Triton, largest moon of Neptune, taken from the Voyager 2 spacecraft. It is one of only three objects in the solar system with a nitrogen-dominated atmosphere - the other two being Earth and Saturn's giant moon Titan. It also has the coldest surface known in the solar system at -391 degrees Fahrenheit. At that temp, nitrogen condenses as frost.
Triton is, with a diameter of 2,700 km, the largest of Neptune’s 13 moons. It is the only large moon in the Solar System with a retrograde orbit (an orbit in the opposite direction to its planet’s rotation), which cannot have formed out of the same region as Neptune, so it must have been captured from elsewhere. Because of its retrograde orbit and composition similar to Pluto’s, Triton is thought to have been captured from the Kuiper belt.