The Atmosphere of Titan

Abstract 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.