The structure of the inside of Neptune is thought to be very similar to the structure of the inside of Uranus. There is likely to be a core, thought to be about 15 Earth masses. It is made up of molten rock and metal surrounded by rock, water, ammonia, and methane. This mixture is referred to as icy. It is called a water-ammonia ocean. More mixtures of methane, ammonia, and water are found in the lower areas of the atmosphere. 