Scientists have identified what may be the first direct evidence of material from “proto-Earth,” a primordial version of our planet that existed before a colossal Moon-forming impact reshaped it forever.
The study, published Tuesday, October 14, in the journal Nature Geoscience, suggests that tiny chemical clues to this phenomenon proto-Earth have survived deep within the Earth’s rocks, virtually unchanged, for billions of years. The results 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 times.
“This may be the first direct evidence that we have preserved proto-Earth materials,” Nicole Niéassistant professor of earth and planetary sciences at MIT who co-led the new paper, said in a statement statement.
About 4.5 billion years ago, young solar system was a swirling cloud of gas and dust that formed the first asteroids and planets, including the young Earth, and then a hot, molten sphere probably bubbling with oceans of lava.
Less than 100 million years later, an asteroid the size of Mars collided with proto-Earth in an event so violent that it melted and remixed almost the entire planet, create the moon in the process. It was the latest event to cause large-scale melting of Earth’s mantle, the new study notes, and scientists have long suspected that this “giant impact” erased almost all chemical traces of what came before it.
But Nie and his colleagues discovered a subtle imbalance of potassium isotopes in ancient rocks, particularly a deficiency of potassium-40. According to the researchers, this anomaly is a potential imprint of material that survived from proto-Earth itself.
“We see a piece of the very ancient Earth, even before the giant impact,” Nie said in the release. “This is surprising because we would expect this very first signature to be slowly erased over the course of Earth’s evolution.”
Potassium occurs naturally as 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 in the solar system and were collected around Earth. The researchers found subtle potassium isotopic differences among them, meaning 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 looked for similar potassium anomalies in Earth’s oldest and deepest rocks. These included samples collected in Greenland, Alexo in the Abitibi belt of Canada, and the Winnipegosis komatiite belt of Manitoba; Hawaii’s Kama’ehuakanaloa and Mauna Loa volcanoes; and Newberry Volcano in the Cascade Range of the northwest United States.
“If this potassium signature is preserved, we would like to search for it in the depths of time and 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 constructed differently,” Nie said in the release.
To detect such a tiny signal, researchers dissolved the powdered rocks in acid, isolated the resulting potassium, then used an ultra-sensitive mass spectrometer to precisely measure the ratios of the three isotopes of the element, according to the new study.
The researchers also performed computer simulations to test whether known geological or cosmic processes, such as asteroid impacts, convection of material from Earth’s mantle to its surface, or large-scale planetary melting, could explain the potassium isotope ratios they observed. But in each modeled scenario, the simulated compositions contained slightly more potassium-40 than was contained in actual rock samples from Canada, Greenland and Hawaii.
This deficit represents the primitive proto-Earth mantle that largely escaped the mixing caused by the giant impact and which still exists deep within the Earth today, the researchers say.
Although the meteorites studied in the team’s previous work also had potassium anomalies, they did not have exactly the same deficiency, suggesting that the materials from which the proto-Earth originated have not yet been discovered.
“Scientists have attempted to understand the original chemical composition of the Earth by combining the compositions of different groups of meteorites,” Nie said in the same statement.
“But our study shows that the current inventory of meteorites is not complete and that there is still much to learn about the origin of our planet.”