Scientists at the Massachusetts Institute of Technology (MIT) have discovered that "proto-Earth" material from the earliest stages of Earth's formation may still be preserved deep inside the Earth. Scientists have long puzzled over why the Earth's overall composition doesn't exactly match the mix of materials in ancient meteorites. One hypothesis is that the material absorbed by the Earth during the "proto-Earth" period (that is, in the early stages before the formation of the moon) is not the same as the material obtained after the giant impact.
To verify this point of view, the MIT team teamed up with multiple research institutions to use thermal ionization mass spectrometry to accurately measure potassium-40 and other isotopes in earth rocks at different times and in different depths. Research has found that in the earliest days of the Earth, the Earth was a hot, rocky planet, and then there was a big collision: a Mars-sized object violently collided with the young Earth. The impact was so powerful that it completely melted the Earth's interior and changed its chemical composition. The collision has long been widely believed in the scientific community to have wiped out all traces of Earth's original existence.
However, the latest research from MIT gives a different answer. Scientists have discovered unique chemical clues in deep ancient rocks that have potassium isotope combinations that are markedly different from modern Earth material. Neither subsequent space impacts nor current geological activities can explain this imbalance. It is speculated that this feature is most likely a remnant of the primitive Earth period, miraculously escaping the assimilation and rewriting of the original devastating impact.
Dr. Nicole Nie of MIT said: "This may be the first direct confirmation that the original earth material has been preserved. We see extremely old earth 'fragments' that even predate the giant impact. This is amazing because in theory this earliest earth feature should gradually disappear over the long evolution."
In 2023, Dr. Nie's team studied meteorites from all over the world. These meteorites were formed in different parts of the solar system and recorded the key to the chemical evolution of the solar system. Scientists have discovered that potassium has three isotopes (39, 40, and 41). On the earth, 39 and 41 are dominant, and 40 is extremely rare. However, the proportions of potassium isotopes in meteorites are different. This "potassium anomaly" implies that they contain primitive materials before the formation of the earth.
This study conducted an in-depth analysis of Earth samples, including Archean mafic rocks and modern ocean island basalts from Greenland, Canada, and Hawaii. They dissolved rock samples in acid to extract the potassium and used a mass spectrometer to precisely measure the isotope ratios. The results show that these ancient rocks have unusually low levels of potassium-40, which is more scarce than elsewhere on Earth and rarely seen like the brown sand between the yellow particles on a beach.
The team also used simulations to analyze the changing trends of potassium-40 during the evolution of the Earth from its formation to subsequent impacts, heating and mixing. Models show that potassium-40 levels, either from large impacts or subsequent geological processes, usually increase slightly, but these ancient samples maintain unusually low levels and are considered rare, unaltered remnants of the early Earth.
The study pointed out that rocks with abnormally low potassium-40 content may indeed be leftover from the "primitive Earth", and their chemical characteristics are still different from existing meteorite samples. Dr. Nie said: "Scientists have been trying to infer the original chemical structure of the Earth through the composition of various types of meteorites, and our research shows that the existing meteorite catalog is far from complete, and there is still much to be discovered about the Earth's origin story."
Relevant papers were published in the journal Nature Geoscience.