Detection of Ozone on Ganymede

An absorption band at 260 nanometers on the trailing hemisphere of Ganymede, identified as the Hartley band of ozone (O_3), was measured with the Hubble Space Telescope. The column abundance of ozone, 4.5 x 10¹⁶ per square centimeter, can be produced by ion impacts or by photochemical equilibrium with previously detected molecular oxygen (O_2). An estimated number density ratio of [O_3]/[O_2] ≈ 10^-4 to 10^-3 requires an atmospheric density orders of magnitude higher than upper limits from spacecraft occultation experiments. Apparently, this O_2-O_3 "atmosphere" is trapped in Ganymede's surface ice, an inference consistent with the shift and broadening of the band compared with the gas-phase O_3 band.