Detection of Ozone on Ganymede

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Science  19 Jul 1996:
Vol. 273, Issue 5273, pp. 341-343
DOI: 10.1126/science.273.5273.341


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 × 1016 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.