Ganymede's ozone-like absorber : Observations by the Galileo ultraviolet spectrometer

We report on Ultraviolet Spectrometer observations of Ganymede from four orbits of the Galileo primary mission. During 12 observation sequences, regions on Ganymede's leading, trailing, and anti-Jovian hemispheres were observed at several latitudes and varying observational geometries. The measurements show an ozone-like absorber concentrated in the polar regions and near sunrise and sunset at low latitudes, with a clear correlation between ozone abundance and solar zenith angle. The absorption maximum is shifted to the red of the gaseous ozone absorption maximum (near 2600 A), confirming disk-integrated observations by other researchers. Ganymede's absorber may be ozone trapped in the ice lattice, which we propose is due to charged particle bombardment of the surface. At high Sun (small solar zenith angles), the ozone-like absorber is destroyed. The ozone in the polar regions is likely to be long-lived, on account of the large solar zenith angles.