Surface of the Atlantic ocean meeting the Earth's planetary boundary layer and troposphere

An ocean (from Ancient Greek Ὠκεανὸς (Okeanos); the World Ocean of classical antiquity^[1]) is a body of saline water that composes a large part of a planet's hydrosphere.^[2] In the context of Earth, it also refers to major divisions of the planet's World Ocean, such as the Atlantic Ocean.^[3][4] The word "sea" is often used interchangeably with "ocean", but a sea is a body of saline water (possibly a division of the World Ocean) in a more inland location.^[5]

Earth's global ocean is the largest confirmed surface ocean on all observable planets. Approximately 71% of the planet's surface (~3.6x10⁸ km²) is covered by saline water that is customarily divided into several principal oceans and smaller seas. Because 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. The total volume is approximately 1.3 billion cubic kilometres (310 million cu mi)^[6] with an average depth of 3,790 metres (12,430 ft). It is the habitat of 230,000 species known to science, however much of the ocean's depths remain unexplored and it is estimated that over two million marine species may exist.^[7] The origin of Earth's oceans is still unknown though they are believed to have first appeared in the Hadean period and may have been the point of origin for the emergence of life.

Extraterrestrial oceans may be composed of a wide range of elements and compounds. The only confirmed large stable bodies of extraterrestrial surface liquids are the lakes of Titan though there is evidence for the existence of oceans elsewhere in the Solar System. Early in their geologic histories, Mars and Venus are theorised to have had large water oceans. The Mars ocean hypothesis suggests that nearly a third of the surface of Mars was once covered by water, though the water on Mars is no longer oceanic, and a runaway greenhouse effect may have boiled away the global ocean of Venus. Compounds such as salts and ammonia mixed water are known to lower the freezing point potentially allowing water to exist in large quantities in extraterrestrial environments as brine or convecting ice. The Solar System's gas giant planets are also believed to possess liquid atmospheric layers of yet to be confirmed compositions. Unconfirmed oceans are speculated beneath the surface of many dwarf planets and natural satellites, notably the ocean of Europa is believed to have over twice the water volume of Earth. Oceans may also exist on exoplanets and exomoons, including surface oceans of liquid water within a circumstellar habitable zone. Ocean planets are a hypothetical type of planet with a surface completely covered with liquid.

Earth's global ocean[link]

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History[link]

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 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.

Artwork which depicts marine-related themes is known as maritime art, a term which particularly applies to common styles of European painting between the 17th and 19th centuries.

Origin of oceans[link]

There are several acknowledged theories as to how Earth's oceans were formed over the past 4.4 billion years.^[8] Some of the most likely contributory factors to the origin of the oceans are as follows:

• The cooling of the primordial Earth to the point where the outgassed volatile components were held in an atmosphere of sufficient pressure for the stabilization and retention of liquid water.
• Comets, trans-Neptunian objects or water-rich meteorites (protoplanets) from the outer reaches of the main asteroid belt colliding with the Earth may have brought water to the world's oceans. Measurements of the ratio of the hydrogen isotopes deuterium and protium point to asteroids, since similar percentage impurities in carbon-rich chondrites were found to oceanic water, whereas previous measurement of the isotopes' concentrations in comets and trans-Neptunian objects correspond only slightly to water on the earth.
• Biochemically through mineralization and photosynthesis.
• Gradual leakage of water stored in hydrous minerals of the Earth's rocks.
• Photolysis: radiation can break down chemical bonds on the surface.

Formation of oceans[link]

Geologically, Earth's ocean is the area of oceanic crust covered by water. Oceanic crust is the thin layer of solidified volcanic basalt that covers the mantle. The ocean floor spreads from mid-ocean ridges where two plates adjoin. Where two plates move towards each other, one plate subducts under another plate (oceanic or continental), leading to an oceanic trench. From this perspective, the earth has three oceans: the World Ocean, the Caspian Sea, and Black Sea. The latter two were formed by the collision of Cimmeria with Laurasia.

The Mediterranean Sea is at times a discrete ocean, because tectonic plate movements have repeatedly broken its connection to the World Ocean through the Strait of Gibraltar. The Black Sea is connected to the Mediterranean through the Bosporus, but the Bosporus is a natural canal that was cut through continental rock around 7,000 years ago, rather than a piece of oceanic sea floor like the Strait of Gibraltar.

Ancient oceans[link]

Genesis of an ocean

Continental drift continually reconfigures the oceans, joining and splitting bodies of water.^{[citation needed]} 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.
• Poseidon Ocean
• 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.

Divisions[link]

Various ways to divide the World Ocean

Though generally described as several separate oceans, these waters comprise one global, interconnected body of salt water sometimes referred to as the World Ocean or global ocean.^[9] This concept of a continuous body of water with relatively free interchange among its parts is of fundamental importance to oceanography.^[10]

The major oceanic divisions are defined in part by the continents, various archipelagos, and other criteria. These divisions are (in descending order of size):

• Pacific Ocean, which separates Asia and Oceania from the Americas
• Atlantic Ocean, which separates the Americas from Europe and Africa
• Indian Ocean, which washes upon southern Asia and separates Africa and Australia
• Southern Ocean, sometimes considered an extension of the Pacific, Atlantic and Indian Oceans,^[11] which encircles Antarctica
• Arctic Ocean, sometimes considered a sea of the Atlantic, which covers much of the Arctic and washes upon northern North America and Eurasia

The Pacific and Atlantic may be further subdivided by the equator into northern and southern portions. A smaller region of the ocean can be called other names, such as sea, gulf, bay, and strait.

Physical properties[link]

The total mass of the hydrosphere is about 1,400,000,000,000,000,000 metric tons (1.5×10¹⁸ short tons) or 1.4×10²¹ kg, which is about 0.023 percent of the Earth's total mass. Less than 3 percent is freshwater; the rest is saltwater, mostly in the ocean. The area of the World Ocean is 361 million square kilometres (139 million square miles),^[12] and its volume is approximately 1.3 billion cubic kilometres (310 million cu mi).^[6] This can be thought of as a cube of water with an edge length of 1,111 kilometres (690 mi). Its average depth is 3,790 metres (12,430 ft), and its maximum depth is 10,923 metres (6.787 mi).^[12] Nearly half of the world's marine waters are over 3,000 metres (9,800 ft) deep.^[9] The vast expanses of deep ocean (anything below 200 metres (660 ft)) cover about 66% of the Earth's surface.^[13] This does not include seas not connected to the World Ocean, such as the Caspian Sea.

The bluish color of water is a composite of several contributing agents. Prominent contributors include dissolved organic matter and chlorophyll.^[14]

Sailors and other 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, they had obtained photographic evidence of this glow.^[15] It is most likely caused by bioluminescence.^[16][17][18]

Zones and depths[link]

The major oceanic divisions

Oceanographers divide the ocean into different zones depending on the present physical and biological conditions. The pelagic zone includes all open ocean regions, and can be divided into further regions categorized by depth and light abundance. The photic zone covers the oceans from surface level to 200 metres down. This is the region where photosynthesis can occur and therefore is the most biodiverse. Since plants require photosynthesis, life found deeper than this must either rely on material sinking from above (see marine snow) or find another energy source; hydrothermal vents are the primary option in what is known as the aphotic zone (depths exceeding 200 m). 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 according to temperature. The mesopelagic is the uppermost region. Its lowermost boundary is at a thermocline of 12 °C (54 °F), which, in the tropics generally lies at 700–1,000 metres (2,300–3,300 ft). Next is the bathypelagic lying between 10 and 4 °C (50 and 39 °F), typically between 700–1,000 metres (2,300–3,300 ft) and 2,000–4,000 metres (6,600–13,000 ft) Lying along the top of the abyssal plain is the abyssalpelagic, whose lower boundary lies at about 6,000 metres (20,000 ft). The last zone includes the deep trenches, and is known as the hadalpelagic. This lies between 6,000–11,000 metres (20,000–36,000 ft) and is the deepest oceanic zone.

Along with pelagic aphotic zones there are also benthic aphotic zones. These correspond to the three deepest zones of the deep-sea. The bathyal zone covers the continental slope down to about 4,000 metres (13,000 ft). 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.

Exploration[link]

Map of large underwater features (1995, NOAA)

Ocean travel by boat dates back to prehistoric times, but only in modern times has extensive underwater travel become possible.

The deepest point in the ocean is the Mariana Trench, located in the Pacific Ocean near the Northern Mariana Islands. Its maximum depth has been estimated to be 10,971 metres (35,994 ft) (plus or minus 11 meters; see the Mariana Trench article for discussion of the various estimates of the maximum depth.) The British naval vessel, Challenger II surveyed the trench in 1951 and named 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 ocean bottom remains unexplored and unmapped. A global image of many underwater features larger than 10 kilometres (6.2 mi) was created in 1995 based on gravitational distortions of the nearby sea surface.^{[citation needed]}

Climate[link]

A map of the global thermohaline circulation; blue represent deep-water currents, while red represent surface currents

Ocean currents greatly affect the Earth's climate by transferring heat from the tropics to the polar regions, and transferring warm or cold air and precipitation to coastal regions, where winds may carry them inland. Surface heat and freshwater fluxes create global density gradients that drive the thermohaline circulation part of large-scale ocean circulation. It plays an important role in supplying heat to the polar regions, and thus in sea ice regulation. Changes in the thermohaline circulation are thought to have significant impacts on the Earth's radiation budget. Insofar as the thermohaline circulation governs the rate at which deep waters reach the surface, it may also significantly influence atmospheric carbon dioxide concentrations.

For a discussion of the possibilities of changes to the thermohaline circulation under global warming, see shutdown of thermohaline circulation.

It is often stated that the thermohaline circulation is the primary reason that the climate of Western Europe is so temperate. An alternate hypothesis claims that this is largely incorrect, and that Europe is warm mostly because it lies downwind of an ocean basin, and because atmospheric waves bring warm air north from the subtropics.^[19][20]

The Antarctic Circumpolar Current encircles that continent, influencing the area's climate and connecting currents in several oceans.

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).

Biology[link]

The ocean has a significant effect on the biosphere. Oceanic evaporation, as a phase of the water cycle, is the source of most rainfall, and ocean temperatures determine climate and wind patterns that affect life on land. Life within the ocean evolved 3 billion years prior to life on land. Both the depth and the distance from shore strongly influence the biodiversity of the plants and animals present in each region.^[21]

Lifeforms native to the ocean include:

• Fish;
• Radiata, such as jellyfish (Cnidaria);
• Cetacea, such as whales, dolphins, and porpoises;
• Cephalopods, such as octopus and squid;
• Crustaceans, such as lobsters, clams, shrimp, and krill;
• Marine worms;
• Plankton; and
• Echinoderms, such as brittle stars, starfish, sea cucumbers, and sand dollars.

Economic value[link]

The oceans are essential to transportation: most of the world's goods move by ship between the world's seaports.

Oceans are also the major supply source for the fishing industry. Some of the more major ones are shrimp, fish, crabs and lobster.

Extraterrestrial oceans[link]

See also Extraterrestrial liquid water

Two models for the composition of Europa predict a large sub-surface ocean of liquid water. Similar models have been proposed for other celestial bodies in the Solar System

While Earth is the only known planet with large stable bodies liquid water on its surface and the only one in our the Solar System, other celestial bodies are believed to possess large oceans.

Planets[link]

The gas giants, Jupiter and Saturn, are thought to lack surfaces and instead have a stratum of liquid hydrogen, however their planetary geology is not well understood. Likewise the ice giants of Uranus and Neptune may also possess vast oceans of liquid water under their thick atmospheres, though their internal structure has not been confirmed.

There is currently much debate over whether Mars once had an ocean in its northern hemisphere, and over what happened to it; recent findings by the Mars Exploration Rover mission indicate Mars had 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 later vanished via resurfacing.

Natural satellites[link]

A global layer of liquid water thick enough to decouple the crust from the mantle is believed to be present on Titan, Europa and, with less certainty, Callisto, Ganymede^[22] and Triton.^[23][24] A magma ocean is thought to be present on Io. Geysers have been found on Saturn's moon Enceladus, though their origins are not well understood. Other icy moons may also have internal oceans, or have once had internal oceans that have now frozen.^[22]

Large bodies of Liquid hydrocarbons are thought to be present on the surface of Titan, though they are not large enough to be described as oceans and are sometimes referred to as lakes or seas. 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. Cassini's more recent fly-by of Titan offers radar images that strongly suggest hydrocarbon lakes near the colder polar regions. Titan is thought to have a subterranean water ocean under the ice and hydrocarbon mix that forms its outer crust.

Dwarf planets and trans-Neptunian objects[link]

Diagram showing a possible internal structure of Ceres

Ceres appears to be differentiated into a rocky core and icy mantle and may harbour a liquid water ocean under its surface.^[25][26]

Not enough is known of the larger Trans-Neptunian objects to determine whether they are differentiated bodies capable of possessing oceans, although models of radioactive decay suggest that Pluto,^[27] Eris, Sedna, and Orcus have oceans beneath solid icy crusts at the core-boundary approximately 100 to 180 km thick.^[22]

Extrasolar[link]

Rendering of a hypothetical large extrasolar moon with surface liquid water oceans

Some planets and natural satellites beyond the Solar System are likely to possess oceans, including possible water ocean planets similar to Earth in the habitable zone or "Liquid Water belt". The detection of oceans, even through the spectroscopy method, however is likely to prove extremely difficult and inconclusive.

Theoretical models have been used to predict with high probability that GJ 1214 b, detected by transit, is composed of exotic form of ice VII, making up 75% of its mass.^[28]

Other possible candidates are merely speculated based on their mass and position in the habitable zone include planet though little is actually known of their composition. Some scientists have speculated Kepler-22b to be an "ocean-like" planet.^[29] Models have been proposed for Gliese 581 d that could include surface oceans. Gliese 436 b is speculated to have an ocean of "hot ice".^[30] Extrasolar moons orbiting planets, particularly gas giants within their parent star's habitable zone may theoretically possess surface oceans.

See also[link]

	Ecology portal
	Environment portal
	Geography portal
	Weather portal

References[link]

Further reading[link]

• Matthias Tomczak and J. Stuart Godfrey. 2003. Regional Oceanography: an Introduction. (see the site)
• "Origins of the oceans and continents". UN Atlas of the Oceans.
• Pope, F. 2009. From eternal darkness springs cast of angels and jellied jewels. in The Times. November 23. 2009 p. 16–17.

External links[link]

Wikimedia Commons has media related to: Ocean

Wikiquote has a collection of quotations related to: Oceans

Look up ocean in Wiktionary, the free dictionary.

• Oceans at the Open Directory Project
• Smithsonian Ocean Portal

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