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Planetary oceanography, also called astro-oceanography or exo-oceanography,[1] is the study of
oceans on planets and moons other than
Earth. Unlike other planetary sciences like
astrobiology,
astrochemistry, and
planetary geology, it only began after the discovery of underground oceans in Saturn's moon
Titan[2] and Jupiter's moon
Europa.[3] This field remains speculative until further missions reach the oceans beneath the rock or ice layer of the moons. There are many theories about oceans or even
ocean worlds of celestial bodies in the
Solar System, from oceans made of diamond in Neptune to a gigantic ocean of liquid hydrogen that may exist underneath Jupiter's surface.[4][5]
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 to exist 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's. 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.[6][7]
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, which are made of hydrocarbons instead of water. However, there is strong evidence for subsurface water oceans' existence elsewhere in the
Solar System. The best-established candidates for subsurface water oceans in the Solar System are Jupiter's moons
Europa,
Ganymede, and
Callisto; and Saturn's moons
Enceladus and
Titan.[8]
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 giantsUranus 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,[19] though some studies have suggested that exotic oceans of liquid
diamond are possible.[20]
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. Some astronomers now propose that
Venus may have had liquid water and perhaps oceans for over 2 billion years. [21]
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 satellitesTitan,
Europa,
Enceladus,
Callisto,
Ganymede,[22][23] and, with less certainty,
Mimas,[24]Miranda,
Ariel,[25] and
Triton.[26][27] A
magma ocean is thought to be present on
Io.[28]Geysers or
fumaroles have been found on Saturn's moon Enceladus, possibly originating from an ocean about 10 kilometers (6.2 mi) beneath the surface ice shell.[17] Other
icy moons may also have internal oceans, or may once have had internal oceans that have now frozen.[29]
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.
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,[32]Eris,
Sedna, and
Orcus have oceans beneath solid icy crusts approximately 100 to 180 km thick.[29] 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.[33][34]
Extrasolar
Some planets and natural satellites outside the Solar System are likely to have 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 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,[35]
making it an
ocean 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 speculate
Kepler-22b may be an "ocean-like" planet.[36] 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".[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]
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.[39] 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 solution.[40][41][42][43][44]
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.[49] The gaseous outer layers of
Jupiter and
Saturn transition smoothly into oceans of
supercriticalhydrogen.[50][51] The
atmosphere of Venus is 96.5%
carbon dioxide, and is a supercritical fluid at the surface.
^Vance, Steve; Bouffard, Mathieu; Choukroun, Mathieu; Sotina, Christophe (12 April 2014). "Ganymede's internal structure including thermodynamics of magnesium sulfate oceans in contact with ice". Planetary and Space Science. 96: 62–70.
Bibcode:
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doi:
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^McKinnon, William B.; Kirk, Randolph L. (2007).
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483–502.
ISBN978-0-12-088589-3.
^Castillo-Rogez, J. C.; McCord, T. B.; Davis, A. G. (2007).
"Ceres: evolution and present state"(PDF). Lunar and Planetary Science. XXXVIII: 2006–2007. Retrieved 2009-06-25.
^"The Inside Story". pluto.jhuapl.edu — NASA New Horizons mission site. Johns Hopkins University Applied Physics Laboratory. 2013. Archived from
the original on 13 November 2014. Retrieved 2 August 2013.
^McKinnon, William B.; Kirk, Randolph L. (2007).
"Triton". In Lucy Ann Adams McFadden; Lucy-Ann Adams; Paul Robert Weissman; Torrence V. Johnson (eds.). Encyclopedia of the Solar System (2nd ed.). Amsterdam; Boston: Academic Press. p.
485.
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^Coustenis, A.; Lunine, Jonathan I.; Lebreton, J.; Matson, D.; et al. (2008). "The Titan Saturn System Mission". American Geophysical Union, Fall Meeting. 21: 1346.
Bibcode:
2008AGUFM.P21A1346C. the Titan system, rich in organics, containing a vast subsurface ocean of liquid water
^Nimmo, F.; Bills, B. G. (2010). "Shell thickness variations and the long-wavelength topography of Titan". Icarus. 208 (2): 896–904.
Bibcode:
2010Icar..208..896N.
doi:
10.1016/j.icarus.2010.02.020. observations can be explained if Titan has a floating, isostatically-compensated ice shell
^Goldreich, Peter M.; Mitchell, Jonathan L. (2010). "Elastic ice shells of synchronous moons: Implications for cracks on Europa and non-synchronous rotation of Titan". Icarus. 209 (2): 631–638.
arXiv:0910.0032.
Bibcode:
2010Icar..209..631G.
doi:
10.1016/j.icarus.2010.04.013.
S2CID119282970. A number of synchronous moons are thought to harbor water oceans beneath their outer ice shells. A subsurface ocean frictionally decouples the shell from the interior