Scientists have long wondered why Earth’s overall makeup doesn’t fully match the mix of materials found in ancient meteorites. One hypothesis has been that our proto-Earth (what the earliest stages of the Earth is referred to, before the Moon formed) gathered different kinds of material than it did after the giant impact that created the Moon.

To explore this idea, scientists at MIT and elsewhere carefully measured potassium-40 and other potassium isotopes in rocks from deep within the Earth and from different times in Earth’s history. To do this they used a very precise method called thermal ionization mass spectrometry.

In the first chapter of our planet’s long story, Earth was a hot, rocky world bubbling with lava. Then, there was a huge collision: a Mars-sized object hit the young celestial object. The impact was so powerful that it melted the inside of the planet and changed its chemical makeup. For a long time, scientists thought this giant collision erased all signs of what Earth was originally made of.

But new research from MIT tells a different tale. Scientists discovered a unique chemical clue in ancient rocks. This unusual mix of potassium isotopes differs from most of today’s Earth materials. Large space impacts or current Earth geology can not explain this imbalance as it may be a remnant from the planet’s early days that somehow survived the massive collision that reshaped everything else.

Nicole Nie from MIT says, “This is maybe the first direct evidence that we’ve preserved the proto-Earth materials. 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 studied meteorites from around the world. These space rocks formed long ago in different parts of the solar system, so they carry clues about how the solar system changed over time. When the scientists looked at the chemical makeup of these meteorites, they found something unusual about potassium, a common element. Potassium comes in three types, called isotopes: 39, 40, and 41. On Earth, we mostly see potassium-39 and potassium-41. Potassium-40 is very rare.

But in the meteorites, the mix of potassium isotopes was different. This “potassium anomaly” suggested the meteorites contain material from before Earth’s current form, possibly from the early proto-Earth, before a giant impact altered its chemistry.

“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.

The current study looked to find signs of similar potassium anomalies in the Earth. The samples included mafic Archaean rocks derived from the Hadean–Eoarchaean mantle (including samples from Isua, Nuvvuagittuq, and the Kaapvaal Craton) and certain modern ocean island basalts (from La Réunion Island and Kama’ehuakanaloa volcano, Hawaii).

The scientists started by dissolving powdered rock samples in acid. Then, they separated the potassium and used a special mass spectrometer to measure how much of each isotope was present. The scientists found that their rock samples had even lower levels of potassium-40 than usual. On Earth, potassium-40 is already rare compared to the other types of potassium, but in these ancient rocks, it was even scarcer, like noticing a single brown grain of sand in a bucket filled with yellow ones.

So the big question is could these rocks be pieces from proto-Earth? The researchers think it’s possible. They believe proto-Earth may have started with material low in potassium-40. But after the massive impact and many smaller meteorite strikes, most of that original material was chemically reshaped, leaving behind the potassium-rich Earth we know today.

The team gathered compositional data from all known meteorites and ran simulations to see how Earth’s potassium-40 levels might have changed over time. They modeled what would happen after major impacts and after long-term geological processes, such as heating and mixing deep inside the Earth. The scientists discovered that the impacts and Earth’s natural changes over time would slightly increase the amount of potassium-40, similar to what we observe in most rocks today. This is why the ancient samples from Canada, Greenland, and Hawaii, which contain even less potassium-40, are unique. They may be rare remnants of Earth from its early days, unaffected by the changes that followed.

The study suggests that rocks with lower than usual potassium-40 levels may be ancient leftovers from proto-Earth.

Interestingly, the chemical signature in these ancient Earth samples doesn’t exactly match any known meteorite. Some meteorites do show unusual potassium patterns, but none show the same kind of potassium-40 shortage found in these suspected proto-Earth rocks.

According to Nie,“Scientists have been trying to understand Earth’s original chemical composition by combining the compositions of different groups of meteorites. 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.”

The study is published in Nature Geoscience.

Source: MIT