Titan's hydrodynamically escaping atmosphere

Abstract

The upper atmosphere of Titan is currently losing mass at a rate ∼(4-5)×10 amus, by hydrodynamic escape as a high density, slow outward expansion driven principally by solar UV heating by CH ₄ absorption. The hydrodynamic mass loss is essentially CH ₄ and H ₂ 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 N ₂ 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 ∼2×10 CHs and 5×10 Hs, or some other combination with a higher H ₂ escape flux, much closer to its limiting value, at the expense of a slightly lower CH ₄ escape rate. Nonthermal escape processes are not required to account for the loss rates of CH ₄ and H ₂, 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].