Scientists at MIT and collaborating institutions have made a groundbreaking discovery that rewrites our understanding of Earth's earliest history. They've identified rare chemical signatures preserved in ancient rocks that originate from the "proto-Earth," the primordial version of our planet that existed 4.5 billion years ago before a catastrophic collision with a Mars-sized object fundamentally altered its composition reports MIT News. This represents the first direct evidence that materials from Earth's original formation survived the Moon-forming giant impact event.
The discovery, published in Nature Geoscience, provides unprecedented insight into the chemical building blocks that formed our planet and challenges long-held assumptions about the complete transformation of Earth's early composition.
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A Rare Isotopic Anomaly From Deep Time
The research team, led by Nicole Nie, Assistant Professor of Earth and Planetary Sciences at MIT, discovered a subtle but significant potassium isotope imbalance in rock samples from some of Earth's most ancient and deepest geological formations. The samples originated from Greenland, Canada, and volcanic deposits from Hawaii - regions where Earth's oldest preserved rocks or deepest mantle materials can be found. Using sophisticated thermal ionization mass spectrometry, the team identified a deficit in potassium-40, one of three naturally occurring isotopes of potassium.
"This is maybe the first direct evidence that we've preserved the proto-Earth materials," explained Professor Nie.
"We see a piece of the very ancient Earth, even before the giant impact. This is amazing because we would expect this very early signature to be slowly erased through Earth's evolution."
Before the Moon-Forming Impact
Approximately 4.5 billion years ago, the early solar system was a swirling disk of gas and dust that gradually coalesced into meteorites, which merged to form proto-Earth and neighboring planets. In this primordial phase, Earth was likely a molten, lava-covered world. Then, less than 100 million years later, a Mars-sized impactor dubbed Theia slammed into the infant planet in what scientists call the "giant impact" event - a catastrophic collision that completely melted and scrambled Earth's interior, effectively resetting its chemistry and producing the Moon.
Until now, researchers believed this violent transformation had completely obliterated any trace of proto-Earth's original composition. The potassium-40 deficit discovered in certain ancient rocks proves otherwise, showing that despite 4.5 billion years of geological activity and mantle convection, some primordial material survived untransformed.
Artist's depiction of a collision between two planetary bodies. (Public Domain)
Materials Unknown to Science
The research team compared their findings with every known meteorite in geological collections worldwide. Remarkably, the proto-Earth samples don't precisely match any existing meteorite group, despite meteorites showing their own range of potassium isotope variations. This means whatever meteorites and materials originally formed the proto-Earth have yet to be discovered - they represent a missing piece of our solar system's geological puzzle.
The team used computer simulations to model how the samples' potassium-40 deficit would change following the giant impact and subsequent smaller meteorite collisions. Their models demonstrated that if proto-Earth originally contained these potassium-40-depleted materials, subsequent impacts and geological processes would gradually increase potassium-40 levels, ultimately producing the composition seen in most modern terrestrial materials. This supports the conclusion that the anomalous samples are genuine proto-Earth remnants.
Implications for Planetary Formation
The discovery indicates that approximately 9-10% of Earth's mass - consistent with giant impact models - came from the impactor, while the remaining material represents proto-Earth. Significantly, the findings suggest proto-Earth was more volatile-depleted than present-day Earth, with up to one-quarter to one-half of our planet's current potassium delivered by the Moon-forming collision. This challenges previous assumptions about Earth's primordial composition and provides the first direct chemical evidence linking ancient Archaean rocks and modern ocean island basalts to pre-giant-impact mantle domains.
The research demonstrates that these primordial reservoirs have persisted in Earth's deep mantle for 4.5 billion years, largely escaping homogenization through mantle convection and occasionally contributing to modern hotspot volcanism in places like Hawaii and La Réunion Island.
The work was supported by NASA and MIT, and future research will continue searching for the elusive meteorites that match proto-Earth's unique chemical signature.
Top image: Artist's impression of the molten proto-Earth during its formation 4.5 billion years ago, before the giant impact that created the Moon. Source: MIT News
By Gary Manners
References
Nie, N. et al. 2025. Potassium-40 isotopic evidence for an extant pre-giant-impact component of Earth's mantle. Available at: https://www.nature.com/articles/s41561-025-01811-3
Chu, J. 2025. Geologists discover the first evidence of 4.5-billion-year-old "proto Earth". Available at: https://news.mit.edu/2025/geologists-discover-first-evidence-45-billion-year-old-proto-earth-1014
