Light hydrogen isotopes in terrestrial core.

The origin of Earth's water remains unresolved. Hydrogen isotopes provide a key tracer of planetary water history, yet the isotope composition of Earth's core-the planet's largest internal reservoir-has remained unconstrained. Here, we use first-principles calculations combined with machine-learning-accelerated path-integral molecular dynamics to quantify hydrogen isotope fractionation between silicate and metallic melts under core-forming conditions. We find that core formation enriched the silicate Earth in deuterium while concentrating isotopically light hydrogen in the core, requiring the proto-Earth to have started with a lower deuterium-to-hydrogen (D/H) ratio than that preserves in the present-day mantle. Our models show that the bulk-Earth D/H ratio can be explained either by direct accretion of enstatite chondrite-dominated material or by isotopic resetting of deuterium-rich planetesimals through interactions with solar nebular gas. These results suggest that Earth's water inventory was established during the earliest stages of accretion.