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Following a world-altering impact with another nascent planet, proto-Earth developed into the world recognized today. (Image credit: MIT News; iStock)
Remnants of the fiery, magma-coated “proto-planet” which came before Earth 4.5 billion years ago have survived intact within ancient rocks, an innovative study suggests.
The fragments display distinct potassium characteristics not observed in other rocks or space rocks assessed by investigators, according to findings shared Oct. 14 in the journal Nature Geoscience. These characteristics should have, theoretically, vanished during the major collision which gave rise to the moon, but it now seems a small number endured this devastating occurrence and successfully resisted the passage of time.
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Proto-Earth existed as an incandescent sphere of bubbling, liquid rock, which originated from cosmic particulates and space rocks within the young solar system. However, after a period of 100 million years, our ancient world experienced a devastating event involving an object roughly the scale of Mars, referred to as Theia. This impact was so forceful that it totally rearranged the interior of proto-Earth and expelled a part of Earth’s early form which became the moon.
Theia also introduced huge volumes of new material to proto-Earth, fundamentally changing the composition of Earth’s predecessor and reshaping it into a world more comparable to today’s. Through time, plate tectonics manifested and material was repeatedly reintegrated into Earth’s core. Consequently, researchers had considered it improbable to uncover undamaged portions of proto-Earth in modern rocks.
Scientists have previously recognized rocks exhibiting unique chemical traits connected to the element ruthenium, potentially preceding the moon-forming impact, yet these signatures may have just as easily emerged after the event, making them less reliable as evidence, as stated by Philip Carter, a computational planetary scientist and astrophysicist from the University of Bristol in the U.K., to Live Science.
In contrast, the freshly discovered potassium traits offer the strongest proof so far that fragments of proto-Earth remain present, added Carter, who was not directly associated with the study. “The most plausible explanation is that this material has endured from before the major impact,” he proposed.
Clues in potassium ratios
These recently identified signatures take the form of subtle differences in the relative amounts of distinct versions, or isotopes, of potassium compared to other substances on Earth. Potassium naturally comprises three isotopes — potassium-39, potassium-40, and potassium-41 — which share the same quantity of protons but vary in their neutron count, thus possessing different atomic weights.
Potassium-39 and potassium-41 are the primary isotopes in Earth’s rocks, whereas potassium-40 is only found in extremely small measures. In prior research, the authors of the current study observed unusual abundances of potassium-40 within space rocks, recording shifting conditions within the solar system across extensive time spans. This hinted that isotopic irregularities in potassium could pinpoint substances that existed before the formulation of modern Earth.
For this latest study, Nie and her fellow researchers obtained samples from ancient rocks found in a selection of locations previously known to have strange ruthenium signatures, including areas in Greenland, Canada, and Hawaii. To identify possible isotopic irregularities in potassium, the researchers pulverized the rocks and diluted them with an acidic solution. Next, they segregated the potassium from within the samples and assessed the proportion of different potassium isotopes via mass spectrometry.
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The researchers observed that the rocks lacked potassium-40 relative to the quantities found in other Earth materials. To determine whether this isotopic anomaly in potassium could be traced back to proto-Earth, the team conducted computational simulations. Using information from every known meteorite that has landed on Earth, they simulated the effects of such impacts and the moon-forming collision on Earth’s composition through the addition of new substances over vast stretches of time.
The simulations indicated that the collision with Theia, specifically, released considerable amounts of potassium-40 onto Earth, which clarifies the higher amount of potassium-40 observed in contemporary rocks. “It necessitates adding a considerable quantity of material to…alter the entire signature and the overall isotopic makeup of potassium in a majority of rocks,” specified Carter. “The moon-forming impact itself is responsible for much of this change; this forms the basis of their argument in the paper.”
When Theia collided with proto-Earth, it brought additional material that continues to influence the composition of our planet now.
The potassium signature identified within the ancient rocks stands apart from what Nie and her colleagues had previously detected in space rocks, rendering it improbable that space rocks could have structured Earth’s current potassium outline after the moon-forming event. “It effectively suggests that proto-Earth originated from a substance that is isotopically distinct from any of the space rocks at our disposal,” Carter clarified.
According to Carter, the moon-forming impact remains the sole recognized occurrence with the capacity to have notably amplified the quantity of potassium-40 in Earth’s rocks. He suggests that this signifies that the potassium-40-deficient rocks discovered in Greenland, Canada, and Hawaii predate the moon-forming impact, linking them back to proto-Earth.
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Martin Schiller, an associate professor of geochemistry at the University of Copenhagen in Denmark, unaffiliated with the study, concurred that the findings seem compelling. He informed Live Science via email, stating, “The truly astonishing/groundbreaking discovery is that the potassium isotope characteristic [within the ancient rocks] cannot be accounted for through a mixture of primeval space rocks.”
The results imply that fragments of proto-Earth persevered through geological processes like continuous mixing within the mantle, the layer of Earth that lies beneath the crust.
Carter noted, “It represents a trait that has been preserved apart from the remaining Earth rocks for a substantial duration.” He also indicated that a higher concentration of this proto-Earth substance likely resides at the base of the mantle, adding, “We are only accessing the smaller quantities that surface.”
TOPICSplate tectonics
Sascha PareSocial Links NavigationStaff writer
Sascha serves as a staff writer for Live Science, located in the U.K. She earned a bachelor’s degree in biology from the University of Southampton in England, coupled with a master’s degree in science communication from Imperial College London. Her published works have appeared in The Guardian and the health website Zoe. Outside of writing, she finds joy in playing tennis, making bread, and exploring second-hand stores in pursuit of hidden treasures.
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