Scientists have identified what may be the first direct evidence of material left over from the “proto-Earth,” a primordial version of our planet that existed before a colossal moon-forming impact reshaped it forever.
The study, published Tuesday (Oct. 14) in the journal Nature Geoscience, suggests that tiny chemical clues of this proto-Earth have survived deep within Earth’s rocks, essentially unaltered, for billions of years. The findings provide a rare window into the planet’s original building blocks and could offer scientists clues about what Earth and its neighboring worlds were like in their earliest eras.
“This is maybe the first direct evidence that we’ve preserved the proto-Earth materials,” Nicole Nie, an assistant professor of Earth and planetary sciences at MIT who co-led the new paper, said in a statement.
Roughly 4.5 billion years ago, the young solar system was a swirling cloud of gas and dust that formed the first asteroids and planets, including the young Earth, then a hot, molten sphere likely bubbling with oceans of lava.
Less than 100 million years later, a Mars-sized asteroid collided with the proto-Earth in an event so violent it melted and remixed nearly the entire planet, creating the moon in the process. It was the last event to cause large-scale melting of Earth’s mantle, the new study notes, and scientists have long suspected that this “giant impact” wiped away nearly all chemical traces of what came before.
But Nie and her colleagues uncovered a subtle imbalance in potassium isotopes in ancient rock, specifically a deficit of potassium-40. This anomaly, the researchers argue, is a potential fingerprint of material that survived from the proto-Earth itself.
“We see a piece of the very ancient Earth, even before the giant impact,” Nie said in the statement. “This is amazing because we would expect this very early signature to be slowly erased through Earth’s evolution.”
Potassium occurs naturally in three isotopes — potassium-39, potassium-40 and potassium-41, which are slightly different versions of the same element with varying numbers of neutrons but the same number of protons.
In 2023, Nie’s team analyzed meteorites that formed at different times and locations across the solar system that had been collected from around Earth. The researchers found subtle potassium isotopic differences among them, which meant that the different isotopes could “be used as a tracer of Earth’s building blocks,” Nie said in the statement.
In the new study, the team hunted for similar potassium anomalies in Earth’s oldest and deepest rocks. These were samples collected from Greenland, Alexo in Canada’s Abitibi belt and the Winnipegosis komatiite belt in Manitoba; Hawaii’s Kama’ehuakanaloa and Mauna Loa volcanoes; and the Newberry volcano in the Cascade Range of the northwestern United States.
“If this potassium signature is preserved, we would want to look for it in deep time and deep Earth,” Nie said in the statement.
The researchers found that these ancient materials contained even less potassium-40 than expected, suggesting that the rocks “were built different,” Nie said in the statement.
To detect such a minute signal, the researchers dissolved the powdered rocks in acid, isolated the resulting potassium, and then used an ultra-sensitive mass spectrometer to precisely measure the ratios of the element’s three isotopes, according to the new study.
The researchers also ran computer simulations to test whether known geological or cosmic processes, such as asteroid impacts, convection of materials from Earth’s mantle to its surface, or large-scale planetary melting could explain the potassium isotope ratios they observed. But in every scenario that was modeled, the simulated compositions contained slightly more potassium-40 than what the actual rock samples from Canada, Greenland and Hawaii contained.
This deficit represents the primitive proto-Earth mantle that largely escaped the mixing caused by the giant impact and still exists deep within Earth today, the researchers say.
While meteorites studied in the team’s earlier work also showed potassium anomalies, they did not exhibit the exact same deficit, suggesting that the materials that originally formed the proto-Earth have yet to be discovered.
“Scientists have been trying to understand Earth’s original chemical composition by combining the compositions of different groups of meteorites,” Nie said in the same statement.
“But our study shows that the current meteorite inventory is not complete, and there is much more to learn about where our planet came from.”
