The ocean began as a gas. For a long time the young Earth was too hot for liquid water, and the water that would one day fill the ocean existed only as vapor. Only when the planet cooled below 212 degrees Fahrenheit, about 3.8 billion years ago, could that vapor condense into rain — and then it rained for centuries.
The ocean formed billions of years ago. Credit: NOAA.
Where the water came from
Most scientists agree that the atmosphere and the ocean built up gradually, over millions and millions of years, through the continual "degassing" of the Earth's interior. According to this theory, water vapor and other gases escaped from the planet's molten rocks into the atmosphere surrounding the cooling Earth.
The first ocean
Once the surface cooled below the boiling point of water, rain began to fall, and kept falling for centuries. The water drained into the great hollows in the Earth's surface, and the primeval ocean came into being — filling the basins we now know as the world ocean.
Why it stayed
Gravity kept the water from escaping the planet.
Sources
Based on "Why do we have an ocean?" from NOAA's National Ocean Service, a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from oceanservice.noaa.gov
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