Composition and structure of Neptune

The internal structure of Neptune:
1. Upper atmosphere, top clouds.
2. Atmosphere consisting of hydrogen, helium, and methane gas.
3. Mantle consisting of water, ammonia, and methane ices.
4. Core consisting of rock and ice.

With a mass of 1.0243×10²⁶ kg,Neptune is an intermediate body between Earth and the larger gas giants: its mass is seventeen times that of the Earth but just 1/19th that of Jupiter.Neptune's equatorial radius of 24,764 km is nearly four times that of the Earth. 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 Jupiter and Saturn.In the search for extrasolar planets Neptune has been used as a metonym: discovered bodies of similar mass are often referred to as "Neptunes",just as astronomers refer to various extra-solar "Jupiters."

Internal structure of Neptune:

Neptune's internal structure resembles that of Uranus. Its atmosphere forms about 5 to 10 percent of its mass and extends perhaps 10 to 20 percent of the way towards the core, where it reaches pressures of about 10 GPa. Increasing concentrations of methane, ammonia, and water are found in the lower regions of the atmosphere.Gradually this darker and hotter region condenses into a superheated liquid mantle, where temperatures reach 2,000 K to 5,000 K. The mantle is equivalent to 10 to 15 Earth masses, and is rich in water, ammonia, methane, and other compounds.As is customary in planetary science, this mixture is referred to as icy even though it is a hot, highly dense fluid. This fluid, which has a high electrical conductivity, is sometimes called a water-ammonia ocean. At a depth of 7,000 km, the conditions may be such that methane decomposes into diamond crystals that then precipitate toward the core.

The core of Neptune is composed of iron, nickel and silicates, with an interior model giving a mass about 1.2 times that of the Earth. The pressure at the centre is 7 Mbar, millions of times more than that on the surface of the Earth, and the temperature may be 5,400 K.