Neptune has a solid, rocky core. It is about 4,600 miles (7,400 km) in radius. That is 30 percent of the planet's total radius. Neptune's mantle is a scalding hot ocean of water, ammonia, and methane. It is about 6,200 miles (10,000 km) deep, or 40 percent of the planet's radius.
Neptune's atmosphere is about 4,600 miles (7,400 km) deep, or 30 percent of the planet's radius. It is made of hydrogen, helium, and methane gases.