The water on Earth (and other terrestrial worlds) most likely was brought over by the comets during the period of heavy bombardment about 4 billion years ago…
These water (and other gases) were trapped in the interior, and released by volcanic activities…by Outgassing
The Atmosphere of Earth
The atmosphere of Earth contains primarily N2 (77%) and O2 (21%).
What happened to all the CO2
Where did all the O2 come from
CO2
CO2 is a colorless gas…
condenses into solid form (dry ice) at -78°C in atmospheric pressure.
condenses into liquid at -57°C at pressure above 5.1 atmospheric pressure.
Atmospheric CO2 is derived from (The sources…)
Volcanic outgassing
burning of organic matter
Respiration of living organisms
…
CO2 can be stored in (The Sinks…)
Highly soluble in water: forms H2CO3
Dissolved CO2 in water can interact with silicate minerals to form carbonated minerals…
…
Carbon Dioxide Cycle
The mechanism by which Earth self-regulates its temperature is called the carbon dioxide cycle, or the CO2 cycle for short.
Starting with the carbon dioxide in the atmosphere:
Volcanoes outgas CO2 into the atmosphere.
Atmospheric carbon dioxide dissolves in the oceans.
At the same time, rainfall erodes rocks on Earth's continents and rivers carry the eroded minerals to the oceans.
In the oceans, the eroded minerals combine with dissolved carbon dioxide and fall to the ocean floor, making carbonate rocks such as limestone.
Over millions of years, the conveyor belt of plate tectonics carries the carbonate rocks to subduction zones, and subduction carries them down into the mantle.
As they are pushed deeper into the mantle, some of the subducted carbonate rock melts and releases its carbon dioxide, which then outgasses back into the atmosphere through volcanoes.
The CO2 Cycle
If Earth warms up a bit, then

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