9.1 Introduction and some history

Titan, with its dense atmosphere, low gravity, weak solar insolation, and complex composition, provides a unique example of a planetary upper atmosphere. The large mass of the atmosphere, coupled with low gravity, results in a greatly extended atmosphere where the plane parallel assumption, nearly universal in terrestrial and giant planet atmosphere studies, no longer applies. Moreover, the weak gravity results in large escape rates that may play a significant role in upper atmospheric thermal balance. The weak solar insolation means that in many cases dynamical processes can dominate over solar processes, while at the same time the complex composition causes radiative cooling processes to be more important than in most other planetary upper atmospheres. Most of the time Titan orbits within Saturn's magnetosphere and the interaction with energetic particle populations may significantly alter the upper atmosphere. Measurements by the Cassini spacecraft have allowed us to greatly extend our knowledge of the thermal balance in Titan's upper atmosphere, although the main result so far may be the realization that the simple descriptions employed before Cassini fail to capture the complexity and variability of this enigmatic atmosphere. To understand the progress enabled by Cassini-Huygens measurements, we first review our knowledge of thermal balance in Titan's upper atmosphere based on observations by the Voyager spacecraft and ground-based telescopes.