Titan, Saturn's largest satellite, is known to harbor one of the richest atmospheric chemistry in the solar system, initiated by the dissociation of the major neutral species (nitrogen and methane) by ultraviolet solar radiation. Until recently, it was believed that the complex organic molecules that would ultimately lead to the formation of micrometer size organic aerosols were produced through neutral chemistry in the stratosphere. However, this understanding of Titan’s atmospheric chemistry is being strongly challenged by recent measurements from the Cassini spacecraft. They revealed an extraordinarily complex ionospheric composition with positive ions extending up to at least hundreds of u/q and negative ions up to at least thousands of u/q, suggesting that ion chemistry plays a much more important role than previously anticipated.
We review our current knowledge of Titan's atmospheric chemistry. We base our discussion on Cassini observations as well as on a new generation of photochemical models and laboratory experiments. We argue that positive ion chemistry is at the origin of complex organic molecules, such as benzene and ammonia. We find that negatively charged macromolecules (m/z ~100) attract the abundant positive ions, which ultimately leads to the formation of the aerosols. We also demonstrate that an incoming flux of oxygen from Enceladus, another Saturn's satellite, is responsible for the presence of oxygen-bearing species in Titan's reductive environment. This input of oxygen could lead to the formation of prebiotic molecules during aerosol production.