The Cassini spacecraft flew by Dione, one of Saturn's icy moons, on 7 April 2010. During that flyby, instruments detected molecular oxygen ions around the moon. Tokar et al. used those measurements to estimate the density of the molecular oxygen ions to be in the range of 0.01 to 0.09 ions per cubic centimeter (or ions per 0.06 cubic inch). These molecular oxygen ions are produced when neutral molecules are ionized; the measurements confirm that a neutral exosphere surrounds Dione.
Source: Geophysical Research Letters, doi:10.1029/2011GL050452, 2012 http://dx.doi.org/10.1029/2011GL050452
Title: Detection of Exospheric O2+ at Saturn's Moon Dione
Authors: R. L. Tokar: Space Science and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico, USA; R. E. Johnson: Engineering Physics, University of Virginia, Charlottesville, Virginia, USA; M. F. Thomsen: Space Science and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico, USA; E. C. Sittler: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA; A. J. Coates: Mullard Space Science Laboratory, University College London, Dorking, UK; R. J. Wilson: Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, Boulder, Colorado, USA; F. J. Crary and D. T. Young: Southwest Research Institute, San Antonio, Texas, USA; G. H. Jones: Mullard Space Science Laboratory, University College London, Dorking, UK, and Centre for Planetary Sciences at UCL/Birkbeck, London, UK.