Neptune’s internal structure is very similar to that of the planet Uranus.
Neptune and Uranus are often considered a sub-class of gas giant termed “ice giants”, due to their smaller size and higher concentrations of volatiles relative to the gas giants Jupiter and Saturn.
The planet Neptune has a partly rocky, but mostly icy, core which may not be completely distinct from its mantle, or middle layer.
The core of Neptune is composed of iron, nickel and silicates.
The mantle seems to be made of water, ammonia, and methane in ice form.
It is surrounded by gaseous forms of hydrogen, helium, and methane in the atmosphere.