Atmospheric Structure and Composition,

Abstract Titan’s atmosphere is predominantly N 2 with CH 4 the next most abundant molecule. It has a mole fraction of 0.05 just above the surface decreasing to 0.014 in the strato-sphere. Above the homopause (~800–850 km), it increases to 0.12 at the exobase. The third abundant molecule is H 2 with a tropospheric mole fraction of 0.001 increasing to 0.004 at ~1000 km and ~0.02 at the exobase (~1500– 1600 km). This chapter reviews the various measurements acquired by the Voyager flybys, Huygens Probe, orbiting Cassini spacecraft, ground-based and orbiting telescopes of the large suite of hydrocarbons, nitriles, other nitrogen and also oxygen bearing compounds. Titan possesses a mostly stable troposphere with a well defined tropopause ( T ~ 70 K at ~44 km) and a lower stratosphere with a high static stability, which is extremely cold over the winter polar regio