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 (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.