Does Titan have an ocean? A review of current understanding of Titan's surface

The nature of Titan's surface has been a fascinating puzzle since the first definitive detection of an atmosphere in 1944. Pre‐Voyager models of the surface based largely on cosmochemistry and the detection of atmospheric methane permitted a range of possibilities from clathrate hydrate to pure methane snow. The Voyager 1 flyby ruled out this last possibility but pointed to a mixed ocean of methane, higher hydrocarbons, and nitrogen about a kilometer in thickness. Subsequent ground‐based radar observations reveal a reflective surface, inconsistent with pure light hydrocarbons. The variation in radar reflectivity from night to night is surprisingly high. Passive radiometry at centimeter wavelengths indicates a surface emissivity distinct from that of the icier Galilean satellites, suggesting that water ice does not dominate the surface either. Observations in the near‐infrared region are consistent with dirty water ice as well as a suite of other materials, including areas of hydrocarbon seas. Reconciling the Voyager, radar and near‐infrared data continue to be a challenge. Possibilities include the following: a hydrocarbon ocean, which is frothy or contains a significant amount of aerosol contaminants, maintained near the surface by stirring; an ocean stored in pore spaces of the ice regolith or in an aquiferlike system; an ocean that is somewhat smaller than estimated from the methane photolysis rate because of impact‐driven chemistry. Models involving significant amounts of dry land exposed above a shallow ocean potentially run afoul of tidal constraints imposed by Titan's nonzero free orbital eccentricity. The Cassini/Huygens mission will investigate the nature of the surface through a variety of techniques and hopefully solve this long‐standing planetary puzzle.