🌍 Discovery of the first evidence of a "proto-Earth" deep within the Earth

At the heart of some of the oldest rocks in the Earth's crust, an international team has identified a chemical signature never observed until now. This major discovery opens a unique window onto the Earth's earliest moments, long before its face was definitively transformed by the cataclysmic event that gave birth to the Moon.

These remnants, carefully preserved for billions of years, offer a direct glimpse into the primitive materials that formed our world.


The quest for our planet's origins often relies on the comparative study of meteorites, considered the elementary building blocks of the Solar System. However, a fine analysis of potassium isotopes in terrestrial samples has revealed anomalies unexplained by current models. These imbalances suggest the existence of a distinct material that did not undergo the chemical reworking that affected the majority of the planet.

This material could constitute the first direct sample of the primitive Earth ever identified.

The search for a lost signature

The element potassium naturally occurs in three isotopes, whose relative proportions are generally constant in terrestrial materials. However, a preliminary study published in Science Advances had shown that some meteorites exhibited potassium isotope signatures different from those commonly observed on Earth. This variation was immediately seen as a potential tracer to distinguish materials of primordial origin from those modified by later geological processes. Potassium thus became a key to traveling back in time.

The research team then undertook to analyze rocks from exceptional geological sites. They collected samples from ancient formations in Greenland and Canada, as well as from lavas originating from volcanic hotspots like Hawaii. These lavas come from the deep mantle, where archaic materials can be preserved from surface mixing. The objective was to search, within these deep rocks, for the trace of the isotopic anomaly detected in meteorites.

Mass spectrometry analyses confirmed the presence of a unique signature. The researchers identified a specific deficit in potassium-40, the rarest isotope, in these samples. This chemical characteristic does not match any signal produced by known meteoritic impacts or by current geological processes. Its persistence shows that these rocks escaped the major reworking that shaped the composition of the rest of the planet.

The remnants of a lost world

To verify the origin of these atypical materials, scientists performed numerical simulations. These models integrated the composition data of all known meteorites and recreated the cumulative effects of cosmic impacts and internal geological evolution over 4.5 billion years. The results, published in Nature Geoscience, show that the giant impact that formed the Moon significantly enriched the Earth's mantle in potassium-40. Therefore, rocks showing a deficit in this isotope could not have formed after this event.

The isotopic composition of these samples does not perfectly match any meteorite cataloged to date. This divergence indicates that the materials that constituted the Earth's primitive core are not yet represented in scientific collections. They might belong to a family of celestial objects still unidentified or having completely disappeared. This discovery implies that the inventory of planetary materials is incomplete.

The preservation of these shreds of the primitive mantle demonstrates that some deep zones of the Earth have remained remarkably stable. Protected from intense convection and mixing, they have retained the chemical imprint of the planet's first few million years. These time capsules now offer geochemists a concrete reference point for reconstructing the conditions that prevailed during the Earth's initial accretion.