Earth has a long, 4.5 billion history full of momentous twists and turns.
Multiple prominent events played leading roles in Earth’s story.
One of them is the catastrophic impact with another planetesimal early in Earth’s history that not only created the Moon, but altered Earth’s chemistry forever.
Isotopic evidence tells much of Earth’s story, and scientists scour the Earth in search of the oldest rocks in existence to study their isotopes.
Some of the planet’s oldest rocks are in Greenland, Canada, and South Africa, and those rocks play a central role in new research into the proto-Earth, the Earth that existed prior to the Moon-creating impact.
The isotopic ratios in those rocks illuminate Earth’s story.
New research in Nature Geoscience examined potassium-40 (⁴⁰K) in samples from certain ancient and modern rocks and compared them with typical terrestrial samples and with meteorites to try to understand what the proto-Earth was like before the Giant Impact.
It’s titled “Potassium-40 isotopic evidence for an extant pre-giant-impact component of Earth’s mantle.](https://www.nature.com/articles/s41561-025-01811-3)” The lead author is Nicole Nie, an Assistant Professor of Earth and Planetary Sciences at MIT.
“Earth’s bulk composition has elemental and isotopic characteristics that cannot be fully reconciled with a mixture of known primitive meteorite compositions,” the authors write in their research, alluding to the fact that Earth formed from the same material as meteorites.
“One potential explanation for this is that the proto-Earth accreted materials with isotopic signatures distinct from those accreted after the Moon-forming giant impact.”
Earth was once a magma ocean, a ball of molten lava too hot to cool and solidify.
This was during the Hadean eon, and at some point during the Hadean, a massive protoplanet about the size of Mars slammed into Earth.
The protoplanet was called Theia, and this impact gave birth to the Moon. The impact also delivered a lot of material to Earth, and it had a slightly different chemistry than the Earth.
The researchers work began in 2023 when they analyzed meteorites from different locations around the world.
These rocks represent the times and locations in the young Solar System where they formed, and in a way, they document the ongoing changes in the Solar System.
When they compared them to Earth rocks, they found a different ratio of potassium isotopes. There are three naturally occurring potassium isotopes: 39K, 40K and 41 K. Each isotope has a different number of neutrons in its nucleus.
The potassium on Earth is dominated by potassium-39 (93.26%) and potassium-41 (6.73%) , with only tiny traces of potassium-40 (0.0117%). When compared to the meteorites, their ratio of potassium isotopes was found to be different from most of Earth. The discrepancy between Earth’s potassium and the meteorites’ potassium indicates that any samples with a similar ratio most likely predates the current composition of Earth. So a potassium imbalance in any Earthly sample strongly suggests that the sample comes from the proto-Earth prior to the impact that created the Moon.
“In that work, we found that different meteorites have different potassium isotopic signatures, and that means potassium can be used as a tracer of Earth’s building blocks,” lead author Nie explained in a press release.
Their current research builds off of their 2023 meteorite research into potassium isotopes. Nie and her fellow researchers looked for different isotope ratios in samples from Earth. They examined some of the oldest rocks on Earth from Greenland and Canada, and lava samples from Hawaiian volcanoes, where rock from deep within Earth’s mantle was brought to the surface. The hope was that these ancient samples would have a different potassium isotope signature.
“If this potassium signature is preserved, we would want to look for it in deep time and deep Earth,” Nie says.
Their laboratory analysis of the three samples showed that they have a different isotope anomaly from other Earth samples. It’s all about potassium-40, the rarest of the potassium isotopes on Earth that only exists in trace amounts.
The ancient samples from Greenland, Canada, and Hawaiian lava contained even less potassium-40. They showed a deficit of 65 parts per million. This means that these ancient materials have a different history than the rest of Earth. The next question the researchers asked is, are these ancient samples relics of the proto-Earth?
The different potassium-40 concentrations suggest that the ancient and deep samples are from proto-Earth. The researchers reasoned that the rest of Earth’s rock has been subjected to huge numbers of impacts throughout the planet’s 4 billion plus years of existence since the Giant Impact.
The cumulative effect of all of those impacts was to change the chemistry of the rock, creating the samples with greater concentration of potassium-40.
To test that reasoning, the team gathered compositional data on every known meteorite and then simulated how their potassium-40 concentrations changed due to impact. They also simulated all of the geological changes the Earth has gone through, including how the mantle has been heated and churned.
The simulations supported their conclusion, showing that the rock ended up with higher potassium-40 concentrations. Not only that, but the overall composition of the simulated samples agreed with the composition of most modern samples of Earth.
All together, it shows that the ancient rock samples are most likely remnants of the proto-Earth that have somehow survived for billions of years. By extension, any materials that display the same potassium-40 deficit are likely proto-Earth relics.
“This is maybe the first direct evidence that we’ve preserved the proto Earth materials,” sa 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.”
But the samples’ signature also doesn’t precisely mirror any known meteorites. While the meteorites in the researchers’ previous work have potassium anomalies, the anomaly isn’t exactly the same as in the proto-Earth samples. That means that the original materials and meteorites that formed Earth remain undiscovered.
“Scientists have been trying to understand Earth’s original chemical composition by combining the compositions of different groups of meteorites,” Nie says. “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.”
Written by Evan Gough/Universe Today.
