Other Types of Ocean
Oceans, seas, lakes, etc., can be composed of liquids other than water: e.g. the hydrocarbon lakes on Titan. The possibility of seas of nitrogen on Triton was also considered but ruled out. Underneath the thick atmospheres of 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. The gaseous outer layers of Jupiter and Saturn transition smoothly into oceans of liquid hydrogen. 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. Our own planet Earth is often called the ocean planet since it is 70% covered in water. The atmosphere of Venus is 96.5% carbon dioxide and at the surface the pressure makes the CO2 a supercritical fluid. 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. It is also possible that terrestrial planets had magma oceans at some point during their formation as a result of giant impacts. Where there are suitable temperatures and pressures, volatile chemicals which 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. Hot Neptunes close to their star could lose their atmospheres via hydrodynamic escape, leaving behind their cores with various liquids on the surface.
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."
Read more about this topic: Ocean Planet
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