Billions of years ago, our solar system began as a vast rotating cloud of gas and dust. Over time, this primordial material coalesced into solid objects, forming the first meteorites. These early meteorites gradually merged through repeated impacts, leading to the formation of the proto-Earth and its neighboring planets. In its infancy, Earth was a molten, lava-covered world, a chaotic environment shaped by immense heat and tumultuous geological processes.
Less than 100 million years after its formation, Earth faced a catastrophic event when a Mars-sized body collided with the young planet in what scientists refer to as a giant impact. This monumental collision melted and mixed the planet's interior, effectively erasing much of its original chemical identity. For decades, the prevailing belief among scientists was that any trace of the proto-Earth had been completely obliterated during this cosmic upheaval. However, groundbreaking research from a team at the Massachusetts Institute of Technology (MIT) challenges this long-held assumption.
The MIT researchers, led by Nicole Nie, have discovered an unusual chemical signature in ancient rock samples that differ from most materials found on Earth today. This signature presents itself as a slight imbalance in potassium isotopes—atoms of the same element that have different numbers of neutrons. Their extensive analysis revealed that this anomaly is inconsistent with later impacts or ongoing geological processes within Earth. The most plausible explanation is that these rocks contain tiny portions of the original material from the proto-Earth, surviving the planet's violent reshaping.
"This is maybe the first direct evidence that we've preserved the proto-Earth materials," says Nie, the Paul M. Cook Career Development Assistant Professor of Earth and Planetary Sciences at MIT. "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."
In 2023, Nie and her team examined numerous well-documented meteorites collected from around the globe. These meteorites formed at different times and locations, capturing the solar system's changing chemistry over billions of years. When the researchers compared their compositions to those of Earth, they noted a peculiar potassium isotopic anomaly. Potassium exists in three isotopic forms—potassium-39, potassium-40, and potassium-41—each differing slightly in atomic mass. On modern Earth, potassium-39 and potassium-41 dominate, while potassium-40 is present only in minute amounts.
Interestingly, the meteorites exhibited distinct isotope ratios that diverged from those typically found on Earth. This finding suggested that any substance displaying the same potassium imbalance must originate from material that existed before the giant impact altered Earth's chemistry, effectively acting as a fingerprint of proto-Earth matter. "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," explains Nie.
In their current study, the team shifted focus from meteorites to the Earth itself. They analyzed rock samples in powder form from Greenland and Canada, where some of the oldest preserved rocks are located. They also examined lava deposits from Hawaii, which bring to the surface some of the Earth's earliest and deepest materials from the mantle—the planet's thickest layer of rock that separates the crust from the core. "If this potassium signature is preserved, we would want to look for it in deep time and deep Earth," Nie states.
The team began by dissolving the various powder samples in acid and isolating the potassium from the rest of the material. Using a specialized mass spectrometer, they measured the ratio of each of potassium's three isotopes. Remarkably, they identified a unique isotopic signature that differed from most materials found on Earth today, particularly noting a deficit in the potassium-40 isotope.
In most Earth materials, potassium-40 is already a negligible fraction compared to its counterparts. However, the samples revealed an even smaller percentage of potassium-40. Detecting this tiny deficit is akin to spotting a single grain of brown sand in a bucket of yellow sand. The team confirmed the presence of the potassium-40 deficit, indicating that these materials are indeed "built different," according to Nie, compared to most of what we encounter on Earth today.
But could these samples be rare remnants of the proto-Earth? To explore this possibility, the researchers postulated that if the proto-Earth was originally composed of such potassium-40-deficient materials, subsequent chemical changes—due to the giant impact and smaller meteorite impacts—would have led to the materials we see today, which contain more potassium-40.
The team utilized compositional data from every known meteorite and conducted simulations to understand how the potassium-40 deficit would evolve following these cosmic events. Their simulations demonstrated a composition with a slightly higher fraction of potassium-40 compared to the samples from Canada, Greenland, and Hawaii. More significantly, the simulated compositions align closely with those of most modern-day materials, suggesting that materials with a potassium-40 deficit likely represent leftover original material from the proto-Earth.
Curiously, the signature found in the samples does not precisely match any other meteorite in existing collections. While the meteorites examined in previous studies exhibited potassium anomalies, they do not correspond to the deficit observed in the proto-Earth samples. This indicates that the materials that originally formed the proto-Earth remain undiscovered. "Scientists have been trying to understand Earth's original chemical composition by combining the compositions of different groups of meteorites," Nie concludes. "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."
This pioneering research was supported, in part, by NASA and MIT and opens new avenues for understanding the origins of our planet and the processes that shaped its early development.
