Io, its Atmosphere and Optical Emissions

Io is surrounded by a halo of atoms which radiate in emission lines. This was discovered by groundbased observations in 1973, a year before Pioneer 10 passed through the Jovian system. Earlier optical and radio observers had reported other anomalies associated with Io, and the Pioneer spacecraft discovered an ultraviolet emission cloud around the satellite and a substantial ionosphere. A new field of planetary research is dedicated to integrating these phenomena into a model of Io and of the Jovian environment with which it strongly interacts. Sodium dominates the optical emission cloud around Io. The production rate is very large and has not yet been satisfactorily explained. Sputtering by charged particle bombardment may play an important role. The primary excitation mechanism is the resonant scattering of sunlight. While the sodium cloud is brightest near Io, it extends around the entire orbit and is present at low levels in the general Jovian environment. The ionospheric electron density on Io is comparable to that on Mars, a surprising result since the solar flus is much weaker. Here, again, Io’s charged particle environment may play an important part. The observed electron profiles plus the constraints provided by the emission cloud are consistent with at least two models of Io’s neutral atmosphere