“Where is the impactor?” The question, raised during a planetary science lecture back in 2019, struck a chord with geophysicist Qian Yuan. The answer, he suspected, was neither in an asteroid belt nor lost to space it was buried inside Earth itself.
Seismic imaging over the past four decades has revealed two enormous anomalies residing deep in the mantle, perched just above the core–mantle boundary. This pair of vast, continent-scale mantle features, known as large low-velocity provinces, LLVPs, are situated beneath Africa and the Pacific Ocean. Their defining characteristic is the way they slow seismic waves, a sign of unusual composition and density. Advanced tomography now demonstrates that they each account for up to 9% of the mantle’s volume, rise more than 1,000 km above the core, and are characterized by sharp, compositionally distinctive boundaries. The prevailing mystery has been their origin.
New high-resolution simulations, which couple planetary impact dynamics with mantle convection models, offer a radical solution: the LLVPs are remnants of Theia, a Mars-sized protoplanet that collided with the proto-Earth approximately 4.5 billion years ago. That same giant impact has long been accepted as the Moon’s birth event and would have plunged Theia’s iron-rich mantle into Earth’s own. Because it was denser-2 to 3.5 % heavier-than the surrounding rock, the mantle material of Theia sank into the lower mantle, where it has managed to survive billions of years of convective stirring.
The simulations show that the energy from the impact was deposited in the upper mantle, while the lower mantle remained relatively cool and did not melt. This thermal shielding enabled Theia’s fragments, tens of kilometers in diameter, to plunge intact into the deeper mantle. On geological timescales, these convection currents piled the material into two gigantic LLVPs we find today. “It makes sense, therefore, to investigate next what consequences they had for Earth’s earliest evolution,” said Caltech’s Paul Asimow, not least their interaction with the process of subduction, continent formation, and magnetic field generation.
Seismology and geochemistry intersect on the uniqueness of the LLVPs. The shear-wave velocity anomalies reach as low as -3.0% in their deepest parts, with very high dlnVS/dlnVP, compatible with the enrichment in Fe³⁺-bearing bridgmanite-a dense mineral phase stable under extreme lower mantle pressures. The laboratory experiments synthesizing such Fe³⁺-rich bridgmanite indicate that it has notably lower sound velocities and higher density compared to the typical mantle rock, matching the seismic signature of LLVPs. Such mineralogical properties would contribute to anchoring the structures against convective mixing.
Complementary mantle circulation models suggest further complexity. The Pacific LLVP is younger and enriched in subducted oceanic crust, continually fed by the circum-Pacific subduction zones over the past 300 million years, while the African LLVP is much older, more diffuse, and composed of well-mixed material with minimal fresh input. Despite their similar temperatures-temperature being the dominant control on seismic speed-their density differences may explain why the African LLVP rises higher above the core. This asymmetry in deep mantle composition could affect how heat is extracted from the core and, in turn, the stability of Earth’s geomagnetic field.
Theia’s buried remains could also retain volatile “fingerprints” of the early Solar System. Lava derived from above LLVPs is enriched in helium and xenon isotopes, indicating the existence of primordial reservoirs that have not been erased by later mantle homogenization. If future samples of the lunar mantle share these isotopic signatures, it would reinforce the connection between the Moon and the LLVPs, joining surface and deep-Earth expressions of the same cataclysm.
The implications reach beyond Earth: Giant impacts were common in the early Solar System, shaping the interiors of terrestrial planets. If dense, unmixed relics can persist for billions of years, similar deep-mantle heterogeneities may exist on Mars, Venus, or large exoplanets, influencing their tectonics, volcanism, and magnetic histories. As Yuan 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.”
