The Earth is the only planet in our Solar System with huge amounts of water that has mostly remained in the same liquid state ever since it first formed. The latest theory explains that, after the world formed – but before it had developed an atmosphere – a variety of gases were released from the interior.
This ‘degassing’ process lasted for about 100 million years, after which time enough gases (including carbon dioxide, methane and ammonia) existed to form the atmosphere, as well as the vast oceans which cover so much of our planet today (70 per cent).
Gravity held the gases to the surface, and the temperature lowered below the boiling point so the gases could condense into water. Other possible contributors include water-rich meteorites and comets colliding with the Earth’s atmosphere, and the photosynthetic processes of bacteria that existed early in the planet’s life.