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

Abstract

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 mole fraction profile from the Cassini Ion Neutral Mass Spectrometer (INMS) measurements implies a small positive gradient in the H mixing ratio from the tropopause region to the lower thermosphere (∼950-1000 km), which drives a downward H flux into Titan's surface comparable to the H escape flux out of the atmosphere (∼2 × 10 ¹⁰ cm ^-2 s ^-1 referenced to the surface) and requires larger photochemical production rates of H 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 mole fractions are incompatible by a factor of ∼2. The measurements imply that the downward H surface flux is in substantial excess of the speculative threshold value for methanogenic life consumption of H (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 mixing ratio.