Charge-coupled device spectra of the Galilean satellites: Molecular oxygen on Ganymede

We have obtained 3200–7800 Å CCD spectra of the Galilean satellites at a variety of orbital longitudes, with spectral resolution between 3 and 18 Å and signal‐to‐noise ratios of up to 2000 at 6000 Å. Despite the higher resolution and signal‐to‐noise ratio than previous published spectra, no new features are seen on Io, Europa, and Callisto. However, Ganymede shows an unusual and previously unreported shallow absorption feature at 5773 Å, which is much stronger on the trailing side, and a weaker band at 6275 Å. The features are apparently due to diatomic oxygen, and require simultaneous electronic transitions in two adjacent molecules: they are the strongest visible‐wavelengt h absorption bands in solid or liquid oxygen. Because condensed pure oxygen is not stable at Ganymede surface temperatures and pressures, the oxygen must be trapped in other surface materials, perhaps in water ice, with the constraint that O2 molecules must be close enough together for simultaneous electronic transitions. Magnetospheric bombardment of water ice is a plausible production mechanism for the O2. Continuum shapes on all four satellites are more clearly seen than in previous spectra. The shape of the continuum is very similar on Ganymede and Callisto but is strikingly different on Europa, indicating a different origin for the nonice component on Europa. The shape of the Europa continuum suggests that allotropes or compounds of sulfur may be the dominant spectrally active materials in the visible spectrum. No convincing changes since 1978 are visible in Io's spectrum, despite the high resurfacing rates, suggesting that volcanic resurfacing tends to overpaint areas with more material of the same composition.