Atmospheric Structure and Composition

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Titan's atmosphere is predominantly N2 with CH4 the next most abundant molecule. It has a mole fraction of 0.05 just above the surface decreasing to 0.014 in the stratosphere. Above the homopause (~800–850 km), it increases to 0.12 at the exobase. The third abundant molecule is H2 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 region (currently northern hemisphere) and warm over the summer pole. Remarkably in the middle stratosphere, the warmest temperatures occur at the equator and the largest meridional temperature gradients are found in the winter hemisphere. The stratopause from the summer pole to about 45° N remains at a relatively constant pressure of 0.1 mbar/300 km and then it rises rapidly upward to ~0.01 mbar/400 km at the winter north pole, where it is the warmest region in the entire atmosphere. One possible interpretation of the Huygens Atmospheric Structure Instrument (HASI) temperature profile is that Titan's atmosphere is essentially isothermal ~170 K from 500–1100 km, with large amplitude thermal waves (10 K) superimposed. The existence and location of a well defined mesopause is an open question. The chemistry of Titan's atmosphere is driven by CH4 photolysis in the thermosphere and catalytic reactions in the stratosphere, and by N2 dissociation due to both UV photons and energetic electrons. Ethane is the most abundant gas product and HCN is the dominant nitrile. The mixing ratios of all photochemical species, except C2H4 , increase with altitude at equatorial and southern latitudes, indicative of transport from a high-altitude source to a condensation sink in the lower stratosphere. Northward of 45° N, most product compounds are enriched as a consequence of subsidence in the winter polar vortex, particularly for nitriles and more complex hydrocarbons than C2H6 and C2H2. North of 45° N, most products have lower increases with altitude than at low latitudes.