The oceans formed on Earth 3.8 billion years ago. At this time the Solar System was about 1 billion years old. To explain the formation of the oceans, it is necessary to briefly discuss the evolution of the Solar System.
The Solar System started as a swirling cloud of dust and gases. The dust and gas particles aggregated into bodies that became the Sun, planets, comets, and asteroids. The planets closest to the Sun are the most dense and consist mostly of rock and metal. These terrestrial planets include Mercury, Venus, Earth, and Mars. The outer planets, Jupiter, Saturn, Uranus, Neptune, and Pluto, are called the jovian planets. They are primarily composed of gases and ice, and are less dense than the terrestrial planets.
4 billion years ago, the Earth could be described as a very large, hot rock without a trace of water on its surface. Water on the young Earth came from two sources, outgassing from within the Earth and bombardment by comets. Outgassing is the process whereby gases are released from molten rock in the mantle of the planet by volcanic activity. This was probably the primary source of gases for the early atmosphere. Comets and meterorites also bring with them gases which contributed to the Earth's atmosphere.
Some of the gases in the new atmosphere were methane (CH3), ammonia (NH3), water vapor (H2O), and carbon dioxide (CO2). The water on Earth stayed in gaseous form until the planet's surface cooled below 100 degrees Celsius. At this time, 3.8 billion years ago, water condensed into rain and poured onto the land. Water collected in low lying areas which gradually became the primitive oceans.
As water entered the oceans from the atmosphere, it brought with it dissolved gasses released from the mantle by volcanoes and geysers. Water also flowed as runoff from the land, bringing in dissolved minerals from the rocks on the surface. These minerals include the salts which make seawater taste salty.
The geochemical cycles had their beginnings here, with minerals entering the
oceans from the land and sky and minerals leaving the oceans through
tectonic activity and by evaporation/deposition processes. These cycles were well
established about 1 billion years ago. Since then the overall composition
of the oceans (chemically, that is) has remained very constant.
This completes How did the Oceans form?
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