Triton is the largest natural satellite of the planet Neptune and was the first Neptunian moon to be discovered). It is the only large moon in the Solar System with a retrograde orbit, an orbit in the direction opposite to its planets rotation). Triton is thought to have been a dwarf planet captured from the Kuiper belt because of its retrograde orbit and composition similar to Pluto). Triton consists of a crust of frozen nitrogen over an icy mantle believed to cover a core of rock and metal, and has a density about twice that of water. Triton is one of the few moons in the Solar System known to be geologically active, as well as suspected to contain an internal, active layer of liquid ocean, similar to the moons of Jupiter and Saturn). Its surface is relatively young, with few obvious impact craters, and intricate cryovolcanic and tectonic terrains suggest a complex geological history). Tritons thin atmosphere is composed mainly of nitrogen with small amounts of methane, and it is one of the coolest objects in our solar system, with surface temperatures of -391 degrees Fahrenheit (-235 degrees Celsius) ). The active ice volcanoes on Triton are thought to be caused by liquid nitrogen, dust, or methane compounds from the center of the moon.