Simulating the one-dimensional structure of Titan's upper atmosphere: 3. Mechanisms determining methane escape

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.

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 .…”