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 H2H2 mole fraction profile from the Cassini Ion Neutral Mass Spectrometer (INMS) measurements implies a small positive gradient in the H2H2 mixing ratio from the tropopause region to the lower thermosphere (∼950–1000 km), which drives a downward H2H2 flux into Titan’s surface comparable to the H2H2 escape flux out of the atmosphere (∼2 × 1010 cm−2 s−1 referenced to the surface) and requires larger photochemical production rates of H2H2 than obtained by previous photochemical models. From detailed model calculations based on known photochemistry with eddy, molecular, and thermal diffusion, the tropospheric and thermospheric H2H2 mole fractions are incompatible by a factor of ∼2. The measurements imply that the downward H2H2 surface flux is in substantial excess of the speculative threshold value for methanogenic life consumption of H2H2 (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 H2H2 mixing ratio.