Geologists discover first evidence of 4.5 billion-year-old ‘proto-Earth’

Scientists at MIT and elsewhere have discovered extremely rare remains of the “proto-Earth,” which formed about 4.5 billion years ago, before a colossal collision irreversibly changed the composition of the early planet and gave rise to Earth as we know it today. Their findings, reported today in the journal Natural geoscienceswill help scientists piece together the primordial ingredients that forged the beginnings of Earth and the rest of the solar system.

Billions of years ago, the early solar system was a swirling disk of gas and dust that eventually clumped together and accumulated to form the first meteorites, which in turn coalesced to form proto Earth and its neighboring planets.

In this first phase, the Earth was probably rocky and seething with lava. Then, less than 100 million years later, a Mars-sized meteorite crashed into the fledgling planet in a singular “giant impact” that completely scrambled and melted the planet’s interior, resetting its chemistry. The original material from which proto-Earth was made was thought to have been completely transformed.

But the MIT team’s findings suggest otherwise. Researchers have identified a chemical signature in ancient rocks that is unique compared to most other materials found on Earth today. The signature appears in the form of a subtle imbalance of potassium isotopes discovered in samples of very old and very deep rocks. The team determined that the potassium imbalance could not have been produced by large impacts or previous geological processes currently occurring on Earth.

The most likely explanation for the chemical composition of the samples is that they must be remnants of material from proto-Earth that remained unchanged, even though most of the first planet was impacted and transformed.

“This may be the first direct evidence that we preserved proto-Earth materials,” says Nicole Nie, the Paul M. Cook Career Development Assistant Professor of Earth and Planetary Sciences at MIT. “We see a piece of the very ancient Earth, even before the giant impact. It’s amazing because we would expect this very early signature to be slowly erased over the course of Earth’s evolution.”

Other authors of the study include Da Wang of Chengdu University of Technology in China, Steven Shirey and Richard Carlson of the Carnegie Institution for Science in Washington, Bradley Peters of ETH Zurich in Switzerland, and James Day of the Scripps Institution of Oceanography in California.

A curious anomaly

In 2023, Nie and his colleagues analyzed many major meteorites collected from sites around the world and carefully studied. Before impacting Earth, these meteorites likely formed at different times and locations in the solar system and therefore represent changing conditions in the solar system over time. When the researchers compared the chemical composition of these meteorite samples to that of Earth, they identified a “potassium isotope anomaly” among them.

Isotopes are slightly different versions of an element that have the same number of protons but a different number of neutrons. The element potassium can exist in one of three natural isotopes, with mass numbers (protons plus neutrons) of 39, 40, and 41, respectively. Wherever potassium has been found on Earth, it exists as a characteristic combination of isotopes, with potassium 39 and potassium 41 being predominantly dominant. Potassium 40 is present, but at a tiny percentage in comparison.

Nie and his colleagues found that the meteorites they studied had different potassium isotope balances than most materials on Earth. This potassium anomaly suggests that any material with a similar anomaly likely predates Earth’s current composition. In other words, any potassium imbalance would be a strong sign of the presence of material from proto-Earth, before the giant impact changed the chemical composition of the planet.

“In this work, we found that different meteorites have different isotopic signatures of potassium, which means that potassium can be used as a tracer of Earth’s building blocks,” says Nie.

“Built differently”

In the current study, the team looked for signs of potassium anomalies not in meteorites, but within Earth. Their samples include rocks, in powder form, from Greenland and Canada, where some of the oldest preserved rocks are found. They also analyzed lava deposits collected in Hawaii, where volcanoes have extracted some of Earth’s oldest and 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 like to search for it in the depths of time and the Earth,” says Nie.

The team first dissolved the different powder samples in acid, then carefully isolated any potassium from the rest of the sample and used a special mass spectrometer to measure the ratio of each of the three potassium isotopes. Remarkably, they identified an isotopic signature in the samples different from that found in most materials on Earth.

Specifically, they identified a potassium-40 isotope deficiency. In most materials on Earth, this isotope already constitutes an insignificant fraction compared to the other two isotopes of potassium. But the researchers found that their samples contained an even lower percentage of potassium-40. Detecting this small deficit is like spotting a single grain of brown sand in a bucket rather than a spoon full of yellow sand.

The team found that, indeed, the samples were deficient in potassium 40, showing that the materials “were constructed differently,” Nie says, compared to much of what we see on Earth today.

But could the samples be rare remnants of proto-Earth? To answer this question, the researchers hypothesized that this might be the case. They reasoned that if proto-Earth had originally been made from potassium-40-deficient materials, then most of those materials would have undergone chemical changes – due to the giant impact and subsequent impacts of smaller meteorites – that would ultimately have resulted in the higher-potassium-40-containing materials we see today.

The team used compositional data from all known meteorites and ran simulations of how the samples’ potassium-40 deficiency would change following impacts from these meteorites and the giant impact. They also simulated geological processes that Earth has experienced over time, such as mantle warming and mixing. Ultimately, their simulations produced a composition with a slightly higher fraction of potassium 40 than samples from Canada, Greenland, and Hawaii. More importantly, the simulated compositions matched those of most modern materials.

The work suggests that materials with a potassium-40 deficiency are likely remnants of original materials from proto-Earth.

Curiously, the signature of the samples does not exactly match that of another meteorite present in the geologists’ collections. Although meteorites from the team’s previous work showed potassium anomalies, they do not exactly match the deficit seen in proto-Earth samples. This means that meteorites and materials originally formed on Earth have not yet been discovered.

“Scientists have attempted to understand the original chemical composition of the Earth by combining the compositions of different groups of meteorites,” says Nie. “But our study shows that the current inventory of meteorites is not complete and that there is still much to learn about the origin of our planet.”

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news in MIT research, innovation and education.