Where Did Earth's Water Come From? Volcanoes and Asteroids

Earth's water—oceans, lakes, rivers, ice, groundwater—originated from two primary cosmic and geological sources.

1. Volcanic Outgassing (Endogenous)

Early Earth's intense volcanism released massive amounts of water vapor (H₂O) from the mantle.
As the planet cooled, this steam condensed into rain, falling for millions of years, filling basins that became the first oceans.
Continues today: mid-ocean ridges and arc volcanoes add ~1 km³/yr (minor vs. total inventory).

2. Carbonaceous Chondrite Impacts (Exogenous)

Primitive meteorites (CI, CM chondrites) contain 5–20% water by weight (hydrated minerals).
Late Heavy Bombardment (~4.1–3.8 Ga) delivered vast quantities.
Isotopic match: Earth's D/H ratio (~150 ppm) aligns with chondrites, not comets (which are ~300 ppm).

Ongoing Cycle

Since then, water has cycled continuously: evaporation → precipitation → runoff → infiltration → storage (oceans, ice, aquifers, atmosphere, biota). The total planetary inventory (~1.386 billion km³) has been remarkably stable for ~3.8 billion years.