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Planetary oceanography, also calledastro-oceanography orexo-oceanography,^[1] is the study ofoceans on planets and moons other thanEarth. This field developed after the discovery ofsub-surface oceans in Saturn's moonTitan^[2] and Jupiter's moonEuropa^[3] during theVoyager missions. TheCassini mission observed surface lakes of liquid methane on Titan, and directly sampled a plume of sub-surface ocean water fromEnceladus.

Early in their geologic histories,Mars andVenus are theorized to have had large water oceans. TheMars ocean hypothesis suggests 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 assalts andammonia, when dissolved in water, will lower water's freezing point, so that water might exist in large quantities in extraterrestrial environments as brine, orconvectingice. Oceans are thought to exist beneath the surfaces of manydwarf planets and natural satellites; notably, the ocean of the moonEuropa is estimated to have over twice the water volume of Earth's.

The Solar System'sgiant planets are thought to have liquid or supercriticalatmospheric layers of yet-to-be-confirmed compositions. Oceans may also exist onexoplanets andexomoons, including surface oceans of liquid water within acircumstellar habitable zone.Ocean planets are a hypothetical type of planet with a surface completely covered with liquid.^[4][5]

Extraterrestrial oceans may be composed of water, or otherelements andcompounds. The only confirmed large, stable bodies of extraterrestrial surfaceliquids are thelakes of Titan, which are made of hydrocarbons instead of water. However, there is strong evidence for the existence of subsurface water oceans elsewhere in the Solar System. The best-established candidates for subsurface water oceans in the Solar System are Jupiter's moonsEuropa,Ganymede, andCallisto, and Saturn's moonsEnceladus andTitan.^[6]

Although Earth is the only knownplanet with large stable bodies of liquid water on its surface, and the only such planet in the Solar System, other celestial bodies are thought to have large oceans.^[7] In June 2020,NASA scientists reported that it is likely thatexoplanets with oceans may be common in theMilky Way galaxy, based onmathematical modeling studies.^[8][9]

The inner structure ofgas giants remain poorly understood. Scientists suspect that, under extreme pressure,hydrogen would act as asupercritical fluid, hence the likelihood of oceans of liquid hydrogen deep in the interior of gas giants likeJupiter.^[10][11] Oceans of liquidcarbon have been hypothesized to exist onice giants, notablyNeptune andUranus.^[12][13]Magma oceans exist during periods ofaccretion on any planet and somenatural satellites when the planet or natural satellite is completely or partly molten.^[14]

Thegas giants,Jupiter andSaturn, are thought to lack surfaces and instead have a stratum ofliquid hydrogen; however theirplanetary geology is not well understood. The possibility of theice giantsUranus andNeptune 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,^[17] though oceans of metallic liquid carbon are possible.

TheMars 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 theice caps). The possibility continues to be studied along with reasons for their apparent disappearance. Some astronomers now propose thatVenus may have had liquid water and perhaps oceans for over 2 billion years.^[18]

A global layer of liquid water thick enough to decouple the crust from the mantle is thought to be present on thenatural satellitesTitan,Europa,Enceladus,Ganymede,^[19][20] andTriton;^[21][22] and, with less certainty, inCallisto,^[23][24]Mimas,^[25]Miranda, andAriel.^[26] Amagma ocean is thought to be present onIo.^[27]Geysers orfumaroles have been found on Saturn's moon Enceladus, possibly originating from an ocean about 10 kilometers (6 mi) beneath the surface ice shell.^[15] Othericy moons may also have internal oceans, or may once have had internal oceans that have now frozen.^[28]

Largebodies of liquid hydrocarbons are thought to be present on the surface ofTitan, although they are not large enough to be considered oceans and are sometimes referred to aslakes or seas. TheCassini–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. Additionally,Mimas is hypothesized to have a subsurface liquid-water ocean underneath its icy crust due to its crust moving as it rotates around Saturn, though any solid proof of such an ocean is as of now unconfirmed.^[29]

Ceres appears to bedifferentiated into arockycore and icymantle and may harbour a liquid-water ocean under its surface.^[30][31]

Not enough is known of the largertrans-Neptunian objects to determine whether they are differentiated bodies capable of supporting oceans, although models of radioactive decay suggest thatPluto,^[32]Eris,Sedna, andOrcus have oceans beneath solid icy crusts approximately 100 to 180 kilometers (60 to 110 mi) thick.^[28] In June 2020, astronomers reported evidence that thedwarf planet Pluto may have had asubsurface ocean, and consequently may have beenhabitable, when it was first formed.^[33][34]

Some planets and natural satellites outside the Solar System are likely to have oceans, including possible water ocean planets similar to Earth in thehabitable zone or "liquid-water belt". The detection of oceans, even through thespectroscopy method, however is likely extremely difficult and inconclusive.

Theoretical models have been used to predict with high probability thatGJ 1214 b, detected by transit, is composed of exotic form ofice VII, making up 75% of its mass,^[35]making it anocean planet.

Other possible candidates are merely speculative based on their mass and position in the habitable zone include planet though little is actually known of their composition. Some scientists speculateKepler-22b may be an "ocean-like" planet.^[36] Models have been proposed forGliese 581 d that could include surface oceans.Gliese 436 b is speculated to have an ocean of "hot ice".^[37]Exomoons orbiting planets, particularly gas giants within their parent star's habitable zone may theoretically have surface oceans.

Terrestrial planets will acquire water during their accretion, some of which will be buried in the magma ocean but most of it will go into a steam atmosphere, and when the atmosphere cools it will collapse on to the surface forming an ocean. There will also be outgassing of water from the mantle as the magma solidifies—this will happen even for planets with a low percentage of their mass composed of water, so "super-Earth exoplanets may be expected to commonly produce water oceans within tens to hundreds of millions of years of their last major accretionary impact."^[38]

Oceans, seas, lakes and other bodies of liquids can be composed of liquids other than water, for example thehydrocarbon lakes onTitan. The possibility of seas ofnitrogen onTriton was also considered but ruled out.^[39] There is evidence that the icy surfaces of the moonsGanymede,Callisto,Europa, Titan andEnceladus are shells floating on oceans of very dense liquid water or water–ammonia solution.^{[40][41][42][43][44]}

Extrasolarterrestrial planets that are extremely close to their parent star will betidally locked and so one half of the planet will be amagma ocean.^[45] It is also possible that terrestrial planets had magma oceans at some point during their formation as a result ofgiant impacts.^[46]Hot Neptunes close to their star could lose their atmospheres viahydrodynamic escape, leaving behind their cores with various liquids on the surface.^[47] Where there are suitable temperatures and pressures, volatile chemicals that might exist as liquids in abundant quantities on planets (thalassogens) includeammonia,argon,carbon disulfide,ethane,hydrazine,hydrogen,hydrogen cyanide,hydrogen sulfide,methane,neon,nitrogen,nitric oxide,phosphine,silane,sulfuric acid, andwater.^[48]

Supercritical fluids, although not liquids, do share various properties with liquids. Underneath the thick atmospheres of the planetsUranus andNeptune, it is expected that these planets are composed of oceans of hot high-density fluid mixtures of water, ammonia and other volatiles.^[49] The gaseous outer layers ofJupiter andSaturn transition smoothly into oceans ofsupercriticalhydrogen.^[50][51] Theatmosphere of Venus is 96.5%carbon dioxide, and is a supercritical fluid at the surface.

• Extraterrestrial liquid water
• Lava planet
• List of largest lakes and seas in the Solar System
• Magma ocean
• Ocean world

• Space science
• Oceanography
• Extraterrestrial water

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