Absorption Coefficients of Condensed O₂: Application to Ganymede

Two absorption features in the visible reflectance spectra of the icy Galilean satellites are associated with dense phase molecular oxygen (O 2 ). These were identified from ground-based and Hubble Space Telescope data on Ganymede, Europa, and Callisto. Molecular oxygen is the radiolytic product of energetic particle bombardment and breakdown of water ice. The oxygen features are strongest near the equator on the trailing hemisphere of Ganymede. We review previous laboratory measurements of high-pressure gas, liquid, and solid phases of O 2 , noting differences that arise with increased density and phase changes. We measure transmission spectra of condensed liquid and solid- γ phase O 2 from 455 to 5000 nm (22,000–2000 cm −1 ) using a Fourier transform infrared spectrometer at 1 cm −1 spectral resolution. This spectral range includes the strongest visible and near-infrared absorption features. The measured absorption is dependent on both phase and temperature increasing with colder temperatures and in the solid form. We use these data to determine the absorption coefficients of the liquid and solid phases. These are then compared with absorption by water ice, the dominant constituent of Ganymede’s surface. O 2 has an absorption coefficient that is ∼2 orders of magnitude stronger than water ice in the visible. Using simple reflectance models we estimate that a fractional abundance of 4%–5% over the path length in the surface is needed to create the observed visible absorptions, and this fraction does not contribute noticeable features in the infrared.