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.

Developing a self‐consistent description of Titan's upper atmosphere without hydrodynamic escape

“…Our efforts here differ significantly from our work in Bell et al [2010aBell et al [ , 2010bBell et al [ , 2011b. Previously, we accounted for both diurnal and sun-Saturn orbital distance variations over the course of our simulations, which are not accounted for in the collective works of Cui et al [2008], Strobel [2008Strobel [ , 2009Strobel [ , 2010Strobel [ , 2012, and Yelle et al [2008].…”

“…We approximate a diurnal average by dividing the solar fluxes by a factor of 2.0. This approach differs from the settings in Bell et al [2010aBell et al [ , 2010bBell et al [ , 2011b, but it matches other 1-D investigations of Titan's upper atmosphere [e.g., Krasnopolsky, 2009Krasnopolsky, , 2010Strobel, 2010;Yelle et al, 2008].…”

“…While most studies agree on the H 2 escape rates, Strobel [2010,2012] found that 1-D simulations could not simultaneously match both the INMS H 2 measurements and those of CIRS and GCMS. Moreover, Strobel [2012] concluded that this mismatch between the INMS H 2 measurements above 1000 km and those below 200 km is exacerbated when using the high-methane homopause and low-methane escape rates of Bell et al [2010bBell et al [ , 2011b.…”

“…We will do this by building upon our previous work in Bell et al [2010aBell et al [ , 2010bBell et al [ , 2011b], where we identified three key aspects of our 1-D simulations that determined the amount of atmospheric escape of CH 4 and H 2 needed to match INMS measurements: (1) the amount of turbulence used in the model, (2) the net chemical destruction of methane (net production of H 2 ) included in the model (i.e., the sum over direct photolytic, ion-neutral, and neutral-neutral chemical losses), and (3) the inclusion of a self-consistent thermal balance calculation. We have designed a series of numerical experiments that explore each of these aspects sequentially, building from highly simplified simulations to fully coupled simulations that combine all three aspects into a global mean description of Titan's upper atmosphere.…”

“…Tucker et al [2013], using a direct simulation Monte Carlo method, have reproduced the INMS measurements in the upper atmosphere while using only thermal escape rates of methane (less than 1.0 × 10 9 CH 4 m −2 s −1 ). Similarly, Bell et al [2010aBell et al [ , 2010bBell et al [ , 2011b] used a 1-D version of the Titan Global Ionosphere-Thermosphere Model (T-GITM) to reproduce the INMS densities and mixing ratios using a methane homopause near 1000 km and escape rates of ∼1.0 × 10 8 CH 4 m −2 s −1 .…”