Is there really another planet hiding deep inside our Earth?

For centuries, scientists have grappled with the enigma of the Moon's birth. The widely accepted theory proposes that approximately 4.5 billion years ago, a colossal collision, termed the "giant impact," unfolded between the fledgling Earth, known as Gaia, and a proto-planet the size of Mars named Theia. This catastrophic encounter between Gaia and Theia resulted in a substantial amount of planetary debris, which, in all likelihood, came together to give rise to our lunar companion.

Yet, the quandary lies in the remarkable similarity in composition between Earth and the Moon, casting doubt on this conventional theory of lunar genesis. Some scientists have also pondered whether this cosmic clash may have thoroughly mixed Earth's materials in the distant past, but we've lacked definitive evidence to confirm this hypothesis. Fortunately, recent research has illuminated these perplexing theories.

By employing an inventive computational fluid dynamics approach referred to as Meshless Finite Mass, researchers have successfully simulated how materials on and within the Earth may have intermingled and diffused post the colossal impact. Intriguingly, the team has ascertained that the upper mantle of the Earth underwent a transformation, forming a molten ocean containing materials from both Gaia and Theia, while the lower mantle has remained relatively untouched.

This dichotomy within the mantle may still endure to this day. The lower mantle of the Earth could still be predominantly composed of Gaian materials from before the impact, characterised by a distinct elemental composition featuring higher silicon content, in contrast to the upper mantle.

As a result, it is proposed that, contrary to the previous notion of Earth becoming more uniform after the Moon-forming collision, this momentous event may have initiated heterogeneity within the early mantle, thus establishing the groundwork for the Earth's geological evolution over the course of 4.5 billion years.

Another puzzle within Earth's inner depths pertains to the origin of Large Low Velocity Provinces (LLVPs), which are peculiar regions extending for thousands of kilometres at the base of the mantle. These regions significantly retard the speed of seismic waves passing through them, but their origins have remained shrouded in mystery.

The researchers have put forward the notion that LLVPs may have evolved from Theian materials that infiltrated the lower mantle of Gaia. After scrutinising prior simulations of giant impacts and conducting fresh, high-precision simulations, the research team has uncovered that a substantial amount of Theian mantle material, approximately 2% of Earth's mass, found its way into Gaia's lower mantle. This dense, iron-rich Theian material descended to the lowermost reaches of the mantle, eventually crystallising into the stable LLVP regions over a staggering 4.5 billion years of geological transformations.

All in all, the striking diversity in the Earth's interior beckons intriguing implications. This newfound understanding not only deepens our grasp of mantle evolution, the chronicles of supercontinents, and the intricacies of Earth's tectonic plates but also furnishes insights into the ancient Earth, Gaia, and Theia, thereby shedding light on the entire narrative of inner solar system formation.

Moreover, this fresh comprehension of Earth's profound heterogeneity also provides additional clues concerning the formation of features like Hawaii and Iceland, which likely emerged from mantle plumes—elongated, upward-moving thermal currents driven by mantle convection. Geochemists have unearthed that these regions house components that deviate from the usual surface materials, thereby unveiling vestiges of heterogeneity in the deep mantle, traces of which date back more than 4.5 billion years.