Oxygen on Ganymede: Laboratory Simulations

R. A. Baragiola, R. A. Vidal, D. A. Bahr (University of Virginia), M. Peters (Teikyo University)

To test proposals for the origin of oxygen absorption bands in the visible reflectance spectrum of Ganymede we measured the reflectance of condensed films of pure tex2html_wrap_inline11 and tex2html_wrap_inline11 -water mixtures and the evolution of tex2html_wrap_inline11 from the films as a function of temperature. Absorption band shapes and positions for oxygen at 26 K are similar to those reported for Ganymede whereas those for the mixtures are slightly shifted. The band intensity drops by more than two orders of magnitude when the ice mixture is warmed to 100 K, although about 20% of the tex2html_wrap_inline11 remains trapped in the ice, suggesting that at these temperatures tex2html_wrap_inline11 molecules dissolve in the ice rather than aggregate in clusters or bubbles. The experiments suggest that the absorption bands in Ganymede’s spectrum are not produced in the relatively warm surface of the satellite but in a much colder source. We discuss the possibility that the solid tex2html_wrap_inline11 exists in a cold subsurface layer or in an atmospheric haze.