Figure 7-14 The Internal Structures of Uranus and Neptune Uranus and Neptune each have a rocky core, resembling a terrestrial planet and probably about the size of planet Earth. A mantle of liquid water with ammonia dissolved in it surrounds the core, and an outer layer of liquid molecular hydrogen and liquid helium surrounds the mantle. Above this, the atmospheres of Uranus and Neptune are very thin shells on top of the hydrogen-helium layers. Uranus and Neptune have about the same diameter, but Neptune is more massive, suggesting it might have a somewhat larger core.