The Plumes and Atmosphere of Io

The current state of knowledge of Io’s atmosphere and plumes is summarized, with an emphasis on research conducted since the Galileo era. While the primary source of Io’s atmosphere is sublimation of SO2 frost, at times and/or locally volcanoes have a substantial effect on the atmosphere (and sometimes dominate), as shown both via observations and model simulations. The effect of sputtering is less clear, but may be important at high latitudes, and perhaps at night and during eclipse. Photodissociation is likely the primary source of SO, Na, K, and Cl but S2’s main source is volcanic, primarily from “Pele-type” volcanism. While the primary source of NaCl and KCl is most likely volcanic, it is not clear how much sputtering may contribute to the emissions. Some of the NaCl and KCl appears to originate from volcanic vents that do not expel much, if any, SO2 gas. A comparison between data and models shows that atmospheric collapse upon eclipse ingress and its reformation upon egress happens much faster than models indicate. The observed brightening and expansion of the SO2 emissions near volcanic plumes upon eclipse egress, however, does agree with models. Data nor models (at the same resolution) show an enhancement in column density above a plume. As shown in this review, although considerable progress has been made towards both a characterization and understanding of Io’s atmosphere, there are some fundamental questions that need to get answered before we can truly understand and simulate all physical processes that contribute to both the composition of and dynamics in Io’s atmosphere.