Where did Earth's water come from?
In plain words
Earth's oceans match the hydrogen chemistry of some water-bearing meteorites, but water could also have come from rocks like those that built Earth or from the gas around the young Sun. The share from each source and its timing are unknown.
Precise statement
Earth's ocean D/H is about 1.5e-4, similar to CI and CM carbonaceous chondrites and to enstatite chondrites, while nebular hydrogen has D/H near 2e-5. Determine the fractional contributions of carbonaceous material, inner-disk (enstatite-like) material, nebular ingassing and comets to bulk Earth hydrogen, and whether delivery preceded or followed the Moon-forming impact. An answer is a mixing solution consistent with H, N, noble gas and nucleosynthetic isotope data.
What would settle it
Multi-isotope mass balance (H, N, Ne, Xe, Mo, Ru) of bulk Earth combined with dynamical delivery models.
Status in the literature
Unverified note
2025 metal-silicate partitioning experiments (arXiv:2508.17740) revise bulk-Earth hydrogen and carbon budgets.