How did Earth’s early atmosphere and oceans form?

Earth’s early atmosphere and oceans formed through complex processes involving planetary accretion, volcanic outgassing, and cooling.

Formation of the Atmosphere

Earth formed approximately 4.6 billion years ago through the accretion of cosmic dust and planetesimals. While the planet’s initial atmosphere may have consisted of captured primordial gases like hydrogen and helium, these were largely lost to space. As Earth’s interior warmed due to radioactive decay and compressional heating, it underwent differentiation, forming a core, mantle, and crust. This process triggered the release of volatile gases from the interior—a process known as volcanic outgassing. Early volcanic gases were likely rich in water vapour, carbon dioxide, carbon monoxide, and nitrogen. Over time, photodissociation of water vapour and the consumption of carbon dioxide through weathering altered the atmospheric composition, eventually allowing for the rise of oxygen through photosynthesis.1234567

Formation of the Oceans

The origin of Earth’s water remains a subject of scientific debate. Theories suggest that water was either delivered by comet and meteor impacts or released from the interior via volcanic outgassing. As the planet’s surface cooled below 100 °C (212 °F), water vapour in the atmosphere condensed into rain, accumulating in surface depressions to form the nascent oceans. Evidence from zircon grains suggests that liquid water may have been present as early as 4.4 billion years ago. These early oceans were likely acidic due to high levels of dissolved carbon dioxide, which facilitated the weathering of surface rocks and increased the water's salt content.1267