Upper Limits for Condensed O2on Saturn's Icy Satellites and Rings

Abstract

New 4800–7200 Å spectra of Saturn's rings, and the icy saturnian satellites Enceladus, Tethys, Dione, Rhea, and Iapetus, do not show the weak 5773 and 6275Å absorption bands due to high-density condensed O2that are seen, with a maximum depth of 1.8%, on Ganymede's trailing side. The 5773Å band depth must be ≤0.6% (and very probably ≤0.3%) on the rings, Tethys, Dione, Rhea, and the trailing side of Iapetus, and ≤1.2% on the trailing side of Enceladus. The lack of detectable absorption does not rule out abundant O2molecules that are separated too well to produce these absorption bands. The observations show that the presence of O3, recently detected on Rhea and Dione (Nollet al.1997b,Nature388, 45–47), does not require the presence of detectable quantities of high-density O2. I discuss the implications of the new observations for models of O2formation on Ganymede.