Earth still carries traces from a time before the Moon existed. Those traces do not sit on the surface as fossils or bones. They hide in tiny differences between atoms deep inside rocks.
A group of researchers has studied those atoms and now argues that parts of “proto-Earth,” the early version of our planet, still survive inside the mantle.
They base that claim on careful measurements of potassium in very old rocks and in lava from a few unusual volcanoes.
Earth, Theia, and “proto-Earth”
About 4.5 billion years ago, the young solar system was a cloud of gas and dust. Small bits of rock and metal collided, stuck together, and grew into larger bodies that slowly built the planets.
One of those growing worlds was an early Earth, often called proto-Earth by scientists.
At some point, a Mars-sized body, sometimes called Theia, slammed into proto-Earth. The impact released huge amounts of energy, melted much of the outer layers, and threw debris into space that later formed the Moon.
Many scientists expected that colossal impact to have completely “reset” Earth’s mantle, mixing it so thoroughly that no chemical trace of the earlier planet remained.
Yet the overall composition of Earth does not match any simple mix of known meteorites, which are samples of early solar system material. That mismatch suggests that there may be hidden regions inside Earth with unusual chemistry.
Potassium and its tiny differences
Potassium plays a key role in this story. It is a common element in rocks and comes in three main isotopes: potassium-39, potassium-40, and potassium-41.
These isotopes behave almost the same way in chemical reactions, but potassium-40 is rare and radioactive. It decays over billions of years and provides a natural way to trace very old processes and heat sources inside planets.
Earlier research showed that different groups of meteorites have slightly different potassium isotope ratios. Those ratios act as identifiers for the materials that built bodies in the solar system.
That result led the new team to ask whether parts of Earth’s deep interior could still preserve a potassium “barcode” of “proto-Earth” from before the Moon-forming impact.
Rocks hold proto-Earth clues
To explore that idea, the researchers went looking for rocks that either formed very early in Earth’s history or came from great depth.
One set of target rocks consisted of very ancient, dark mafic rocks from some of the oldest crust on the planet, in places such as Greenland, Canada, and South Africa.
Some of these rocks formed more than three billion years ago, so they can record what the mantle was like not long after the surface first solidified.
They also studied younger volcanic rocks called ocean island basalts. These samples came from La Réunion Island in the Indian Ocean and from Kama’ehuakanaloa, a submarine volcano near Hawaii.
These volcanoes sit above mantle plumes, which are narrow, hot upwellings that rise from deep inside the mantle and bring up material that may have stayed isolated for long periods.
Counting potassium atoms
In the laboratory, the team ground each rock into powder, dissolved the powder in strong acids, and separated potassium from the other elements.
They then used a technique called thermal ionization mass spectrometry. In that method, they heated and ionized the potassium so that an instrument could count how many atoms of each isotope were present.
The measurements were precise enough to detect differences of only a few dozen parts per million.
Most of the rocks they analyzed had potassium isotope ratios that matched the average mantle of Earth, the “normal” potassium composition used as a reference.
A specific group of samples stood out, though. Very ancient mafic rocks from those old crustal regions, along with some hotspot lavas from La Réunion and Kama’ehuakanaloa, all contained slightly less potassium-40 than the reference value, by about 65 parts per million.
That difference equals roughly 65 “special” atoms out of one million potassium atoms, and the measurements show that this small shift is real and repeatable rather than random noise.
More proto-Earth evidence needed
When rocks melt, cool, and crystallize, isotopes can separate in subtle ways. These shifts usually follow predictable patterns that depend on mass.
The researchers tested whether common magmatic processes like partial melting or crystallization could produce the specific potassium-40 deficit they saw in their data.
Their calculations showed that ordinary geological processes inside Earth could not easily create this signal.
Because of that result, they turned to models that track the planet’s early evolution. In these models, they started with an early planet that already had a potassium-40-poor composition similar to the unusual rocks.
They then added the effects of the giant impact, later smaller impacts, and billions of years of mantle convection and mixing.
Hidden pockets inside the mantle
In the simulations, most of the mantle gradually moved toward the same composition seen in ordinary rocks at the surface. Deep regions that avoided strong stirring kept the older, potassium-40-poor pattern.
These long-lived mantle domains formed early and stayed relatively isolated despite the violence of planetary evolution.
The team argues that rocks with the potassium-40 deficit come from these domains, which formed before the Moon-forming impact and somehow survived it. The domains likely sit deep in the mantle and occasionally feed mantle plumes that rise to the surface.
The shared potassium isotope pattern in both extremely ancient crustal rocks and modern hotspot lavas suggests that these very different samples tap the same hidden reservoir.
Proto-Earth comes into focus
The potassium isotope pattern in these suspected “proto-Earth” rocks does not match any known meteorite group.
Scientists usually treat meteorites as pieces of the original material that built the planets, so this outcome hints that the meteorites we have do not represent every type of body that contributed to Earth.
Some of those original bodies may no longer send fragments our way, or their pieces may still sit undiscovered.
For you as a student, the main takeaway is about method. By carefully measuring tiny differences in potassium isotopes, scientists can test ideas about events that happened more than 4.5 billion years ago inside Earth.
Studies like this bring together chemistry, physics, and geology to connect tiny measurements in the lab with the long history of the planet we live on today.
The full study was published in the journal Nature Geoscience.
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