Rapid melting of ice to form a flow of liquid or gas, which then escapes onto the surface of a planetary body. Formations such as lava tubes, ejecta blankets, and plumes that are seen in terrestrial volcanoes all have cryovolcanic equivalents. In the outer Solar System water ice is a common cryovolcanic material, for example in the eruptions seen on Saturn’s moon Enceladus. On colder moons cryovolcanic processes are driven by other ices, such as nitrogen, which melt or vaporize at temperatures below 0°C. The nitrogen geysers of Neptune’s moon Triton are an example of cryovolcanism involving a substance other than water. Other cryovolcanic materials include ammonia and methane. Notable cryovolcanoes in the Solar System include Doom Mons on Saturn’s moon Titan, 1.5 km high and 63 km wide, and Wright Mons on Pluto, about 150 km across and 4 km high, the largest cryovolcano in the outer Solar System. Cryovolcanism may be an important resurfacing mechanism for moons. Possible energy sources for cryovolcanoes include tidal friction and gravity distortion causing the heating which leads to eruptions.