What connect peanut butter, cosmic crashes, and earthquake waves? They could all contribute to solving the mystery of why Earth’s deepest secrets are stranger and more alien than anyone ever dreamed, according to a daring new hypothesis.
Seismic imaging has long shown two continent-scale anomalies hiding at the bottom of Earth’s mantle, under Africa and the Pacific Ocean. These complexes, referred to as large low-velocity provinces (LLVPs), are so extensive that if they were found at the surface, they would roughly 100 kilometers thick around the entire planet. Their characteristic feature is their capacity to retard seismic waves, something that has turned them into a nagging mystery ever since they were first discovered in the 1980s. But a new paper, out this week in Nature, is making waves by suggesting that these LLVPs are not mere chemical anomalies these might be the last surviving fragments of Theia that slammed into Earth and created the Moon deep within Earth’s mantle lie two enormous, continent-sized structures known as LLVPs.
The giant-impact theory, the best so far for the origin of the Moon, assumes that a body, Theia, roughly the same size as Mars, collided with the proto-Earth about 4.5 billion years ago. Although the origin of the Moon from the wreckage of this impact is accepted almost universally, Theia’s fate was not a mystery that puzzled scientists. Everybody supposed that whatever survived was completely mixed into the Earth’s interior, obliterated by millennia of mantle convection. But as Dr. Qian Yuan, author of the new work, describes, “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’s realization initiated a cross-disciplinary study using high-resolution smoothed particle hydrodynamics (SPH) simulations in conjunction with geochemical and seismic data.
These new SPH models, with 100 to 1,000 times more resolution than previous attempts, modeled the Moon-forming impact in unprecedented detail. The simulations were dramatic: a considerable portion of Theia’s iron-enriched mantle might have survived impact, sinking into Earth’s lower mantle where it accreted into dense, stable anomalies the very LLVPs found by seismic tomography. The models suggest that Theia’s mantle material is 2–3.5% denser than the surrounding mantle, a margin enough to hold these pieces in place for billions of years on top of the core-mantle boundary LLVPs may represent buried relics of Theia mantle material (TMM) that was preserved in proto-Earth’s mantle after the Moon-forming giant impact.
But why did Theia’s material not just merge into Earth’s mantle? The models yield one important clue: the impact energy was not evenly distributed. A lot of it stayed in the upper mantle, leaving the lower mantle relatively cool and viscous “like peanut butter sitting on a very hot stove,” study coauthor Dr. Steve Desch describes. That enabled the denser, iron-rich pieces to exist intact, slowly falling and merging into the LLVPs. If the lower mantle had been more intensely melted, the residue would have been scattered and eliminated any evidence of Theia.
Seismic tomography, the primary method for imaging these deep structures, shows that not only are the LLVPs enormous spanning 25–30% of the core-mantle boundary but also chemically unique. The African LLVP is more vertical and smeared, whereas the Pacific LLVP is denser and younger, having been enriched with recent subducted oceanic crust. This compositional heterogeneity is corroborated by 3-D mantle circulation models that model the transport and stacking of subducted material for the past billion years. These models repeatedly determine that young crust stocks the Pacific LLVP, whereas the African LLVP has older, more mixed material a distinction potentially responsible for their differing heights and densities.
The significance of these observations resonates far beyond whether or not Theia survived. If LLVPs are truly ancient planetary fossils, then they could have had a deep influence on the tectonic history of the Earth, the emergence of continents, and even the stability of the planet’s magnetic field. Their elevated temperatures and positioning affect how heat is withdrawn from the core, which in turn affects convection flows driving the geodynamo. As Dr. Paula Koelemeijer notes, “The fact that these two LLVPs differ in composition, but not in temperature, is key to the story and explains why they appear to be the same seismically. It is also fascinating to see the links between the movements of plates on the Earth’s surface and structures 3000 km deep in our planet” the Pacific LLVP is more youthful and oceanic crust-enriched as a result of proximity to active subduction zones, whereas the African LLVP is older and more scattered.
Not all are willing to give up on alternative explanations, including the belief that LLVPs are simply accumulations of subducted oceanic crust. As Dr. Seth Jacobson warns, I doubt the advocates for other hypotheses (about LLVP formation) are going to abandon them just because this one has appeared. I think we’ll be debating this for quite some time. Yet the new models offer a compelling synthesis, bridging the Moon’s violent birth with the hidden architecture of Earth’s deep interior.
As computational modeling and seismic tomography evolve further, the mute witnesses to Theia’s destruction those giant iron-rich provinces at the bottom of the mantle can, in the not-too-distant future, reveal even more secrets. For the time being, they remain a reminder of the violent origin of the planet, and of the fact that the history of Earth is, quite literally, not of this world.
