Simulating the one-dimensional structure of Titan's upper atmosphere: 2. Alternative scenarios for methane escape

Abstract

In Bell et al. (2010) (paper 1), we provide a series of benchmark simulations that validate a newly developed Titan Global Ionosphere-Thermosphere Model (T-GITM) and calibrate its estimates of topside escape rates with recent work by Cui et al. (2008), Strobel (2009), and Yelle et al. (2008). Presently, large uncertainties exist in our knowledge of the density and thermal structure of Titan's upper atmosphere between the altitudes of 500 km and 1000 km. In this manuscript, we explore a spectrum of possible model configurations of Titan's upper atmosphere that are consistent with observations made by the Cassini Ion-Neutral Mass Spectrometer (INMS), Composite Infrared Spectrometer, Cassini Plasma Spectrometer, Magnetospheric Imaging Instrument, and by the Huygens Gas Chromatograph Mass Spectrometer and Atmospheric Science Instrument. In particular, we explore the ramifications of multiplying the INMS densities of Magee et al. (2009) by a factor of 3.0, which significantly alters the overall density, thermal, and dynamical structures simulated by T-GITM between 500 km and 1500 km. Our results indicate that an entire range of topside CH4 escape fluxes can equivalently reproduce the INMS measurements, ranging from ∼108-1.86 × 1013 molecules m-2 s -1 (referred to the surface). The lowest topside methane escape rates are achieved by scaling the INMS densities by a factor of 3.0 and either (1) increasing the methane homopause altitude to ∼1000 km or (2) including a physicochemical loss referred to as aerosol trapping. Additionally, when scaling the INMS densities by a factor of 3.0, we find that only Jeans escape velocities are required to reproduce the H2 measurements of INMS. Copyright 2010 by the American Geophysical Union.