In the enigmatic tapestry of Earth's ancient history, a recent breakthrough in geophysical research has unveiled a tantalizing narrative: deep within the Earth's fiery mantle lie the remnants of a bygone celestial body, possibly a relic from the same colossal event that gave birth to our Moon.

This discovery not only reshapes our understanding of Earth's formative years but also invites us to reimagine our planet not as a solitary entity but as a guardian to a deep-seated, hidden companion.

The origins of this discovery trace back to Earth's tumultuous early years, marked by relentless cosmic ballets and massive collisions with planetesimals—space-faring bodies that pepper our solar system.

Among the most significant of these celestial dances was a cataclysmic encounter with Theia, a Mars-sized protoplanet whose union with Earth is believed to have birthed a torus of debris, culminating in the formation of our Moon.

Illustration - arteck

This theory, known as the Giant Impact Hypothesis, suggests that remnants of Theia may still reside within Earth's cavernous interior, a testament to our planet's violent infancy.

The Earth's infancy unfolded during the Hadean eon—a time when our planet was a crucible of magma, its mantle viscous and tumultuous, enriched with radiogenic heat and a fluidity augmented by an abundance of water.

Such conditions were conducive to the deep burial of celestial intruders, leading to one of the 20th century's most profound geophysical discoveries: the existence of two continent-sized anomalies known as Large Low-Shear-Velocity Provinces (LLSVPs).

Located under Africa and the Pacific Ocean, these behemoths, rich in iron, stand out in the seismic landscape due to their capacity to alter the speed of seismic waves—thus betraying their presence to our instruments.

For years, the origins of these LLSVPs confounded scientists—until now. A recent paper titled "Moon-forming impactor as a source of Earth’s basal mantle anomalies," published in the journal Nature, spearheaded by Qian Yuan of Caltech's Seismological Laboratory, has illuminated the possibility that these enigmatic structures could indeed be the remains of Theia.

In the 1980s, scientists discovered two large regions deep inside the Earth that have low shear velocity. These regions contain high amounts of iron, which produce distinct seismic wave patterns. Image Credit: Edward Garnero.

Yuan's epiphany, sparked by a seminar on planet formation, led to a series of high-resolution simulations that reconstructed the fateful impact with unprecedented detail.

These simulations narrate a story of immense energy unleashed by Theia's descent into Earth's nascent form. Not all of the molten detritus from the impact escaped Earth's gravity—some coalesced to form our Moon, while some melded into the Earth's molten layers.

The twist in this tale lies in the revelation that much of the impact's energy was confined to Earth's upper mantle, sparing the core and thus preventing complete dissolution of Theia's remains. The cooler lower mantle allowed Theia's remnants, now termed Theia Mantle Material (TMM), to clump together, giving rise to the seismic signatures we observe today as the LLSVPs.

This groundbreaking research posits that the distinct iron-rich LLSVPs could be thermal and chemical vestiges from the impact, their survival to the present day a natural outcome of that ancient celestial collision.

This schematic from the research illustrates the formation of the LLSVPs. Image Credit: Yuan et al. 2023.

The implications of this are staggering, suggesting that these primordial fragments may have played a pivotal role in shaping Earth's geological and biological evolution—from the stirring of tectonic plates to the whispers of climate change and the profound influences on life's inexorable march through time.

This discovery is not merely a chapter added to Earth's history but a gateway to myriad questions about the planet's past and future. As we probe the depths of Earth's mantle, we come face to face with the residues of cosmic encounters that have not only sculpted our physical world but have also, perhaps, laid the groundwork for life itself to flourish.

With each seismic wave that travels through these ancient remnants, we decode a little more of our planet's epic saga—a saga in which Earth is both the stage and the storyteller of a celestial drama spanning billions of years.