UREY-PRIZE-LECTURE - IOS ATMOSPHERE - NOT YET UNDERSTOOD

The atmosphere of Io has been detected at infrared, millimeter, and ul traviolet wavelengths. The observations indicate a predominantly SO2 t enuous atmosphere on both the leading and trailing daysides, probably variable but permanently detectable. The observed SO2 pressure is on t he order of 1 nanobar. There are several indications that the atmosphe re is patchy rather than spatially uniform. The observable atmosphere may be produced from sublimation of SO2 frost or directly from volcani c output, while surface sputtering may produce a more tenuous atmosphe ric component in specific regions. The large horizontal pressure gradi ents probably drive a supersonic meteorology that affects the pressure distribution on both local and large scales and the sublimation/conde nsation exchanges. SO appears to be a significant minor component alon g with SO2, but its spatial distribution (global or localized) remains uncertain. Fundamental issues are unresolved, as the detailed interpr etation of some observations is still ambiguous. The surface pressure and areal extent of the atmosphere are only known to within factors of similar to 10 and 5, respectively. The vertical structure is extremel y poorly characterized. We do not know for sure whether Io's atmospher e is in hydrostatic equilibrium or if the gas is rapidly ejected by vo lcanic venting. The gas temperature is very difficult to constrain fro m observations, although detailed models are available. The dominant s ource of Io's atmosphere remains uncertain. Current data sets would su ggest a preference for the concept of an atmosphere in dynamical equil ibrium with volcanic sources, but this conclusion must be viewed as te ntative. The existence, composition, thickness, and extent of the atmo sphere on the nightside are unknown. (C) 1996 Academic Press, Inc.