Composition and chemistry of Titan's stratosphere.

Our present knowledge of the composition and chemistry of Titan's stratosphere is reviewed. Thermal measurements by the Cassini spacecraft show that the mixing ratios of all photochemical species, except ethylene, increase with altitude at equatorial and southern latitudes, reflecting transport from a high-altitude source to a condensation sink in the lower stratosphere. Most compounds are enriched at latitudes northward of 45 degrees N, a consequence of subsidence in the winter polar vortex. This enrichment is much stronger for nitriles and complex hydrocarbons than for ethane and acetylene. Titan's chemistry originates from breakdown of methane due to photodissociation in the upper atmosphere and catalytical reactions in the stratosphere, and from destruction of nitrogen both by UV photons and electrons. Photochemistry also produces haze particles made of complex refractory material, albeit at a lower rate than ethane, the most abundant gas product. Haze characteristics (vertical distribution, physical and spectral properties) inferred by several instruments aboard Cassini/Huygens are discussed here.