Planetary oceanography, also called astro-oceanography or exo-oceanography, is the study of oceans on planets and moons other than Earth. This field developed after the discovery of sub-surface oceans in Saturn's moon Titan and Jupiter's moon Europa during the Voyager missions. The Cassini mission observed surface lakes of liquid methane on Titan, and directly sampled a plume of sub-surface ocean water from Enceladus.
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, when dissolved in water, will lower water's freezing point, so that water might exist in large quantities in extraterrestrial environments as brine, or convecting ice. Oceans are thought to exist beneath the surfaces 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.
