Oxygen on Ganymede: laboratory studies

To test proposals for the origin of oxygen absorption bands in the visible reflectance spectrum of Ganymede, the reflectance of condensed films of pure oxygen (O2) and O2-water mixtures and the evolution of O2 from the films as a function of temperature were determined. Absorption band shapes and positions for oxygen at 26 kelvin were similar to those reported for Ganymede, whereas those for the mixtures were slightly shifted. The band intensity dropped by more than two orders of magnitude when the ice mixture was warmed to 100 kelvin, although about 20 percent of the O2 remained trapped in the ice, which suggested that at these temperatures O2 molecules dissolve in the ice rather than aggregate in clusters or bubbles. The experiments suggest that the absorption bands in Ganymede's spectrum were not produced in the relatively warm surface of the satellite but in a much colder source. Solid O2 may exist in a cold subsurface layer or in an atmospheric haze.

Detection of ozone on Ganymede

An absorption band at 260 nanometers on the trailing hemisphere of Ganymede, identified as the Hartley band of Ozone (O3), was measured with the Hubble Space Telescope. The column abundance of ozone, 4.5 x 10(16) per square centimeter, can be produced by ion impacts or by photochemical equilibrium with previously detected molecular oxygen (O2). An estimated number density ratio of [O3]/[O2] = 10(-4) to 10(-3) requires an atmospheric density orders of magnitude higher than upper limits from spacecraft occultation experiments. Apparently, this O2-O3 "atmosphere" is trapped in Ganymede's surface ice, an inference consistent with the shift and broadening of the band compared with the gas-phase O3 band.