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Titan, Saturn’s largest satellite, is one of the most intriguing moons in our Solar System, in particular because of its dense and extended nitrogen-based and organic-laden atmosphere. Other unique features include a methanological cycle similar to the Earth’s hydrological one, surface features similar to terrestrial ones, and a probable under-surface liquid water ocean. Besides the dinitrogen main component, the gaseous content includes methane and hydrogen, which, through photochemistry and photolysis, produce a host of trace gases such as hydrocarbons and nitriles. This very advanced organic chemistry creates layers of orange-brown haze surrounding the satellite. The chemical compounds diffuse downward in the form of aerosols and condensates and are finally deposited on the surface. There is very little oxygen in the atmosphere, mainly in the form of H2O, CO, and CO2. The atmospheric chemical and thermal structure varies significantly with seasons, much like on Earth, albeit on much longer time scales. Extensive analysis of Titan data from ground, Earth-orbiting observatories, and space missions, like those returned by the 13-year operating Cassini-Huygens spacecraft, reveals a complex system with strong interactions among the atmosphere, the surface, and the interior. The processes operating in the atmosphere are informative of what occurs on Earth and give hints as to the origin and evolution of our outer Solar System.

The observed composition of a planetary atmosphere is the product of planetary formation and evolution, including the chemical and physical processes shaping atmospheric abundances into the present day. In the solar system, the gas giant planets Jupiter, Saturn, Uranus, and Neptune possess massive molecular envelopes consisting mostly of H2 and He along with various minor amounts of heavy elements such as C, N, and O (present as CH4, NH3, and H2O, respectively) and numerous additional minor species. The terrestrial planets Venus, Earth, and Mars each possess a relatively thin atmospheric envelope surrounding a rocky surface. The atmospheres of Mars and Venus are characterized by abundant CO2 with a small amount of N2, whereas the atmosphere of the Earth is dominated by N2 and O2. Such differences provide clues to the divergent pathways of atmospheric evolution. Numerous closely coupled physical and chemical processes give rise to the abundances observed in the planetary atmospheres of the solar system. These processes include the maintenance of thermochemical equilibrium, reaction kinetics, atmospheric transport, photochemistry, condensation (including cloud formation) and vaporization, deposition and sublimation, diurnal and seasonal effects, greenhouse effects, surface–atmosphere reactions, volcanic activity, and (in the case of Earth) biogenic and anthropogenic sources. The present understanding of the chemical composition of planetary atmospheres is the result of over a century of observations, including ground-based, space-based, and in situ measurements of the major, minor, trace, and isotopic species found on each planet. These observations have been accompanied by experimental studies of planetary materials and the development of theoretical models to identify the key processes shaping atmospheric abundances observed today.