10.1 Introduction

The focus of this chapter is primarily on Titan's exosphere, escape processes, and rates. The composition and structure of the exosphere are intimately linked to the flow of mass, momentum, and energy from the thermosphere as well documented by analyses of the Cassini Ion Neutral Mass Spectrometer (INMS) data (de la Haye et al., 2007a; Cui et al., 2011; Westlake et al., 2011). In addition to the direct in-situ INMS measurements of exospheric densities, these densities are also remotely sensed by their interaction with the energetic ions. Energetic neutral atoms (ENAs) are created and imaged by the Ion and Neutral Camera (INCA) sensor of the Magnetosphere Imaging Instrument (MIMI) (Garnier et al., 2007; Brandt et al., 2012). For the extended exosphere of Titan – that is, for altitudes above ~ 10,000 km where direct detection is not feasible – the ENA method is the only one capable of detecting these populations (Brandt et al., 2012).

A review of basic observational facts on Titan's thermosphere is also necessary to understand the coupling of these two regions. The transition from the thermosphere to the exosphere represents a transition from a region where the atmosphere can be treated as a fluid, because the mean free path, the distance a molecule or atom travels before making a collision, is shorter than the smallest macroscopic length scale, which is the pressure scale height H, that characterizes the exponential decay of pressure with altitude, to a quasi-collisionless region known as the exosphere where the mean free path exceeds the atmospheric scale height.