Interior of Titan

There is very little direct information on Titan's interior and even the precisely known mean density allows large uncertainties in volatile content (defined as all components more volatile than H_2O). Volatile-poor models, in which Titan is like Ganymede or Callisto with a thin (observed) volatile veneer, are discussed and discarded. Volatile-rich models are described in which the present Titan consists of a methane clathrate shell overlying a deep water-ammonia ocean and solid ammonia hydrate. The central core of rock (radius ≈ 1900 km) comprises about one half the total mass. The surficial hydrocarbon "ocean" is stored in an "aquifer" (subsurface caverns and pore space) thereby reconciling previous arguments for an ocean with radar and tidal constraints