Molecular hydrogen in Titan’s atmosphere: Implications of the measured tropospheric and thermospheric mole fractions

The third most abundant species in Titan’s atmosphere is molecular hydrogen with a tropospheric/lower stratospheric mole fraction of 0.001 derived from Voyager and Cassini infrared measurements. The globally averaged thermospheric H 2 mole fraction profile from the Cassini Ion Neutral Mass Spectrometer (INMS) measurements implies a small positive gradient in the H 2 mixing ratio from the tropopause region to the lower thermosphere (∼950–1000 km), which drives a downward H 2 flux into Titan’s surface comparable to the H 2 escape flux out of the atmosphere (∼2 × 1010 cm−2 s−1 referenced to the surface) and requires larger photochemical production rates of H 2 than obtained by previous photochemical models. From detailed model calculations based on known photochemistry with eddy, molecular, and thermal diffusion, the tropospheric and thermospheric H 2 mole fractions are incompatible by a factor of ∼2. The measurements imply that the downward H 2 surface flux is in substantial excess of the speculative threshold value for methanogenic life consumption of H 2 (McKay, C.P., Smith, H.D. [2005], Icarus 178, 274–276. doi:10.1016/j.icarus.2005.05.018), but without the extreme reduction in the surface H 2 mixing ratio.