Sequestration of ethane in the cryovolcanic subsurface of Titan

Abstract

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.