[1] We employ a newly developed Navier-Stokes model, the Titan Global Ionosphere-Thermosphere Model (T-GITM) to address the one dimensional (1-D) coupled composition, dynamics, and energetics of Titan's upper atmosphere. Our main goals are to delineate the details of this new theoretical tool and to present benchmark calibration simulations compared against the Ion-Neutral Mass Spectrometer (INMS) neutral density measurements. First, we outline the key physical routines contained in T-GITM and their computational formulation. Then, we compare a series of model simulations against recent 1-D work by Cui et al. (2008), Strobel (2008 in order to provide a fiducial for calibrating this new model. In paper 2 and a future paper, we explore the uncertainties in our knowledge of Titan's atmosphere between ∼500 km and 1000 km in order to determine how the present measurements constrain our theoretical understanding of atmospheric structures and processes.

Titan, the largest moon of Saturn and similar to Earth in many aspects, has unique orangeyellow colour that comes from its atmospheric haze, whose formation and dynamics are far from well understood. Present models assume that Titan's tholin-like haze formation occurs high in atmosphere through gas-phase chemical reactions initiated by high-energy solar radiation. Here we address an important question: Is the lower atmosphere of Titan photochemically active or inert? We demonstrate that indeed tholin-like haze formation could occur on condensed aerosols throughout the atmospheric column of Titan. Detected in Titan's atmosphere, dicyanoacetylene (C 4 N 2 ) is used in our laboratory simulations as a model system for other larger unsaturated condensing compounds. We show that C 4 N 2 ices undergo condensed-phase photopolymerization (tholin formation) at wavelengths as long as 355 nm pertinent to solar radiation reaching a large portion of Titan's atmosphere, almost close to the surface.

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.

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.