Figure 14-7: The Internal Structures of Uranus and Neptune In the model shown here, both Uranus and Neptune have a rocky core (resembling a terrestrial planet), a mantle of liquid water with ammonia dissolved in it, and an outer layer of liquid molecular hydrogen and liquid helium. The atmospheres are very thin shells on top of the hydrogen-helium layer. Uranus and Neptune have about the same diameter, but since Neptune is more massive it may have a somewhat larger core.