Distribution and escape of molecular hydrogen in Titan's thermosphere and exosphere

Abstract

We present an in‐depth study of the distribution and escape of molecular hydrogen (H2) on Titan, based on the global average H2 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 H2 distribution can be described by an isothermal diffusion model, with a most probable flux of (1.37 ± 0.01) × 1010 cm−2 s−1, referred to the surface. This is a factor of ∼3 higher than the Jeans flux of 4.5 × 109 cm−2 s−1, corresponding to a temperature of 152.5 ± 1.7 K, derived from the background N2 distribution. The H2 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 H2, 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 H2 escape flux considerably higher than the Jeans value. Nonthermal processes may not be required to interpret the H2 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.