Titan's hydrodynamically escaping atmosphere

The upper atmosphere of Titan is currently losing mass at a rate ∼(4–5)×1028 amus−1, 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 ∼2×1027 CH4s−1 and 5×1027 H2s−1, 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].