MIT finds traces of a lost world deep within planet Earth

Billions of years in the past, the solar system was a vast rotating cloud of gas and dust. Over time, this material coalesced into solid objects, forming the first meteorites. These meteorites gradually merged through repeated impacts to create the proto Earth and its neighboring planets.

During its infancy, Earth was a molten, lava-covered world. Less than 100 million years later, it experienced a catastrophic event when a Mars-sized body struck the young planet in what scientists call a "giant impact." The collision melted and mixed the planet's interior, wiping out much of its original chemical identity. For decades, scientists believed that any trace of the proto Earth had been completely destroyed in that cosmic upheaval.

However, the MIT team's new results challenge that assumption. The researchers found an unusual chemical signature in ancient, deep rock samples that differs from most materials found on Earth today. This signature appears as a slight imbalance in potassium isotopes -- atoms of the same element with different numbers of neutrons. After extensive analysis, the scientists concluded that the anomaly could not have been created by later impacts or by ongoing geological processes within Earth.

The most plausible explanation is that these rocks preserve tiny portions of the proto Earth's original material, somehow surviving the planet's violent reshaping.

"This is maybe the first direct evidence that we've preserved the proto Earth materials," says Nicole 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."

Nie's co-authors include Da Wang of Chengdu University of Technology (China), Steven Shirey and Richard Carlson of the Carnegie Institution for Science (Washington, D.C.), Bradley Peters of ETH Zürich (Switzerland), and James Day of the Scripps Institution of Oceanography (California).

A curious anomaly

In 2023, Nie and her team examined numerous well-documented meteorites collected from around the world. These meteorites formed at different times and locations throughout the solar system, capturing its changing chemistry over billions of years. When the researchers compared their compositions to that of Earth, they noticed a peculiar "potassium isotopic anomaly."

Potassium occurs naturally 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 exists only in minute amounts. Yet the meteorites displayed isotope ratios distinct from those typically seen on Earth.

This finding suggested that any substance showing the same kind of potassium imbalance must come from material that existed before the giant impact altered Earth's chemistry. In essence, the anomaly could serve 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," Nie explains.

"Built different"

In the current study, the team looked for signs of potassium anomalies not in meteorites, but within the Earth. Their samples include rocks, in powder form, from Greenland and Canada, where some of the oldest preserved rocks are found. They also analyzed lava deposits collected from Hawaii, where volcanoes have brought up some of the Earth's earliest, 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 says.

The team first dissolved the various powder samples in acid, then carefully isolated any potassium from the rest of the sample and used a special mass spectrometer to measure the ratio of each of potassium's three isotopes. Remarkably, they identified in the samples an isotopic signature that was different from what's been found in most materials on Earth.

Specifically, they identified a deficit in the potassium-40 isotope. In most materials on Earth, this isotope is already an insignificant fraction compared to potassium's other two isotopes. But the researchers were able to discern that their samples contained an even smaller percentage of potassium-40. Detecting this tiny deficit is like spotting a single grain of brown sand in a bucket rather than a scoop full of of yellow sand.

The team found that, indeed, the samples exhibited the potassium-40 deficit, showing that the materials "were built different," says Nie, compared to most of what we see on Earth today.

But could the samples be rare remnants of the proto Earth? To answer this, the researchers assumed that this might be the case. They reasoned that if the proto Earth were originally made from such potassium-40-deficient materials, then most of this material would have undergone chemical changes -- from the giant impact and subsequent, smaller meteorite impacts -- that ultimately resulted in the materials with more potassium-40 that we see today.

The team used compositional data from every known meteorite and carried out simulations of how the samples' potassium-40 deficit would change following impacts by these meteorites and by the giant impact. They also simulated geological processes that the Earth experienced over time, such as the heating and mixing of the mantle. In the end, their simulations produced a composition with a slightly higher fraction of potassium-40 compared to the samples from Canada, Greenland, and Hawaii. More importantly, the simulated compositions matched those of most modern-day materials.

The work suggests that materials with a potassium-40 deficit are likely leftover original material from the proto Earth.

Curiously, the samples' signature isn't a precise match with any other meteorite in geologists' collections. While the meteorites in the team's previous work showed potassium anomalies, they aren't exactly the deficit seen in the proto Earth samples. This means that whatever meteorites and materials 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 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."

This work was supported, in part, by NASA and MIT.