An ocean (from Ancient Greek Ὠκεανός,transc. Okeanós, the sea of classical antiquity) is a body of saline water that composes much of a planet's hydrosphere.On Earth, an ocean is one of the major conventional divisions of the World Ocean, which covers almost 71% of its surface. These are, in descending order by area, thePacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans. The word sea is often used interchangeably with "ocean" inAmerican English but, strictly speaking, a sea is a body of saline water (generally a division of the world ocean) partly or fully enclosed by land.
Saline water covers approximately 72% of the planet's surface (~3.6×108 km2) and is customarily divided into several principal oceans and smaller seas, with the ocean covering approximately 71% of Earth's surface and 90% of the earth's biosphere. The ocean contains 97% of Earth's water, andoceanographers have stated that less than 5% of the World Ocean has been explored.The total volume is approximately 1.35 billion cubic kilometers (320 million cu mi) with an average depth of nearly 3,700 meters (12,100 ft).
As it is the principal component of Earth's hydrosphere, the world ocean is integral to all known life, forms part of the carbon cycle, and influences climate and weather patterns. It is the habitat of 230,000 known species, although much of the oceans depths remain unexplored, and over two million marinespecies are estimated to exist. The origin of Earth's oceans remains unknown; oceans are thought to have formed in the Hadeanperiod and may have been the impetus for theemergence of life.
Extraterrestrial oceans may be composed of water or other elements and compounds. The only confirmed large stable bodies of extraterrestrial surface liquids are the lakes of Titan, although there is evidence for the existence of oceans elsewhere in the Solar System. Early in their geologic histories, Marsand Venus are theorized to have had large water oceans. The Mars ocean hypothesissuggests that nearly a third of the surface of Mars was once covered by water, and arunaway greenhouse effect may have boiled away the global ocean of Venus. Compounds such as salts and ammonia dissolved in water lower its freezing point, so that water might exist in large quantities in extraterrestrial environments as brine or convecting ice. Unconfirmed oceans are speculated beneath the surface of manydwarf planets and natural satellites; notably, the ocean of Europa is estimated to have over twice the water volume of Earth. The Solar System's giant planets are also thought to have liquid atmospheric layers of yet to be confirmed compositions. Oceans may also exist on exoplanets and exomoons, including surface oceans of liquid water within acircumstellar habitable zone. Ocean planetsare a hypothetical type of planet with a surface completely covered with liquid.[11