Neptune, like the rest of the gas giant planets in the Solar System, can be broken up into various layers. The composition of Neptune changes depending on which of these layers you’re looking at. Let’s see what Neptune is made of.
The outermost layer of Neptune is the atmosphere, forming about 5-10% of the planet’s mass, and extending up to 20% of the way down to its core. Neptune’s atmosphere is about 80% hydrogen, 19% helium, with trace amounts of other ices, like methane, ammonia and water ice.
Beneath the atmosphere is the planet’s large mantle. This is a superheated liquid region where temperatures can reach 2,000 K to 5,000 K. It contains about 10 to 15 times the mass of the Earth, and probably consists of water, ammonia, methane, and other compounds. Even though the mantle of the Neptune is extremely hot, astronomers refer to the region as icy; it’s really a hot, dense fluid.
At the very center of Neptune is the planet’s core. The core of Neptune is made of iron, nickel and other silicates that make up about 1.2 times the mass of the Earth.
We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.