic Evidence: Isotopic evidence from Iceland and Samoa supports the idea that LLSVPs have existed since the Moon-forming impact, offering credibility to the proposal.
  5. Reevaluation of Impact: Recent studies suggest Theia, the impactor, may have been as large as Earth, challenging previous assumptions about its size and composition.

Scientists widely agree that the Moon resulted from a colossal impact between Earth and a protoplanet named Theia around 4.5 billion years ago. Recent research proposes a compelling theory that remnants of Theia may be concealed in two massive layers of rock submerged deep within Earth’s mantle.

Seismologists have long grappled with these enormous structures, known as large low-shear velocity provinces (LLSVPs), situated beneath West Africa and the Pacific Ocean. Spanning up to 1000 kilometers in height and several times that in width, they represent the most substantial features within Earth’s mantle. Seismic waves passing through these layers experience a sudden slowdown, indicating their density and chemical dissimilarity from the surrounding mantle rock.

Referred to as “continent-size blobs,” the LLSVPs have provoked speculation about their origin for years. The prevailing theories propose that they crystallized from Earth’s primordial magma ocean or formed as dense pockets of primitive mantle rock surviving the Moon-forming impact. However, recent isotopic evidence and modeling suggest that the LLSVPs might, in fact, be the remnants of Theia itself, presenting a bold and intriguing hypothesis.

The proposal, presented by Qian Yuan, a Ph.D. student in geodynamics at Arizona State University, challenges conventional notions. According to Yuan, this radical idea gains credence through multiple lines of evidence, marking a departure from previous speculations that lacked comprehensive support. The seismic imaging of plumes of magma in Iceland and Samoa, connected to the LLSVPs, adds weight to the argument, as these plumes trace back to the time of the Moon-forming impact.

Building on the conventional impact theory developed in the 1970s, recent studies suggest that Theia, the impacting protoplanet, might have carried dense rock deep into Earth. Contrary to earlier beliefs about Theia’s size, newer research by Steven Desch, an astrophysicist at ASU Tempe, indicates that Theia could have been nearly as large as Earth. This challenges previous assumptions and provides a new perspective on the scale of the Moon-forming impact.

Desch’s work on hydrogen-to-deuterium ratios in Apollo Moon rocks reveals clues about Theia’s composition, suggesting a massive and dry protoplanet. The simulations conducted by Yuan align perfectly with Desch’s findings, indicating that mantle rocks with a density 1.5% to 3.5% greater than Earth’s could have endured, settling near Earth’s core.

The proposal also sheds light on the possible explanation for the scale of the LLSVPs, which collectively contain six times more mass than the Moon. Yuan suggests that only an impactor as substantial as Theia could account for their size. However, the idea is not without its caveats, including uncertainties surrounding the LLSVPs’ existence and structure.

Questions linger about the accuracy of seismic imaging and the possibility that the pilelike structure of the LLSVPs is an illusion. Barbara Romanowicz, a seismologist at UC Berkeley, suggests that the piles might not reach the reported heights and could be a bundle of tubes rather than continuous structures.

Further complicating matters is the suggestion that smaller or less monolithic LLSVPs may be more consistent with a forthcoming analysis. This analysis indicates that the LLSVPs are densest at the bottom, raising doubts about Theia’s purported size and the impact hypothesis.

To validate the proposed theory, researchers could compare the geochemical characteristics of island lavas with rocks from the Moon’s mantle. However, obtaining unaltered mantle samples remains a challenge, highlighting the importance of future lunar missions, including NASA’s and China’s plans to explore the Moon’s south pole.

If Theia’s remnants do reside deep in Earth’s mantle, it opens the possibility that similar remnants from other protoplanets may also exist. Seismologists have identified small, ultradense pockets near the LLSVPs, hinting at the potential presence of iron-rich cores from other miniature planets that collided with early Earth. In this intriguing narrative, Theia becomes just one component of a planetary cemetery waiting to be explored further.

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