The Moon-forming impact was the most violent event in Earth’s history. A Mars-sized body — Theia — struck the proto-Earth roughly 4.5 billion years ago. The collision was energetic enough to vaporize rock, eject a debris disk, and produce the Moon. Standard models assume it also homogenized Earth’s interior — mixing the proto-Earth’s original composition with Theia’s material so thoroughly that no chemical trace of the pre-impact planet should survive.

Nie et al. (Nature Geoscience, October 2025) found one.

Potassium-40 isotope ratios in some of Earth’s oldest rocks — Archean samples from Greenland and Canada, 3.7 to 4.3 billion years old — show a deficit that cannot be explained by any post-impact process. Radioactive decay, mantle mixing, crustal recycling — none produce this specific isotopic signature. The deficit matches what you would expect from the proto-Earth’s original composition: the planet that existed before the impact.

These are chemical remnants of a world that should have been destroyed. Not reprocessed fragments. Not post-impact crystallization products. Direct chemical survivors from the pre-collision planet, preserved in the deep mantle for 4.5 billion years while the rest of Earth’s interior convected and mixed around them.

The implication rewrites the standard model. The giant impact did not fully homogenize Earth’s mantle. Reservoirs of proto-Earth material — chemically distinct from everything that formed after the collision — persisted through billions of years of mantle convection. Some of Earth’s deepest geochemical anomalies, previously attributed to post-impact differentiation, may instead be inheritance. The planet remembers what it was before the collision.

The most violent event in Earth’s history failed to erase what came before.