Earth was a broiling mass of lava and rock before it took the form that it has today, and scientists have for the first time discovered traces of that very early ‘proto Earth’, hidden away in the deepest and oldest rocks on our planet.
The clues have survived some 4.5 billion years, so it’s an astonishing find. The international team of researchers behind the discovery compares it to picking out a single grain in a bucket of sand.
As small and as ancient as these traces are, they can now be used to explore the environmental conditions that would have dominated Earth when it was still a brand-new planet. The discovery could also help reveal how planets like ours might have come into existence.
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“This is maybe the first direct evidence that we’ve preserved the proto Earth materials,” says geochemist Nicole Nie, from the Massachusetts Institute of Technology (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.”
This proto Earth only existed for a small sliver of time: perhaps around 100 million years. At that point, a huge collision with an incoming Mars-sized meteorite called Theia changed the composition of our planet, and gave us the Moon as a next-door neighbor.
Nie and colleagues were looking for a deficit in the potassium-40 isotope. A previous study of meteorites had identified variations in this element as a suitable way of establishing the origins of rocks, by comparing them with other rocks found on Earth, delivered from elsewhere in the Solar System.
“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,” says Nie.
Through a detailed analysis of ancient rock samples from Greenland, Canada, and Hawaii – where volcanic activity brings up material from deep in the mantle – the researchers found a unique potassium signature that hadn’t been seen before.
This chemical signature or fingerprint doesn’t show up in studies of other large impacts on Earth, or in any geological processes happening on our planet now. The most likely explanation: these rocks are remnants from the dawn of Earth’s history.
Computer simulations based on existing meteorite data ran through how these rocks would have been altered by 4.5 billion years of geological aging and further collisions. These changes were then matched up with the signature found by the team – further evidence that this material is from the proto Earth.
There’s another takeaway from this research: there are types of meteorites out there that we haven’t found yet. The proto Earth rock must have come from somewhere, as clumps of gas and dust gradually formed the planet, and that’s something that future studies can look into.
“Scientists have been trying to understand Earth’s original chemical composition by combining the compositions of different groups of meteorites,” says Nie.
“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 research has been published in Nature Geosciences.
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