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

Abstract

Hubble Space Telescope observations of Callisto with the Space Telescope Imaging Spectrograph were per-formed at both eastern and western elongations to search for the UV emissions diagnostic of the presence of O 2 , CO 2 , and/or CO atmospheres. We report upper limits of photons cm Ϫ2 s Ϫ1 or 15 R for a uniform disk Ϫ5 5 # 10 the diameter of Callisto on emissions of O i l1304, O i l1356, C i l1561, C ii l1335, and CO fourth positive bands. These upper limits yield upper bounds on O 2 , CO 2 , and CO atmospheres far in excess of the detected CO 2 atmosphere by R. W. Carlson. Our results are interpreted in terms of a strong electrodynamic interaction with the Jovian magnetosphere, which drives ∼ A through Callisto's highly conducting (∼10 4 mho) 5 1.5 # 10 ionosphere and generates a highly reduced ionospheric electric field, severely retarded ionospheric convection (∼0.1 km s Ϫ1), 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.