Simulating the one-dimensional structure of Titan's upper atmosphere: 3. Mechanisms determining methane escape
Bar-Nun, Akiva; Bougher, Stephen W.; Mandt, Kathleen E.; Gell, David; Toth, Gabor; Magee, Brian A.; Westlake, Joseph; Waite, J. Hunter, Jr.; Bell, Jared M.; Ridley, Aaron J.; DeJong, Anna D.; Jacovi, Ronen; De La Haye, Virginie; Fletcher, Gregory
United States, Israel
Abstract
This investigation extends the work presented by Bell et al. (2010a, 2010b). Using the one-dimensional (1-D) configuration of the Titan Global Ionosphere-Thermosphere Model (T-GITM), we quantify the relative importance of the different dynamical and chemical mechanisms that determine the CH4 escape rates calculated by T-GITM. Moreover, we consider the implications of updated Huygens Gas Chromatograph Mass Spectrometer (GCMS) determinations of both the 40Ar mixing ratios and 15N/14N isotopic ratios in work by Niemann et al. (2010). Combining the GCMS constraints in the lower atmosphere with the Ion Neutral Mass Spectrometer (INMS) measurements in work by Magee et al. (2009), our simulation results suggest that the optimal CH4 homopause altitude is located at 1000 km. Using this homopause altitude, we conclude that topside escape rates of 1.0 × 1010 CH4 m-2 s-1 (referred to the surface) are sufficient to reproduce the INMS methane measurements in work by Magee et al. (2009). These escape rates of methane are consistent with the upper limits to methane escape (1.11 × 1011 CH4 m-2 s-1) established by both the Cassini Plasma Spectrometer (CAPS) and Magnetosphere Imaging Instrument (MIMI) measurements of Carbon-group ions in the near Titan magnetosphere.