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Surface view of the Atlantic Ocean

An ocean is a body of water that composes much of a planet's hydrosphere.[1] On Earth, an ocean is one of the major conventional divisions of the World Ocean. These are, in descending order by area, the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans.[2][3] The phrases "the ocean" or "the sea" used without specification refer to the interconnected body of salt water covering the majority of the Earth's surface.[2][3] As a general term, "the ocean" is mostly interchangeable with "the sea" in American English, but not in British English.[4] Strictly speaking, a sea is a body of water (generally a division of the world ocean) partly or fully enclosed by land.[5]

Saline seawater covers approximately 361,000,000 km2 (139,000,000 sq mi) 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.[6] The ocean contains 97% of Earth's water, and oceanographers have stated that less than 20% of the World Ocean has been mapped.[6] 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).[7][8][9]

As the world ocean is the principal component of Earth's hydrosphere, it is integral to life, forms part of the carbon cycle, and influences climate and weather patterns. The World Ocean is the habitat of 230,000 known species, but because much of it is unexplored, the number of species that exist in the ocean is much larger, possibly over two million.[10] The origin of Earth's oceans is unknown; oceans are thought to have formed in the Hadean eon and may have been the cause 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 the moon 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 a circumstellar habitable zone. Ocean planets are a hypothetical type of planet with a surface completely covered with liquid.[11][12]


The word ocean comes from the figure in classical antiquity, Oceanus (/ˈsənəs/; Greek: Ὠκεανός Ōkeanós,[13] pronounced [ɔːkeanós]), the elder of the Titans in classical Greek mythology, believed by the ancient Greeks and Romans to be the divine personification of the sea, an enormous river encircling the world.

The concept of Ōkeanós has an Indo-European connection. Greek Ōkeanós has been compared to the Vedic epithet ā-śáyāna-, predicated of the dragon Vṛtra-, who captured the cows/rivers. Related to this notion, the Okeanos is represented with a dragon-tail on some early Greek vases.[14]

Earth's global ocean

Rotating series of maps showing alternate divisions of the oceans
Various ways to divide the World Ocean

Oceanic divisions

1. Epipelagic zone: surface – 200 meters deep 2. Mesopelagic zone: 200 m – 1000 m 3. Bathypelagic zone: 1000 m – 4000 m 4. Abyssopelagic zone: 4000 m – 6000 m 5. Hadal zone (the trenches): 6000 m to the bottom of the ocean



# Ocean Location Area
Avg. depth
1 Pacific Ocean Separates Asia and Australasia from the Americas[19][NB] 168,723,000
3,970 135,663
2 Atlantic Ocean Separates the Americas from Europe and Africa[20] 85,133,000
3,646 111,866
3 Indian Ocean Borders southern Asia and separates Africa and Australia[21] 70,560,000
3,741 66,526
4 Southern Ocean Encircles Antarctica. Sometimes considered an extension of the Pacific, Atlantic and Indian Oceans,[22][23] 21,960,000
3,270 17,968
5 Arctic Ocean Borders northern North America and Eurasia and covers much of the Arctic. Sometimes considered a sea or estuary of the Atlantic.[24][25] [26] 15,558,000
1,205 45,389
Total – World Ocean 361,900,000
3,688 377,412[27]
Seas and Bays
Sea Location Area

(sq. km)

Arabian Sea Between the Arabian peninsula and the Indian subcontinent 3,862,000 1
Bay of Bengal Between the Indian subcontinent and the Malay Peninsula 2,173,000 2
Sources: Encyclopedia of Earth,[19][20][21][22][26] International Hydrographic Organization,[23] Regional Oceanography: an Introduction (Tomczak, 2005),[24] Encyclopædia Britannica,[25] and the International Telecommunication Union.[27]


Global system

World Distribution of Mid-Oceanic Ridges; USGS
Three main types of plate boundaries.


Physical properties

The total mass of the hydrosphere is about 1.4 quintillion tonnes (1.4×1018 long tons or 1.5×1018 short tons), which is about 0.023% of Earth's total mass. Less than 3% is freshwater; the rest is saltwater, almost all of which is in the ocean. The area of the World Ocean is about 361.9 million square kilometers (139.7 million square miles),[9] which covers about 70.9% of Earth's surface, and its volume is approximately 1.335 billion cubic kilometers (320.3 million cubic miles).[9] This can be thought of as a cube of water with an edge length of 1,101 kilometers (684 mi). Its average depth is about 3,688 meters (12,100 ft),[9] and its maximum depth is 10,994 meters (6.831 mi) at the Mariana Trench.[29] Nearly half of the world's marine waters are over 3,000 meters (9,800 ft) deep.[16] The vast expanses of deep ocean (anything below 200 meters or 660 feet) cover about 66% of Earth's surface.[30] This does not include seas not connected to the World Ocean, such as the Caspian Sea.

The bluish ocean color is a composite of several contributing agents. Prominent contributors include dissolved organic matter and chlorophyll.[31] Mariners and other seafarers have reported that the ocean often emits a visible glow which extends for miles at night. In 2005, scientists announced that for the first time, they had obtained photographic evidence of this glow.[32] It is most likely caused by bioluminescence.[33][34][35]

Oceanic zones

Drawing showing divisions according to depth and distance from shore
The major oceanic zones, based on depth and biophysical conditions









False color photo
Map of large underwater features (1995, NOAA)


Oceanic maritime currents

Oceanic surface currents (U.S. Army, 1943).
Amphidromic points showing the direction of tides per incrementation periods along with resonating directions of wavelength movements.

Oceanic maritime currents have different origins. Tidal currents are in phase with the tide, hence are quasiperiodic; they may form various knots in certain places,[clarification needed] most notably around headlands. Non-periodic currents have for origin the waves, wind and different densities.

The wind and waves create surface currents (designated as “drift currents”). These currents can decompose in one quasi-permanent current (which varies within the hourly scale) and one movement of Stokes drift under the effect of rapid waves movement (at the echelon of a couple of seconds).).[36] The quasi-permanent current is accelerated by the breaking of waves, and in a lesser governing effect, by the friction of the wind on the surface.[37]

This acceleration of the current takes place in the direction of waves and dominant wind. Accordingly, when the sea depth increases, the rotation of the earth changes the direction of currents in proportion with the increase of depth, while friction lowers their speed. At a certain sea depth, the current changes direction and is seen inverted in the opposite direction with current speed becoming null: known as the Ekman spiral. The influence of these currents is mainly experienced at the mixed layer of the ocean surface, often from 400 to 800 meters of maximum depth. These currents can considerably alter, change and are dependent on the various yearly seasons. If the mixed layer is less thick (10 to 20 meters), the quasi-permanent current at the surface adopts an extreme oblique direction in relation to the direction of the wind, becoming virtually homogeneous, until the Thermocline.[38]

In the deep however, maritime currents are caused by the temperature gradients and the salinity between water density masses.

In littoral zones, breaking waves are so intense and the depth measurement so low, that maritime currents reach often 1 to 2 knots.










As it is thought that life evolved in the ocean, the diversity of life is immense, including:



Characteristics of oceanic gases [40][41][42]
Gas Concentration of seawater, by mass (in parts per million), for the whole ocean % Dissolved gas, by volume, in seawater at the ocean surface
Carbon dioxide (CO2) 64 to 107 15%
Nitrogen (N2) 10 to 18 48%
Oxygen (O2) 0 to 13 36%
Solubility of oceanic gases (in mL/L) with temperature at salinity of 33‰ and atmospheric pressure[43]
Temperature O2 CO2 N2
0 °C 8.14 8,700 14.47
10 °C 6.42 8,030 11.59
20 °C 5.26 7,350 9.65
30 °C 4.41 6,600 8.26


Generalized characteristics of ocean surface by latitude [44][45][46][47][48][49][50]

Mixing time

Mean oceanic residence time for various constituents [51][52]
Constituent Residence time (in years)
Iron (Fe) 200
Aluminum (Al) 600
Manganese (Mn) 1,300
Water (H2O) 4,100
Silicon (Si) 20,000
Carbonate (CO32−) 110,000
Calcium (Ca2+) 1,000,000
Sulfate (SO42−) 11,000,000
Potassium (K+) 12,000,000
Magnesium (Mg2+) 13,000,000
Sodium (Na+) 68,000,000
Chloride (Cl) 100,000,000


A zone of rapid salinity increase with depth is called a halocline. The temperature of maximum density of seawater decreases as its salt content increases. Freezing temperature of water decreases with salinity, and boiling temperature of water increases with salinity. Typical seawater freezes at around −2 °C at atmospheric pressure.[53] If precipitation exceeds evaporation, as is the case in polar and temperate regions, salinity will be lower. If evaporation exceeds precipitation, as is the case in tropical regions, salinity will be higher. Thus, oceanic waters in polar regions have lower salinity content than oceanic waters in temperate and tropical regions.[54]


Salinity (in ‰) = 1.80655 × Chlorinity (in ‰)

The average chlorinity is about 19.2‰, and, thus, the average salinity is around 34.7‰ [54]

Absorption of light

Absorption of light in different wavelengths by ocean [54]
Color: Wavelength (nm) Depth at which 99 percent of the wavelength is absorbed (in meters) Percent absorbed in 1 meter of water
Ultraviolet (UV): 310 31 14.0
Violet (V): 400 107 4.2
Blue (B): 475 254 1.8
Green (G): 525 113 4.0
Yellow (Y): 575 51 8.7
Orange (O): 600 25 16.7
Red (R): 725 4 71.0
Infrared (IR): 800 3 82.0

Economic value

Many of the world's goods are moved by ship between the world's seaports.[55] Oceans are also the major supply source for the fishing industry. Some of the major harvests are shrimp, fish, crabs, and lobster.[6]

Waves and swell

The motions of the ocean surface, known as undulations or waves, are the partial and alternate rising and falling of the ocean surface. The series of mechanical waves that propagate along the interface between water and air is called swell.[citation needed]

Extraterrestrial oceans

Artist's conception of subsurface ocean of Enceladus confirmed April 3, 2014.[56][57]

Although Earth is the only known planet with large stable bodies of liquid water on its surface and the only one in the Solar System, other celestial bodies are thought to have large oceans.[58] In June 2020, NASA scientists reported that it's likely that exoplanets with oceans may be common in the Milky Way galaxy, based on mathematical modeling studies.[59][60]


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 the ice giants Uranus and Neptune having hot, highly compressed, supercritical water under their thick atmospheres has been hypothesised. Although their composition is still not fully understood, a 2006 study by Wiktorowicz and Ingersall ruled out the possibility of such a water "ocean" existing on Neptune,[61] though some studies have suggested that exotic oceans of liquid diamond are possible.[62]

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 (much of it residing in the ice caps). The possibility continues to be studied along with reasons for their apparent disappearance. Astronomers now think that Venus may have had liquid water and perhaps oceans for over 2 billion years. [63]

Natural satellites

A global layer of liquid water thick enough to decouple the crust from the mantle is thought to be present on the natural satellites Titan, Europa, Enceladus and, with less certainty, Callisto, Ganymede[64][65] and Triton.[66][67] A magma ocean is thought to be present on Io.[68] Geysers have been found on Saturn's moon Enceladus, possibly originating from an ocean about 10 kilometers (6.2 mi) beneath the surface ice shell.[56] Other icy moons may also have internal oceans, or may once have had internal oceans that have now frozen.[69]

Large bodies of liquid hydrocarbons are thought to be present on the surface of Titan, although they are not large enough to be considered 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. Later flybys of Titan provided radar and infrared images that showed a series of hydrocarbon lakes in the colder polar regions. Titan is thought to have a subsurface liquid-water ocean under the ice in addition to the hydrocarbon mix that forms atop its outer crust.

Dwarf planets and trans-Neptunian objects

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.[70][71]

Not enough is known of the larger trans-Neptunian objects to determine whether they are differentiated bodies capable of supporting oceans, although models of radioactive decay suggest that Pluto,[72] Eris, Sedna, and Orcus have oceans beneath solid icy crusts approximately 100 to 180 km thick.[69] In June 2020, astronomers reported evidence that the dwarf planet Pluto may have had a subsurface ocean, and consequently may have been habitable, when it was first formed.[73][74]




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,[75] making it an ocean planet.

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.[76] 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".[77] Exomoons orbiting planets, particularly gas giants within their parent star's habitable zone may theoretically have surface oceans.


Non-water surface liquids

Oceans, seas, lakes and other bodies of liquids can be composed of liquids other than water, for example the hydrocarbon lakes on Titan. The possibility of seas of nitrogen on Triton was also considered but ruled out.[79] There is evidence that the icy surfaces of the moons Ganymede, Callisto, Europa, Titan and Enceladus are shells floating on oceans of very dense liquid water or water–ammonia.[80][81][82][83][84] Earth is often called the ocean planet because it is 70% covered in water.[85][86] Extrasolar terrestrial planets that are extremely close to their parent star will be tidally locked and so one half of the planet will be a magma ocean.[87] It is also possible that terrestrial planets had magma oceans at some point during their formation as a result of giant impacts.[88] Hot Neptunes close to their star could lose their atmospheres via hydrodynamic escape, leaving behind their cores with various liquids on the surface.[89] Where there are suitable temperatures and pressures, volatile chemicals that might exist as liquids in abundant quantities on planets include ammonia, argon, carbon disulfide, ethane, hydrazine, hydrogen, hydrogen cyanide, hydrogen sulfide, methane, neon, nitrogen, nitric oxide, phosphine, silane, sulfuric acid, and water.[90]

Supercritical fluids, although not liquids, do share various properties with liquids. Underneath the thick atmospheres of the planets Uranus and Neptune, it is expected that these planets are composed of oceans of hot high-density fluid mixtures of water, ammonia and other volatiles.[91] The gaseous outer layers of Jupiter and Saturn transition smoothly into oceans of supercritical hydrogen.[92][93] The atmosphere of Venus is 96.5% carbon dioxide, which is a supercritical fluid at its surface.

See also


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