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 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, though oceans of metallic liquid carbon are possible. 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.
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,
Ganymede, and
Triton; and, with less certainty, in
Callisto,
Mimas,
Miranda, and
Ariel. A
magma ocean is thought to be present on
Io.
Geysers or
fumaroles have been found on Saturn's moon
Enceladus, possibly originating from an ocean about beneath the surface ice shell. Other
icy moons may also have internal oceans, or may once have had internal oceans that have now frozen. 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. 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.
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. 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,
Eris,
Sedna, and
Orcus have oceans beneath solid icy crusts approximately thick. 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. 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".
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." == Non-water surface liquids ==