Neptune's interior. Computer artwork showing the interior structure of the planet Neptune. Internally, Neptune is divided into three major layers: a 5000 km-thick gaseous atmosphere (blue), consisting mostly of hydrogen and helium; a 16,500 km-thick liquid mantle (green) of water, ammonia and methane, and a rocky core with a radius of 3000 km. The most dynamic layer is the mantle, which generates the magnetic field of the planet.