Hubble Space Telescope Space Telescope Imaging Spectrograph Search for an Atmosphere on Callisto: A Jovian Unipolar Inductor

Hubble Space Telescope observations of Callisto with the Space Telescope Imaging Spectrograph were performed at both eastern and western elongations to search for the UV emissions diagnostic of the presence of O₂, CO₂, and/or CO atmospheres. We report upper limits of 5 × 10^-5 photons cm^-2 s^-1 or 15 R for a uniform disk the diameter of Callisto on emissions of O I λ1304, O I λ1356, C I λ1561, C II λ1335, and CO fourth positive bands. These upper limits yield upper bounds on O₂, CO₂, and CO atmospheres far in excess of the detected CO₂ atmosphere by R. W. Carlson. Our results are interpreted in terms of a strong electrodynamic interaction with the Jovian magnetosphere, which drives ~1.5 × 10⁵ A through Callisto's highly conducting (~10⁴ mho) ionosphere and generates a highly reduced ionospheric electric field, severely retarded ionospheric convection (~0.1 km s^-1), and a factor of ~1500 reduction in the net electron impact emission rate. Callisto's highly conducting ionosphere renders it the most tenable unipolar inductor of the Galilean satellites.