Discovery Beneath the Earth: Traces of a Primordial Planet Detected in Earth’s Mantle

Investigators have identified remnants of the prehistoric planet Theia within the mantle of the Earth, shedding light on the origins of the Moon and the primordial Earth.

An international collective of researchers from multiple disciplines has uncovered that a significant irregularity within the Earth’s mantle might be a vestige of a cataclysmic impact that occurred around 4.5 billion years ago, which is hypothesized to have led to the Moon’s creation.

This work elucidates not only the structure of Earth’s interior but also the planet’s developmental history and the genesis of the inner solar system.

The findings, based on computational fluid dynamics techniques developed by Prof. Hongping Deng at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, were featured on the cover of the journal Nature on November 2.

The Mystery of Lunar Genesis

For many scientific minds across generations, the origins of the Moon have presented a challenging riddle. The predominant hypothesis posits that a colossal collision between the young Earth (referred to as Gaia) and a Mars-sized body named Theia led to the Moon’s formation from the resulting detritus, around 4.5 billion years ago.

Simulations suggest that most lunar material originated from Theia, while Gaia, being substantially larger, experienced minimal contamination by Theian matter.

This planetary collision simulation illustrates the varying components of Gaia and Theia, indicating that Gaia’s lower mantle, within a specific radius, experienced only slight Theian material contamination.

The assumption has been that the Earth and Moon should exhibit markedly distinct compositions, considering they were predominantly influenced by Gaian and Theian materials, respectively. Yet, precise isotopic analyses later contested this, revealing striking similarities between Earth’s and the Moon’s compositions, challenging the traditional model of the Moon’s formation.

Despite various adaptations to the giant impact theory, each has encountered its own set of challenges.

Unraveling the Mysteries of Earth’s Mantle

Prof. Deng initiated a new phase of research on lunar origins in 2017, concentrating on a computational technique known as Meshless Finite Mass (MFM), which excels in simulating turbulence and the mixing of materials.

Through this method and a series of simulations, Prof. Deng noted that post-impact, Earth’s mantle was stratified, with differing states and compositions for the upper and lower mantle. While the upper mantle became a magma ocean mixed from both Gaian and Theian materials, the lower mantle stayed mostly solid, retaining Gaian characteristics.

This finding shifts focus from the debris disk — the Moon’s precursor — to the consequences of the colossal impact on Earth’s early state.

The LLVPs and the Legacy of Theia

Interactions with geophysicists from the Swiss Federal Institute of Technology in Zurich led Prof. Deng’s team to surmise that such mantle stratification has likely endured, correlating with the seismic reflectors identified within the mid-mantle. The lower mantle, in particular, might still consist largely of pre-impact Gaian substances, possessing different elemental make-up than the upper mantle.

The implications of Large Low Velocity Provinces (LLVPs) beneath the African and Pacific tectonic plates, areas where seismic wave velocity significantly decreases, are profound. Their origins, however, have puzzled scientists.

Dr. Qian Yuan from the California Institute of Technology, with colleagues, suggested that LLVPs might have originated from Theian materials merging into Gaia’s lower mantle. They brought in Prof. Deng to examine the distribution and condition of Theian substances in Earth’s depths post-collision.

Advanced simulations led the team to ascertain that a substantial proportion of Theian mantle, around two percent of the Earth’s mass, was integrated into Gaia’s lower mantle.

Prof. Deng collaborated with computational astrophysicist Dr. Jacob Kegerreis to validate these findings using established Smoothed Particle Hydrodynamics (SPH) methodologies.

The research team deduced that Theian mantle materials, iron-rich like lunar rocks, quickly descended to the mantle’s base. Over billions of years, these materials contributed to the formation of the pronounced LLVP regions, which have remained consistent throughout Earth’s geological evolution.

Traces of Ancient Heterogeneity on the Surface

The presence of ancient heterogeneity in the mantle is manifest not only in mid-mantle reflectors but also in LLVPs, indicating a complex interior far from homogeneity. Mantle plumes can transport these deep-seated variances to the surface, as seen in regions like Hawaii and Iceland.

Isotope studies of Icelandic basalt have found evidence of deep mantle heterogeneity within these samples, illuminating aspects of Earth’s initial conditions and even the development of proximal planets.

Broader Implications and Universal Relevance

Dr. Yuan underscores the potential of such research: by analyzing a broader array of rock specimens and integrating more precise impact and Earth evolution models, we might better understand the material composition and orbital dynamics of the primordial Earth and its celestial counterparts.

Prof. Deng envisages this research having far-reaching implications, potentially aiding our comprehension of the formation and habitability of planets beyond our solar system.

For further information on this subject, please refer to the study “Strange Blobs in Earth’s Deep Mantle Are the Remains of an Ancient Planet.”

For the original study, please see: “Moon-forming impactor as a source of Earth’s basal mantle anomalies” by Qian Yuan et al., dated 32 October 2023, published in Nature.
DOI: 10.1038/s41586-023-06589-1

Frequently Asked Questions (FAQs) about Theia mantle remnants

More about Theia mantle remnants

• Deep Mantle Anomalies and Theia
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• Large Low Velocity Provinces Explained
• Mantle Plumes and Earth’s Surface Features