A team of geologists and Earth scientists have discovered what they believe to be traces of planet Earth, as it was when it first formed, lingering in ancient rocks.
There are various competing theories regarding the formation of Earth and the Moon.
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But the general consensus is that around 4.4 billion years ago, when our planet was no more than 100 million years old, it was struck by a large asteroid or other similarly large spacerock.
Debris from that impact then went into orbit around Earth, gradually coalescing into the familiar satellite – our Moon – that we see when we look up after dark.
Assuming such an impact took place, it would have engendered huge temperatures and pressures that, in turn, would have caused chemical reactions to occur in the very building blocks of our planet.
Earth’s geochemistry would have been forever altered by the impact.
Most scientists have traditionally assumed that any evidence of what Earth’s crust and mantle looked like (chemically) before the impact would have long since eroded away.
Discovering our ancient planet
Now, however, it seems that some traces of the ancient Earth may still linger on present-day Earth after all.
The research team – including geophysicists, geologists, geochemists, oceanographers and planetary scientists – published their findings in the journal Nature Geoscience on 14 October 2025.
Their technique mostly involved looking at the isotopes of potassium found within rocks from a wide variety of sources, including various known outcrops of very ancient rock as well as samples from meteorites.
Potassium occurs naturally in three different istopes: potassium-39, potassium-40 and potassium-41.
All three molecules have the same number of protons in their nucleus, but a different number of neutrons.
Rocks on Earth generally have high levels of potassium-39 and potassium-41, but only small traces of potassium-40.
During the study, the researchers gathered samples from several regions around the world that are known to contain very ancient rock.
These included the Isua Greenstone Belt in Greenland, the Kaapvaal Craton in Western Australia, the Nuvvuagittuq Greenstone Belt in Quebec, Canada, the Kamaʻehuakanaloa volcano in Hawaii and Réunion Island in the Indian Ocean.
They ground the samples to a fine powder, which they then dissolved in acid to isolate the potassium.
The team were then able to determine the balance between the three potassium isotopes within the rock, using an ultra-sensitive mass spectrometer.
What they found was that these ancient rocks contained even less potassium-40 than the rest of Earth’s rock does: the rocks displayed a 'potassium imbalance'.
Earth rocks versus spacerocks
The team then turned their attention to samples from meteorites, which have their own levels of the three different potassium isotopes.
They used computer modelling to see what would happen if similarly-constituted space rocks were to interact with the ancient rocks described above, under the kinds of pressure and temperature conditions you’d expect to see during a giant impact.
The resulting fusions predicted by the computer models looked remarkably similar to the rocks we most commonly see on Earth today.
That led the team to conclude that, while a giant impact likely caused most of the material in Earth’s mantle to transform both physically and chemically, the 'imbalanced' types of rock found in those ancient outcrops – that is, those with less potassium-40 – are relics of an earlier, pre-impact Earth.
"This is maybe the first direct evidence that we’ve preserved the proto-Earth materials," said Nicole Nie, an assistant professor of earth and planetary sciences at MIT who co-led the research.
"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."
More mysteries to solve
What the computer models failed to produce, however, was an exact match for Earth rocks today.
They fed the models data on all of the different potassium balances found in the meteorite samples (each of which is unique, the meteors having formed in different regions of the Solar System at different times) but in no case did a simulated impact produce quite what we see in the real world.
In other words, while the team may now have discovered proof of what Earth looked like before something enormous smashed into it, we still don’t know quite what that something was made of.
"Scientists have been trying to understand Earth’s original chemical composition by combining the compositions of different groups of meteorites," Nie said.
"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."
