Io's tenuous atmosphere

An evaluation is made of observational evidence adduced to date for the various extant models of the Io atmosphere. SO2 and its various photochemical products have been noted to play a major role in the atmosphere; such atomic species as Na and K are present in smaller quantities, and play an important role in the atmosphere's ion chemistry. The vapor pressure equilibrium between condensed SO2 on the surface and the atmospheric gas is an important factor in the global control of SO2 gas distribution, but cannot account for the neutral densities at the terminator required for ionospheric models in view of realistic surface temperature distributions.