Titan's hydrodynamically escaping atmosphere: Escape rates and the structure of the exobase region

摘要

In Strobel [Strobel, D.F., 2008. Icarus, 193, 588-594] a mass loss rate from Titan's upper atmosphere, ～ 4.5 × 10~(28) ? amu s~(- 1), was calculated for a single constituent, N_2 atmosphere by hydrodynamic escape as a high density, slow outward expansion driven principally by solar UV heating due to CH_4 absorption. It was estimated, but not proven, that the hydrodynamic mass loss is essentially CH_4 and H_2 escape. Here the individual conservation of momentum equations for the three major components of the upper atmosphere (N_2, CH_4, H_2) are solved in the low Mach number limit and compared with Cassini Ion Neutral Mass Spectrometer (INMS) measurements to demonstrate that light gases (CH_4, H_2) preferentially escape over the heavy gas (N_2). The lightest gas (H_2) escapes with a flux 99% of its limiting flux, whereas CH_4 is restricted to ≥75% of its limiting flux because there is insufficient solar power to support escape at the limiting rate. The respective calculated H_2 and CH_4 escape rates are 9.2 × 10~(27) and 1.7 × 10~(27) ? s~(- 1), for a total of ～ 4.6 × 10~(28) ? amu s~(- 1). From the calculated densities, mean free paths of N_2, CH_4, H_2, and macroscopic length scales, an extended region above the classic exobase is inferred where frequent collisions are still occurring and thermal heat conduction can deliver power to lift the escaping gas out of the gravitational potential well. In this region rapid acceleration of CH_4 outflow occurs. With the thermal structure of Titan's thermosphere inferred from INMS data by Müller-Wodarg et al. [Müller-Wodarg, I.C.F., Yelle, R.V., Cui, J., Waite Jr., J.H., 2008. J. Geophys. Res. 113, doi:10.1029/2007JE003033. E10005], in combination with calculated temperature profiles that include sputter induced plasma heating at the exobase, it is concluded that on average that the integrated, globally average, orbit-averaged, plasma heating rate during the Cassini epoch does not exceed ～ 5 × 10~8 ? eV cm~(- 2) s~(- 1) (～ 0.0008 ? erg cm~(- 2) s~(- 1)).