It is estimated to be as much as 25% water ice, and its atmosphere has pressure of only about one millibar.  
It is estimated to be as much as 25% water ice, and its atmosphere has pressure of only about one millibar.  The furthest body from the sun known to be volcanically active, its so-called "ice volcanoes" shoot nitrogen or methane into the atmosphere.  As it could not have formed from the Solar Nebula, the capture theory is the most popular theory as to why it is where it is today.  Its orbit currently draws it into the gas giant it rotates, so it will eventually either break up or crash into that planet.  Only five other moons, each less than one tenth its diameter in size, exhibit a retrograde orbit.  Its extremely high albedo makes it the coldest known body in the solar system, even colder than Pluto, with which it has very similar properties. FTP, identify this largest moon of Neptune.
It is estimated to be as much as 25% water ice, and its atmosphere has pressure of only about one millibar.  
{Triton}