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Marine environment

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Marine environment

File:The Ocean - a driving force for Weather and Climate.ogv

An ocean (; 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 refers to one or all of the major divisions of the planet's World Ocean – they are, in descending order of area, the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans.[3][4] The word sea is often used interchangeably with "ocean" in American 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.[5] The general characteristics of seas and oceans are covered at sea.

Earth is the only known planet to have an ocean (or any large amounts of open liquid water). Approximately 72% of the planet's surface (~3.6x108 km2) is covered by saline water that is customarily divided into several principal oceans and smaller seas, with the ocean covering approximately 71% of the Earth's surface.[6] The ocean contains 97% of the Earth's water, and oceanographers have stated that only 5% of the ocean as a whole on Earth has been explored.[6] The total volume is approximately 1.3 billion cubic kilometres (310 million cu mi)[7] with an average depth of 3,682 metres (12,080 ft).[8]

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. It is the habitat of 230,000 known species, although much of the ocean's depths remain unexplored and it is estimated that over two million marine species exist.[9] The origin of Earth's oceans is still unknown, but oceans are believed to have formed in the Hadean period and may have been the impetus for the emergence 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, Mars and Venus are theorized 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, and a runaway 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 many dwarf planets and natural satellites; notably, the ocean of Europa is believed to have over twice the water volume of Earth. The Solar System's gas giant planets are also believed to possess liquid atmospheric layers of yet to be confirmed compositions. 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.[10][11]

Earth's global ocean

Divisions

Further information: Borders of the oceans


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.[11][12] This concept of a continuous body of water with relatively free interchange among its parts is of fundamental importance to oceanography.[13]

The major oceanic divisions are defined in part by the continents, various archipelagos, and other criteria. See the table below for more information; note that the table is in descending order in terms of size.[11][14]

Rank Ocean Notes
1 Pacific Ocean Separates Asia and Oceania from the Americas[14]
2 Atlantic Ocean Separates the Americas from Eurasia and Africa
3 Indian Ocean Washes upon southern Asia and separates Africa and Australia[14][15][16]
4 Southern Ocean Sometimes considered an extension of the Pacific, Atlantic and Indian Oceans,[11][17] which encircles Antarctica
5 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

Further information: Seawater

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),[18] and its volume is approximately 1.3 billion cubic kilometres (310 million cu mi).[7] 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).[18] Nearly half of the world's marine waters are over 3,000 metres (9,800 ft) deep.[12] The vast expanses of deep ocean (anything below 200 metres (660 ft)) cover about 66% of the Earth's surface.[19] 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.[20]

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.[21] It is most likely caused by bioluminescence.[22][23][24]

Zones and depths


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 (6,600–13,100 ft) Lying along the top of the abyssal plain is the abyssopelagic, whose lower boundary lies at about 6,000 metres (20,000 ft). The last zone includes the deep oceanic trench, 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

Main article: Ocean exploration


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.

Climate


Ocean currents greatly affect the Earth's climate by transferring heat from the tropics to the polar regions. 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.[25][26]

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

Further information: Marine biology

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.[27]

Lifeforms native to the ocean include:

Economic value

The oceans are essential to transportation. This is because 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.[6]

Extraterrestrial oceans

See also Extraterrestrial liquid water


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

Planets

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. The possibility of Uranus and Neptune possessing hot, highly compressed, supercritical water under their thick atmospheres has been hypothesised. While their composition is still not fully understood, a 2006 study by Wiktorowicz et al. ruled out the possibility of such an water "ocean" existing on Neptune,[28] though some studies have suggested that exotic oceans of liquid diamond are possible.[29]

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. The possibility continues to be studied along with reasons for their apparent disappearance. 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

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[30] and Triton.[31][32] 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.[30]

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 exist 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


Ceres appears to be differentiated into a rocky core and icy mantle and may harbour a liquid-water ocean under its surface.[33][34]

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,[35] Eris, Sedna, and Orcus have oceans beneath solid icy crusts at the core-boundary approximately 100 to 180 km thick.[30]

Extrasolar


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.[36]

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 speculate Kepler-22b may be an "ocean-like" planet.[37] 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".[38] Extrasolar moons orbiting planets, particularly gas giants within their parent star's habitable zone may theoretically possess surface oceans.

See also

References

Further reading

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

External links

  • DMOZ
  • Smithsonian Ocean Portal
  • NOAA - National Oceanic and Atmospheric Administration - Ocean
  • Ocean :: Science Daily
  • Ocean-bearing Planets: Looking For Extraterrestrial Life In All The Right Places
  • Titan Likely To Have Huge Underground Ocean | Mind Blowing Science
  • UN Atlas of the Oceans.

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