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Abstract: The third most abundant species in Titan’s atmosphere is molecular hydrogen with a tropospheric/lower stratospheric mole fraction of 0.001 derived from Voyager and Cassini infrared measurements. The globally averaged thermospheric mole fraction profile from the Cassini Ion Neutral Mass Spectrometer (INMS) measurements implies a small positive gradient in the mixing ratio from the tropopause region to the lower thermosphere (∼950–1000km), which drives a downward flux into Titan’s surface comparable to the escape flux out of the atmosphere (∼2×1010 cm−2 s−1 referenced to the surface) and requires larger photochemical production rates of than obtained by previous photochemical models. From detailed model calculations based on known photochemistry with eddy, molecular, and thermal diffusion, the tropospheric and thermospheric mole fractions are incompatible by a factor of ∼2. The measurements imply that the downward surface flux is in substantial excess of the speculative threshold value for methanogenic life consumption of (McKay, C.P., Smith, H.D. [2005], Icarus 178, 274–276. doi:10.1016/j.icarus.2005.05.018), but without the extreme reduction in the surface mixing ratio. [Copyright &y& Elsevier]

Abstract: In Strobel [Strobel, D.F., 2008. Icarus, 193, 588–594] a mass loss rate from Titan''s upper atmosphere, , was calculated for a single constituent, N2 atmosphere by hydrodynamic escape as a high density, slow outward expansion driven principally by solar UV heating due to CH4 absorption. It was estimated, but not proven, that the hydrodynamic mass loss is essentially CH4 and H2 escape. Here the individual conservation of momentum equations for the three major components of the upper atmosphere (N2, CH4, H2) are solved in the low Mach number limit and compared with Cassini Ion Neutral Mass Spectrometer (INMS) measurements to demonstrate that light gases (CH4, H2) preferentially escape over the heavy gas (N2). The lightest gas (H2) escapes with a flux 99% of its limiting flux, whereas CH4 is restricted to ⩾75% of its limiting flux because there is insufficient solar power to support escape at the limiting rate. The respective calculated H2 and CH4 escape rates are and , for a total of . From the calculated densities, mean free paths of N2, CH4, H2, and macroscopic length scales, an extended region above the classic exobase is inferred where frequent collisions are still occurring and thermal heat conduction can deliver power to lift the escaping gas out of the gravitational potential well. In this region rapid acceleration of CH4 outflow occurs. With the thermal structure of Titan''s thermosphere inferred from INMS data by Müller–Wodarg et al. [Müller-Wodarg, I.C.F., Yelle, R.V., Cui, J., Waite Jr., J.H., 2008. J. Geophys. Res. 113, doi:10.1029/2007JE003033. E10005], in combination with calculated temperature profiles that include sputter induced plasma heating at the exobase, it is concluded that on average that the integrated, globally average, orbit-averaged, plasma heating rate during the Cassini epoch does not exceed (). [Copyright &y& Elsevier]

Abstract: The upper atmosphere of Titan is currently losing mass at a rate , by hydrodynamic escape as a high density, slow outward expansion driven principally by solar UV heating by CH4 absorption. The hydrodynamic mass loss is essentially CH4 and H2 escape. Their combined escape rates are restricted by power limitations from attaining their limiting rates (and limiting fluxes). Hence they must exhibit gravitational diffusive separation in the upper atmosphere with increasing mixing ratios to eventually become major constituents in the exosphere. A theoretical model with solar EUV heating by N2 absorption balanced by HCN rotational line cooling in the upper thermosphere yields densities and temperatures consistent with the Huygens Atmospheric Science Investigation (HASI) data [Fulchignoni, M., and 42 colleagues, 2005. Nature 438, 785–791], with a peak temperature of ∼185–190 K between 3500–3550 km. This model implies hydrodynamic escape rates of and , or some other combination with a higher H2 escape flux, much closer to its limiting value, at the expense of a slightly lower CH4 escape rate. Nonthermal escape processes are not required to account for the loss rates of CH4 and H2, inferred by the Cassini Ion Neutral Mass Spectrometer (INMS) measurements [Yelle, R.V., Borggren, N., de la Haye, V., Kasprzak, W.T., Niemann, H.B., Müller-Wodarg, I., Waite Jr., J.H., 2006. Icarus 182, 567–576]. [Copyright &y& Elsevier]

Abstract: Tidal waves driven by Titan''s orbital eccentricity through the time-dependent component of Saturn''s gravitational potential attain nonlinear, saturation amplitudes (, , and ) in the upper atmosphere (⩾500 km) due to the approximate exponential growth as the inverse square root of pressure. The gravitational tides, with vertical wavelengths of ∼100–150 km above 500 km altitude, carry energy fluxes sufficient in magnitude to affect the energy balance of the upper atmosphere with heating rates in the altitude range of 500–900 km. [Copyright &y& Elsevier]

Abstract: Titan''s atmospheric winds, like those on Venus, exhibit superrotation at high altitudes. Titan general circulation models have yielded conflicting results on whether prograde winds in excess of 100 m s−1 at the 1 mbar level are possible based on known physical processes that drive wind systems. A comprehensive two-dimensional (2D) model for Titan''s stratosphere was constructed to systematically explore the physical mechanisms that produce and maintain stratospheric wind systems. To ensure conservation of angular momentum in the limit of no net exchange of atmospheric angular momentum with the solid satellite and no external sources and sinks, the zonal momentum equation was solved in flux form for total angular momentum. The relationships among thermal wind balance, meridional circulation, and zonal wind were examined with numerical experiments over a range of values for fundamental input parameters, including planetary rotation rate, radius, internal friction due to wave stresses, and net radiative drive. The magnitude of mid-latitude jets is most sensitive to a single parameter, the planetary rotation rate and results from the conversion of planetary angular momentum to relative angular momentum by the meridional circulation, whereas the strength of meridional circulation is mainly determined by the magnitude of the radiative drive. For Titan''s slowly rotating atmosphere, the meridional temperature gradient is vanishingly small, even when the radiative drive is enhanced beyond reasonable magnitudes, and can be inferred from zonal winds in gradient/thermal wind balance. In our 2D model large equatorial superrotation in Titan''s stratosphere can be only produced through internal drag forcing by eddy momentum fluxes, which redistribute angular momentum within the atmosphere, while still conserving the total angular momentum of the atmosphere with time. We cannot identify any waves, such as gravitational or thermal tides, that are sufficiently capable of generating the required eddy forcing of >50 m s−1 Titan-day−1 to maintain peak prograde winds in excess of 100 m s−1 at the 1 mbar level. [Copyright &y& Elsevier]

We have reanalyzed the Voyager 1 UVS solar occultations by Titan to expand upon previous analyses and to resolve inconsistencies that have been noted in the scientific literature. To do so, we have developed a detailed model of the UVS detector and improved both the data reduction methods and retrieval techniques. In comparison to the values previously determined by Smith et al. (1982, J. Geophys. Res. 87, 1351–1359) we find N2 densities that are 25–60% higher, CH4 densities that are smaller by a factor of 3–7, and C2H2 densities that are roughly two orders of magnitude smaller. Our values for the thermospheric temperature are 153–158 K, which are approximately 20–40 K colder than previous estimates. We also report the first-ever determination from Voyager UVS data of density profile information for C2H4, HCN, and HC3N. Finally, we present a simple engineering model that is consistent with our new results in the upper atmosphere and merges smoothly with the model of Yelle et al. (1997, in: HUYGENS Science, Payload and Mission, in: ESA SP, vol. 1177, pp. 243–256) in the lower atmosphere. Our results provide improved constraints for photochemical models and offer scientists a better understanding of Titan''s upper atmosphere as we head into the Cassini era in the exploration of the saturnian system. [Copyright &y& Elsevier]