Detection of Ozone on Rhea and Dione

K. S. Noll (STScI), T. L. Roush (SFSU; NASA/Ames), D. P. Cruikshank, Y. J. Pendleton (NASA/Ames), R. E. Johnson (U Va)

We have detected an absorption feature with a minimum at 260 nm in spectra of Saturn's satellites Rhea and Dione obtained with the Hubble Space Telescope's Faint Object Spectrograph (FOS). We have identified this feature as the Hartley continuum of ozone. The feature in Rhea and Dione matches the one identified in ratios of Ganymede's trailing hemisphere to leading hemisphere spectra(1). On Ganymede, both ozone and molecular oxygen(2) are concentrated on the trailing hemisphere. Both molecules show evidence of high density in Ganymede and a +6nm shift of the ozone feature is consistent with laboratory observations of ozone trapped in water ice. A similar shift is observed in Rhea and Dione and we conclude that, like on Ganymede, ozone is trapped in small voids in the surface ice. Because ozone is destroyed by ultraviolet photons, the presence of detectable quantities implies that molecular oxygen must also be trapped in the surface ice of Rhea and Dione. Spectra of Iapetus show a broad shallow minimum that differs in shape and in wavelength from the features in Rhea and Dione. Rhea, Dione, and Ganymede intercept similar fluxes of ions from the magnetospheres of Saturn and Jupiter; these ions are presumed to be the source of the oxygen. Iapetus orbits outside Saturn's magnetosphere where it receives a much lower dose of ion radiation. Laboratory studies of the interaction of charged particle radiation with water ice have long predicted the formation of tenuous oxygen atmospheres around icy satellites but did not forsee the trapping of significantly larger quantities of molecular oxygen in the surface ice. The identification of ozone on Rhea and Dione removes the possibility that special circumstances unique to Ganymede might have been required for the production of oxygen and ozone; that process is apparently more general.

(1) K. S. Noll, R. E. Johnson, A. L. Lane, D. L. Domingue, H. A. Weaver, Science 273, 341 (1996)

(2) J. R. Spencer, W. M. Calvin, M. J. Person, JGR 100, 19049 (1995)