Three Theories on the Origin of Earth’s Ocean Water

Where Did All the Water Come From?

1. Outgassing (Degassing) – Volcanic activity released water vapor trapped in Earth’s mantle during planetary cooling. Vapor condensed into rain, filling basins over millions of years.
2. Comet & Asteroid Delivery – Icy planetesimals bombarded early Earth, depositing extraterrestrial water. Isotopic ratios (D/H) in ocean water match some carbonaceous chondrites.
3. Primordial Water – Hydrogen and oxygen in the solar nebula combined directly during Earth’s accretion, forming water that never fully escaped.

Most scientists now favor a hybrid model: outgassing provided the bulk, supplemented by late‑stage asteroid/comet impacts (the “late veneer”).