## Abstract

We present an in-depth study of the distribution and escape of molecular hydrogen (H_{2}) on Titan, based on the global average H_{2} distribution at altitudes between 1000 and 6000 km, extracted from a large sample of Cassini/Ion and Neutral Mass Spectrometer (INMS) measurements. Below Titan's exobase, the observed H_{2} distribution can be described by an isothermal diffusion model, with a most probable flux of (1.37 ± 0.01) × 10^{10} cm^{-2} s^{-1}, referred to the surface. This is a factor of ∼3 higher than the Jeans flux of 4.5 × 10^{9} cm^{-2} s^{-1}, corresponding to a temperature of 152.5 ± 1.7 K, derived from the background N_{2} distribution. The H_{2} distribution in Titan's exosphere is modeled with a collisionless approach, with a most probable exobase temperature of 151.2 ± 2.2 K. Kinetic model calculations in the 13-moment approximation indicate a modest temperature decrement of several kelvin for H_{2}, as a consequence of the local energy balance between heating/cooling through thermal conduction, viscosity, neutral collision, and adiabatic outflow. The variation of the total energy flux defines an exobase level of ∼1600 km, where the perturbation of the Maxwellian velocity distribution function, driven primarily by the heat flow, becomes strong enough to raise the H_{2} escape flux considerably higher than the Jeans value. Nonthermal processes may not be required to interpret the H_{2} escape on Titan. In a more general context, we suggest that the widely used Jeans formula may significantly underestimate the actual thermal escape flux and that a gas kinetic model in the 13-moment approximation provides a better description of thermal escape in planetary atmospheres.

Original language | English (US) |
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Article number | E10004 |

Journal | Journal of Geophysical Research: Planets |

Volume | 113 |

Issue number | 10 |

DOIs | |

State | Published - Oct 20 2008 |

## ASJC Scopus subject areas

- Geochemistry and Petrology
- Geophysics
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science