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Distribution of molecular hydrogen in the atmosphere of Titan

1983; American Geophysical Union; Volume: 88; Issue: A11 Linguagem: Inglês

10.1029/ja088ia11p08716

2156-2202

Jean‐Loup Bertaux, G. Kockarts,

Spacecraft and Cryogenic Technologies

The diffusion problem of light gases in the atmosphere of Titan is reassessed after Voyager encounter. The pre‐Voyager theoretical work of Hunten (1973 a ) is completed by the use of parameters derived from Voyager encounter: the radial distances of turbopause level (3500 km) and critical level (4100 km), the exospheric temperature (186 K) and the volume mixing ratio of 2×10 −3 for H 2 . An analytical expression of the solution of the diffusion equation is given for a spherical geometry and an isothermal atmosphere. It is used to predict an altitude profile of H 2 in the upper atmosphere of Titan. The modification of this profile by a possible return flow of H 2 from a torus to the atmosphere of Titan is addressed, and is found negligible since there is observational evidence from Voyager that the H 2 density is less than 100 cm −3 in the torus, at large distance from Titan. The H 2 concentration at the exobase level is 3.1×10 5 cm −3 , and the thermal escape of H 2 is nearly equal to the diffusion limiting flux of 3.4×10 9 molecules cm −2 s −1 at the turbopause level. Photodissociation of CH 4 has to provide for an equal constant supply of H 2 .