The timescale of forming Earth’s earliest water ocean in the Hadean Eon

The timing of the first water ocean formation on Earth is critical for understanding the onset of habitability, but the duration of this process remains poorly constrained. Using a heat-budget model for the early atmosphere-ocean system, this study quantifies the transition from the final solidification of the magma ocean to the establishment of a stable hydrosphere. The results demonstrate that once the mantle transitioned to solid-state convection, a thick primordial steam atmosphere can radiatively cool and condense into a global water ocean within 10,000 years. Sensitivity tests indicate that this rapid condensation timescale is robust across reasonably oxidized volatile scenarios, even when accounting for cloud feedback and alternative mantle rheological trajectories. This finding suggests that a stable hydrosphere emerged almost contemporaneously with the solidification of Earth’s earliest crust, and provides a physical framework for the existence of liquid water by ∼4.4 Ga on the Earth and other terrestrial planets.