Diagram showing the theoretical interior of the ice giant planet Neptune. It probably looks very similar to the interior of Uranus. At the very centre is a rocky and icy core, similar and size and mass to the Earth. This is encased in a thick inner mantle, a slushy mixture of various ices including methane, ammonia and water. Above this is the outer mantle, which is made up of a mixture of liquid hydrogen and other elements. And finally there is a thick atmosphere, composed chiefly of hydrogen, helium and methane.