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 (similar to 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 (similar to 2 x 10(10) cm's 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 similar to 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. (C) 2010 Elsevier Inc. All rights reserved.