Titan's atmosphere and climate

Titan is the only moon with a substantial atmosphere, the only other thick N-2 atmosphere besides Earth's, the site of extraordinarily complex atmospheric chemistry that far surpasses any other solar system atmosphere, and the only other solar system body with stable liquid currently on its surface. The connection between Titan's surface and atmosphere is also unique in our solar system; atmospheric chemistry produces materials that are deposited on the surface and subsequently altered by surface-atmosphere interactions such as aeolian and fluvial processes resulting in the formation of extensive dune fields and expansive lakes and seas. Titan's atmosphere is favorable for organic haze formation, which combined with the presence of some oxygen-bearing molecules indicates that Titan's atmosphere may produce molecules of prebiotic interest. The combination of organics and liquid, in the form of water in a subsurface ocean and methane/ethane in the surface lakes and seas, means that Titan may be the ideal place in the solar system to test ideas about habitability, prebiotic chemistry, and the ubiquity and diversity of life in the universe. The Cassini-Huygens mission to the Saturn system has provided a wealth of new information allowing for study of Titan as a complex system. Here I review our current understanding of Titan's atmosphere and climate forged from the powerful combination of Earth-based observations, remote sensing and in situ spacecraft measurements, laboratory experiments, and models. I conclude with some of our remaining unanswered questions as the incredible era of exploration with Cassini-Huygens comes to an end. Plain Language Summary Titan, the largest moon of Saturn, is unique in our solar system. It is the only moon with a dense atmosphere and the only other thick nitrogen atmosphere besides that of Earth. In the atmosphere very complicated chemistry, started by sunlight, forms molecules made of carbon, hydrogen, nitrogen, and oxygen (sometimes called organic molecules); these molecules end up on the surface where they are moved by wind and rain to form dunes, lakes, and seas. It is the only world besides Earth that has lakes and seas presently on its surface, although they are made of liquid methane and ethane instead of water. The conditions in Titan's atmosphere are ideal for creation of smog-like particles called haze, which may have molecules that are important for the origin of life. The combination of liquid and organics means that Titan may be the best place in the solar system to test ideas about how life begins and how common it is in the universe. The Cassini-Huygens mission to the Saturn system has provided a wealth of new information allowing us to study Titan as a system. Here I review our current understanding of Titan's atmosphere and climate forged from the powerful combination of Earth-based observations, measurements from spacecraft, laboratory experiments, and computer models. I conclude with some of our remaining unanswered questions as the incredible era of exploration with Cassini-Huygens comes to an end.