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An ocean (from Greek Ωκεανός, Okeanos (Oceanus)) is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface (an area of some 361 million square kilometers) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas. More than half of this area is over 3,000 meters (9,800 ft) deep. Average oceanic salinity is around 35 parts per thousand (ppt) (3.5%), and nearly all seawater has a salinity in the range of 30 to 38 ppt.
Though generally recognized as several 'separate' oceans, these waters comprise one global, interconnected body of salt water often referred to as the World Ocean or global ocean. This concept of a global ocean as a continuous body of water with relatively free interchange among its parts is of fundamental importance to oceanography. The major oceanic divisions are defined in part by the continents, various archipelagos, and other criteria: these divisions are (in descending order of size) the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean (which is sometimes subsumed as the southern portions of the Pacific, Atlantic, and Indian Oceans), and the Arctic Ocean (which is sometimes considered a sea of the Atlantic). The Pacific and Atlantic may be further subdivided by the equator into northerly and southerly portions. Smaller regions of the oceans are called seas, gulfs, bays and other names. There are also some smaller bodies of saltwater that are on land and not interconnected with the World Ocean, such as the Aral Sea, and the Great Salt Lake – though they may be referred to as 'seas', they are actually salt lakes. There are five oceans of the world: the Pacific Ocean the largest and deepest ocean of the world, the Atlantic Ocean the second largest and deepest ocean of the world, the Indian Ocean, the Southern Ocean, and the Arctic Ocean the smallest and shallowest ocean of the world.
The oceans cover ¾ of the earth’s surface and have great impact on the biosphere. The evaporation of these oceans is how we get most of our rainfall, and their temperature determines our climate and wind pattern. Life within the ocean had already evolved 3 billion years prior to the movement of animal and plant life on land. The amount of life and distance from shore (abiotic factor) effects the major distribution of marine biomes. Animals such as algae, branacles and muscles who live within the intertidal zone (land meets ocean) will fix themselves to rocks so they do not get washed from the resulting tides. The ocean is home to many species and consists of several other zones such as pelagic, benthic (sea floor), photic, and aphotic.
Geologically, an ocean is an area of oceanic crust covered by water. Oceanic crust is the thin layer of solidified volcanic basalt that covers the Earth's mantle where there are no continents. From this perspective, there are three oceans today: the World Ocean and the Caspian and Black Seas, the latter two having been formed by the collision of Cimmeria with Laurasia. The Mediterranean Sea is very nearly a discrete ocean, being connected to the World Ocean through the Strait of Gibraltar, and indeed several times over the last few million years movement of the African continent has closed the strait off entirely. The Black Sea is connected to the Mediterranean through the Bosporus, but this is in effect a natural canal cut through continental rock some 7,000 years ago, rather than a piece of oceanic sea floor like the Strait of Gibraltar.
The area of the World Ocean is 361 million square kilometers (139 million sq mi), its volume is approximately 1.3 billion cubic kilometers (310 million cu mi), and its average depth is 3,790 meters (12,430 ft). Nearly half of the world's marine waters are over 3,000 meters (9,800 ft) deep. The vast expanses of deep ocean (anything below 200m) cover about 66% of the Earth's surface. This does not include seas not connected to the World Ocean, such as the Caspian Sea.
The total mass of the hydrosphere is about 1.4 × 1021 kilograms, which is about 0.023% of the Earth's total mass. Less than 2% is freshwater; the rest is saltwater, mostly in the ocean.
A common misconception is that the oceans are blue primarily because the sky is blue. In fact, water has a very slight blue colour that can only be seen in large volumes. While the sky's reflection does contribute to the blue appearance of the surface, it is not the primary cause. The primary cause is the absorption by the water molecules' nuclei of red photons from the incoming light, the only known example of colour in nature resulting from vibrational, rather than electronic, dynamics.
A number of sailors and professional mariners have reported that the ocean often emits a visible glow, or luminescence, which extends for miles at night. In 2005, scientists announced that for the first time, photographic evidence had been obtained of this glow. It may be due to bioluminescence.
Travel on the surface of the ocean through the use of boats dates back to prehistoric times, but only in modern times has extensive underwater travel become possible.
The deepest point in the ocean is the Marianas Trench located in the Pacific Ocean near the Northern Mariana Islands. It has a maximum depth of 10,923 meters (35,838 ft) . It was fully surveyed in 1951 by the British naval vessel, "Challenger II" which gave its name to the deepest part of the trench, the " Challenger Deep". In 1960, the Trieste successfully reached the bottom of the trench, manned by a crew of two men.
Much of the bottom of the world's oceans are unexplored and unmapped. A global image of many underwater features larger than 10 kilometers (6 mi) was created in 1995 based on gravitational distortions of the nearby sea surface.
Oceans are divided into numerous regions depending on the physical and biological conditions of these areas. The pelagic zone includes all open ocean regions, and can be subdivided into further regions categorized by depth and light abundance. The photic zone covers the oceans from surface level to 200 meters down. This is the region where the photosynthesis most commonly occurs and therefore contains the largest biodiversity in the ocean. Since plants can only survive with photosynthesis any life found lower than this must either rely on material floating down from above (see marine snow) or find another primary source; this often comes in the form of hydrothermal vents in what is known as the aphotic zone (all depths exceeding 200m). The pelagic part of the photic zone is known as the epipelagic. The pelagic part of the aphotic zone can be further divided into regions that succeed each other vertically. The mesopelagic is the uppermost region, with its lowermost boundary at a thermocline of 12°C, which, in the tropics generally lies between 700 and 1,000 m. After that is the bathypelagic lying between 10°C and 4°C, or between 700 or 1,000 m and 2,000 or 4,000 m. Lying along the top of the abyssal plain is the abyssalpelagic, whose lower boundary lies at about 6,000 m. The final zone falls into the oceanic trenches, and is known as the hadalpelagic. This lies between 6,000 m and 10,000 m and is the deepest oceanic zone.
Along with pelagic aphotics zones there are also benthic aphotic zones, these correspond to the three deepest zones. The bathyal zone covers the continental slope and the rise down to about 4,000 m. The abyssal zone covers the abyssal plains between 4,000 and 6,000 m. Lastly, the hadal zone corresponds to the hadalpelagic zone which is found in the oceanic trenches. The pelagic zone can also be split into two subregions, the neritic zone and the oceanic zone. The neritic encompasses the water mass directly above the continental shelves, while the oceanic zone includes all the completely open water. In contrast, the littoral zone covers the region between low and high tide and represents the transitional area between marine and terrestrial conditions. It is also known as the intertidal zone because it is the area where tide level affects the conditions of the region.
One of the most dramatic forms of weather occurs over the oceans: tropical cyclones (also called "typhoons" and "hurricanes" depending upon where the system forms). Ocean currents greatly affect the Earth's climate by transferring warm or cold air and precipitation to coastal regions, where they may be carried inland by winds. The Antarctic Circumpolar Current encircles that continent, influencing the area's climate and connecting currents in several oceans.
Lifeforms native to oceans include (among others):
- Cetacea such as whales, dolphins and porpoises,
- Cephalopods such as the octopus
- Crustaceans such as lobsters and shrimp
- Marine worms
- Echinoderms (brittle star, starfish, sea cucumber, sand dollar)
The oceans are essential to transportation: most of the world's goods are moved by ship between the world's seaports. Important ship canals include the Saint Lawrence Seaway, Panama Canal, and Suez Canal. They are also an important source of valuable food items for the fishing industry. Some of these are shrimp, fish, crabs and lobster.
Continental drift has reconfigured the Earth's oceans, joining and splitting ancient oceans to form the current oceans. Ancient oceans include:
- Bridge River Ocean, the ocean between the ancient Insular Islands and North America.
- Iapetus Ocean, the southern hemisphere ocean between Baltica and Avalonia.
- Panthalassa, the vast world ocean that surrounded the Pangaea supercontinent.
- Rheic Ocean
- Slide Mountain Ocean, the ocean between the ancient Intermontane Islands and North America.
- Tethys Ocean, the ocean between the ancient continents of Gondwana and Laurasia.
- Khanty Ocean, the ocean between Baltica and Siberia.
- Mirovia, the ocean that surrounded the Rodinia supercontinent.
- Paleo-Tethys Ocean, the ocean between Gondwana and the Hunic terranes.
- Proto-Tethys Ocean,
- Pan-African Ocean, the ocean that surrounded the Pannotia supercontinent.
- Superocean, the ocean that surrounds a global supercontinent.
- Ural Ocean, the ocean between Siberia and Baltica.
- See also Extraterrestrial liquid water
Earth is the only known planet with liquid water on its surface and is certainly the only one in our own solar system. However, liquid water is thought to be present under the surface of the Galilean moons Europa and, with less certainty, Callisto and Ganymede. Geysers have been found on Enceladus, though these may not involve bodies of liquid water. Other icy moons may have once had internal oceans that have now frozen, such as Triton. The planets Uranus and Neptune may also possess large oceans of liquid water under their thick atmospheres, though their internal structure is not well understood at this time.
There is currently much debate over whether Mars once had an ocean of water in its northern hemisphere, and over what happened to it if it did; recent findings by the Mars Exploration Rover mission indicate it had some long-term standing water in at least one location, but its extent is not known.
Astronomers believe that Venus had liquid water and perhaps oceans in its very early history. If they existed, all trace of them seems to have vanished in later resurfacing.
Liquid hydrocarbons are thought to be present on the surface of Titan, though it may be more accurate to describe them as "lakes" rather than an "ocean." The Cassini-Huygens space mission initially discovered only what appeared to be dry lakebeds and empty river channels, suggesting that Titan had lost what surface liquids it might have had. A more recent fly-by of Titan made by Cassini has produced radar images that strongly suggest hydrocarbon lakes near the polar regions where it is colder. Titan is also thought likely to have a subterranean water ocean under the mix of ice and hydrocarbons that forms its outer crust.
Beyond the solar system, Gliese 581 c is at the right distance from its sun for liquid water to exist on the planet's surface. Since it does not transit its sun, there is no way to know if there is any water there. HD 209458b may have water vapour in its atmosphere--this is currently being disputed. Gliese 436 b is believed to have "hot ice." Neither of these planets are cool enough for liquid water--but if water molecules exist there, they are also likely to be found on planets at a suitable temperature.
The original concept of "ocean" goes back to notions of Mesopotamian and Indo-European mythology, imagining the world to be encircled by a great river. Okeanos, "Ωκεανός" in Greek, reflects the ancient Greek observation that a strong current flowed off Gibraltar and their subsequent assumption that it was a great river. (Compare also Samudra from Hindu mythology and Jörmungandr from Norse mythology.) The world was imagined to be enclosed by a celestial ocean above the heavens, and an ocean of the underworld below (compare Rasā, Varuna).