Like Jupiter, Saturn and Uranus, Neptune is a gas giant with no solid surface. Unlike Jupiter and Saturn the interior is of uniform composition, a mixture of frozen water, frozen methane and frozen ammonia with a rock and ice core. The atmosphere is made of Hydrogen, Helium and Methane.

The lack of an outer core of electrically conducting material means that its magnetic field, like that of Uranus, is off centre and not aligned with the axis of rotation.

Neptune has a number of rings of unknown composition. The rings are thin and made of a dark material. The rings are clumpy giving the impression that the rings are incomplete and made up of ring arcs. Voyager 2 was able to show that in fact the rings are complete. One of the rings has a braided appearance, probably due to the gravity of a shepherd moon or moons.

Neptune has 13 known moons the only one large enough to be round is Triton. The other moons are of irregular shape.

More information about the moons at:

http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune&Display=Moons

Triton is remarkable for a number of reasons. Its orbit is retrograde, that is it moves in its orbit in the opposite direction to the rotation of Neptune. It is the coldest object that has been measured in the solar system with a temperature of -235ºC. It is locked into a synchronous orbit with Neptune keeping the same face towards the planet much as the Moon keeps the same face towards Earth and is slowly spiraling in towards Neptune. Because of this odd behaviour it is thought that Triton is not a natural moon of Neptune but is a captured Kuiper Belt object.

As the Voyager 2 spacecraft passed Triton it photographed mysterious plumes of dark material rising from the surface to a height of 8 kilometres where they encounter upper atmosphere winds which stretches them out for about 150 kilometres. The nature of these plumes is not certain, they may be ice volcanoes or possibly sub-surface Nitrogen gas heated by sunlight, turning to gas and exploding through the surface layer.