Li, and two co-authors, reported in 2021 that the Moon is widely recognized as formed from the “Giant Impact”: when at least a Mars-sized planetary embryo Theia collided with the protoEarth during the last stage of terrestrial planet formation. Such a model is well aligned with some key physical aspects of Earth-Moon system, including anomalous high angular momentum of Earth-Moon system, small iron core of the Moon and its high mass ratio compared to the Earth. However, one of the most critical issues related to this scenario is that no evidence has been found for the existence of the hypothesized planetary embryo Theia.
This is in part because of its widely debated size ranging from 0.1~0.45 Earth mass (M⊕) and enstatite to carbonaceous chondrite composition. Moreover, whereas it is mostly agreed that the core of Theia promptly merged with the proto-Earth core shortly after the impact, what fraction of and how the Theia mantle was preserved into the Earth mantle remain elusive. This post-impact process is not only responsible for the initial thermal and compositional structures of the Earth, but also significantly affects Earth’s long-term chemical evolution.
In the meantime, the continent-sized Large Low Shear Velocity provinces (LLSVPs) above the core/mantle boundary has been seismically identified (more), which play an important role to understand the structure and thermochemical evolution in the deep mantle. Although purely thermal origin of the LLSVPs are not completely ruled out, growing evidences relate them to chemically dense thermochemical piles as the LLSVPs show anti-correlation between shear-wave and bulk-sound velocity and sharp side boundaries.
The correlation between LLSVP margins and surface hotspots together with reconstructed locations of large igneous provinces is better satisfied with a chemically denser component which is more likely to be stable for at least 250 Ma. In this scenario, the dense materials have been proposed to origin from the remnants of ancient differentiation process, the subducted oceanic crust.
All these different hypothetic origins of the dense materials in the lowermost mantle are due to the Earth’s internal differentiation. However, increasing
studies of noble gases, such as neon, xenon and helium, show less degassed primordial characters of some oceanic island basalts (OIBs) than the middle
ocean ridge basalt (MORB) established by 4.45 Gyr ago. If the OIB rocks were indeed derived from the LLSVPs as suggested, these noble gas isotopes suggest LLSVPs should exist since the formation of the Earth and was preserved even after the Moon-forming impact.
Simulations of the Giant Impact suggest that at least some intact pieces of Theia’s mantle may have persisted in the Earth’s mantle throughout Earth’s
history. In this study, the authors hypothesized that, due to its possible enrichment in iron, the left-over Theia mantle materials may sink to the bottom of Earth’s mantle and cause the LLSVPs. The authors provided several lines of evidence and perform numerical modeling experiments to support this hypothesis.
[Pop-Read on above subject: https://www.businessinsider.com/pieces-planet-theia-buried-in-earth-mantle-2021-4]
[And another pop-read: https://futurism.com/the-byte/pieces-alien-planet-buried-inside-earth-millions-of-larger-mount-everest]
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