  Saturn's largest satellite, Titan, has a thick atmosphere dominated by
nitrogen and methane. The dense orange-brown smog hiding the satellite's
surface is produced by photochemical reactions of methane, nitrogen and their
dissociation products with solar ultraviolet, which lead primarily to the
formation of ethane and heavier hydrocarbons. In the years prior to the
exploration of Titan's surface by the Cassini-Huygens spacecraft, the
production and condensation of ethane was expected to have formed a
satellite-wide ocean one kilometer in depth, assuming that it was generated
over the Solar system's lifetime. However, Cassini-Huygens observations failed
to find any evidence of such an ocean. Here we describe the main cause of the
ethane deficiency on Titan: cryovolcanic lavas regularly cover its surface,
leading to the percolation of the liquid hydrocarbons through this porous
material and its accumulation in subsurface layers built up during successive
methane outgassing events. The liquid stored in the pores may, combined with
the ice layers, form a stable ethane-rich clathrate reservoir, potentially
isolated from the surface. Even with a low open porosity of 10% for the
subsurface layers, a cryovolcanic icy crust less than 2300 m thick is required
to bury all the liquid hydrocarbons generated over the Solar system's lifetime.
