Researchers at MIT have identified chemical remnants of Earth's earliest form, preserved deep within the planet's mantle. The discovery reveals a potassium isotope imbalance pointing to material from 4.5 billion years ago, surviving a cataclysmic collision. This finding challenges assumptions about Earth's formative history.
About 4.5 billion years ago, the solar system emerged from a rotating cloud of gas and dust, forming meteorites that coalesced into proto-Earth, a molten world. Less than 100 million years later, a Mars-sized body collided with it in a giant impact, melting the interior and altering its chemistry, which scientists long thought erased any original traces.
A team led by Nicole Nie, Paul M. Cook Career Development Assistant Professor of Earth and Planetary Sciences at MIT, challenged this view. Published on October 14 in Nature Geoscience, their study analyzed ancient rock samples from Greenland and Canada—some of the oldest preserved rocks—and lava deposits from Hawaii, which originate from the mantle, Earth's thickest layer between crust and core.
The researchers detected a deficit in potassium-40 isotopes, distinct from typical Earth materials. "This is maybe the first direct evidence that we've preserved the proto Earth materials," Nie said. "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."
To confirm, the team dissolved samples in acid, isolated potassium, and measured isotopes with a mass spectrometer. Simulations of the giant impact and later meteorite strikes, using data from known meteorites, showed that the deficit matches unaltered proto-Earth material. The signature does not precisely match any collected meteorites, suggesting undiscovered building blocks of Earth.
Co-authors include Da Wang of Chengdu University of Technology in China, Steven Shirey and Richard Carlson of the Carnegie Institution for Science in Washington, D.C., Bradley Peters of ETH Zürich in Switzerland, and James Day of the Scripps Institution of Oceanography in California. The work was supported by NASA and MIT.
"Scientists have been trying to understand Earth's original chemical composition by combining the compositions of different groups of meteorites," Nie noted. "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."