Production, ionization and redistribution of O2 in Saturnʹs ring atmosphere

Molecular oxygen produced by the decomposition of icy surfaces is ubiquitous in Saturnʹs magnetosphere. A model is described for the toroidal O2 atmosphere indicated by the detection of O 2 + and O+ over the main rings. The O2 ring atmosphere is produced primarily by UV photon-induced decomposition of ice on the sunlit side of the ring. Because O2 has a long lifetime and interacts frequently with the ring particles, equivalent columns of O2 exist above and below the ring plane with the scale height determined by the local ring temperature. Energetic particles also decompose ice, but estimates of their contribution over the main rings appear to be very low. In steady state, the O2 column density over the rings also depends on the relative efficiency of hydrogen to oxygen loss from the ring/atmosphere system with oxygen being recycled on the grain surfaces. Unlike the neutral density, the ion densities can differ on the sunlit and shaded sides due to differences in the ionization rate, the quenching of ions by the interaction with the ring particles, and the northward shift of the magnetic equator relative to the ring plane. Although O+ is produced with a significant excess energy, O 2 + is not. Therefore, O 2 + should mirror well below those altitudes at which ions were detected. However, scattering by ion–molecule collisions results in much larger mirror altitudes, in ion temperatures that go through a minimum over the B-ring, and in the redistribution of both molecular hydrogen and oxygen throughout the magnetosphere. The proposed model is used to describe the measured oxygen ion densities in Saturnʹs toroidal ring atmosphere and its hydrogen content. The oxygen ion densities over the B-ring appear to require either significant levels of UV light scattering or ion transmission through the ring plane.