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Researchers at MIT and other institutions have uncovered exceedingly rare traces of “proto Earth,” which emerged roughly 4.5 billion years ago, prior to a massive collision that irrevocably changed the early planet’s makeup and formed the Earth we recognize today. Their discoveries, shared today in the journal Nature Geosciences, will aid scientists in reconstructing the primordial building blocks that shaped the nascent Earth and the rest of the solar system.
Millennia ago, the fledgling solar system existed as a swirling disk of gas and dust that ultimately coalesced to create the initial meteorites, which in turn combined to form the proto Earth and its sibling planets.
In this initial stage, Earth was likely rugged and teeming with lava. However, in less than 100 million years, a Mars-sized meteorite collided with the young planet in a singular “giant impact” event that completely disrupted and melted the planet’s core, essentially resetting its chemical structure. Any original material that made up the proto Earth was presumed to have been completely altered.
Yet, the findings from the MIT team indicate otherwise. The scientists have detected a chemical signature in ancient rocks that deviates from the majority of other materials present on Earth today. This signature takes the form of a subtle imbalance in potassium isotopes identified in samples of extremely old and deeply buried rocks. The team concluded that this potassium imbalance could not have arisen from prior large impacts or geological processes occurring in Earth’s current state.
The most plausible explanation for the chemical composition of these samples is that they represent leftover material from the proto Earth that somehow remained unchanged, despite most of the early planet being affected and altered.
“This may be the first direct evidence we have that preserves the materials of proto Earth,” states Nicole Nie, the Paul M. Cook Career Development Assistant Professor of Earth and Planetary Sciences at MIT. “We observe a fragment of the very ancient Earth, even predating the giant impact. This is astonishing because we would anticipate this early signature to be gradually erased throughout Earth’s evolution.”
The study’s co-authors include Da Wang from Chengdu University of Technology in China, Steven Shirey and Richard Carlson from the Carnegie Institution for Science in Washington, Bradley Peters from ETH Zürich in Switzerland, and James Day from Scripps Institution of Oceanography in California.
An intriguing anomaly
In 2023, Nie and her associates examined many of the prominent meteorites gathered from locations worldwide and conducted meticulous analyses. Before striking the Earth, these meteorites likely formed at various times and places across the solar system, thus reflecting the solar system’s varying conditions over time. When the researchers compared the chemical structures of these meteorite samples to those of Earth, they identified a “potassium isotopic anomaly” among them.
Isotopes are slightly altered forms of an element that contain the same number of protons but differ in the number of neutrons. The element potassium can exist in three naturally occurring isotopes, characterized by mass numbers (the sum of protons and neutrons) of 39, 40, and 41. In any location potassium has been discovered on Earth, it appears in a typical combination of isotopes, with potassium-39 and potassium-41 being overwhelmingly prevalent. Potassium-40 does exist, but at an exceedingly small fraction in contrast.
Nie and her team found that the meteorites they investigated displayed distributions of potassium isotopes that were dissimilar from most Earth materials. This potassium anomaly indicated that any material exhibiting a similar anomaly likely predates the current composition of Earth. In other words, any imbalance of potassium would serve as strong evidence of material from proto Earth, prior to the giant impact that reset the planet’s chemical structure.
“In that investigation, we uncovered that various meteorites possess distinct potassium isotopic signatures, indicating potassium can serve as a tracer for Earth’s foundational components,” Nie elaborates.
“Built distinct”
In the current research, the team sought potassium anomalies not in meteorites, but within the Earth itself. Their samples included powdered rocks from Greenland and Canada, where some of the oldest preserved rocks are located. They also examined lava deposits sourced from Hawaii, where volcanic activity has brought forth some of Earth’s earliest, deepest materials from the mantle (the planet’s thickest layer of rock that separates the crust from the core).
“If this potassium signature is preserved, we would aim to trace it back through deep time and deep Earth,” Nie states.
The team initially dissolved the various powdered samples in acid, then meticulously isolated any potassium from the remainder of the sample and utilized a specialized mass spectrometer to assess the ratio of each of potassium’s three isotopes. Remarkably, they identified an isotopic signature in the samples that differed from what has been found in most materials on Earth.
Specifically, they discovered a deficiency in the potassium-40 isotope. In the majority of Earth’s materials, this isotope is already a negligible fraction compared to potassium’s other two isotopes. Yet the researchers discerned that their samples exhibited an even smaller percentage of potassium-40. Detecting this minimal deficit is akin to identifying a single grain of brown sand in a bucket rather than a scoop full of yellow sand.
The team confirmed that, indeed, the samples displayed the potassium-40 deficiency, indicating that these materials “were built distinct,” Nie asserts, in contrast to most of what we observe on Earth today.
However, could these samples truly be rare remnants of proto Earth? To explore this, the researchers considered this possibility. They reasoned that if the proto Earth had originally been composed of such potassium-40-deficient materials, then much of this material would have undergone chemical modifications — due to the giant impact and subsequent, smaller meteorite impacts — leading to the materials with greater potassium-40 that we observe today.
The team leveraged compositional data from every known meteorite and conducted simulations to examine how the potassium-40 deficit in the samples would shift following impacts from these meteorites and by the giant impact. They also simulated geological processes that Earth experienced over time, such as the heating and mixing of the mantle. Ultimately, their simulations yielded a composition with a slightly higher proportion of potassium-40 compared to the samples from Canada, Greenland, and Hawaii. More crucially, the simulated compositions aligned with those of most contemporary materials.
The research suggests that materials with a potassium-40 deficit likely represent leftover original material from proto Earth.
Interestingly, the samples’ signature is not an exact match with any other meteorite in geologists’ collections. Although the meteorites in the team’s previous research displayed potassium anomalies, they do not align precisely with the deficit observed in the proto Earth samples. This implies that whatever meteorites and materials initially formed the proto Earth remain to be discovered.
“Scientists have been endeavoring to comprehend Earth’s original chemical makeup by amalgamating the compositions of various meteorite groups,” Nie remarks. “However, our study demonstrates that the present inventory of meteorites is incomplete, and there is considerably more to understand about the origins of our planet.”
This research was partially funded by NASA and MIT.
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