| The third most abundant species in Titan's atmosphere is molecular hydrogen. In the troposphere and lower stratosphere the mole fraction is 0.001 based on Voyager IRIS, Cassini CIRS infrared measurements and the Huygens GCMS, whereas the Cassini INMS measures the thermospheric H₂ mole fraction profile above 950 km. Strobel (Icarus, 208, 878-886, doi:10.1016/j.icarus.2010.03.003, 2010) concluded that the combined measurements imply a downward H₂ flux into Titan's surface comparable to the H₂ escape flux out of the atmosphere (rate ~ 1 × e28 s-¹) and required larger photochemical production rates of H₂ than obtained by previous photochemical models. Further detailed model calculations indicate the source must be in the upper atmosphere presumably associated with magnetospheric energetic particle deposition. |