Upper Limits for Condensed O2on Saturn's Icy Satellites and Rings
✍ Scribed by John Spencer
- Publisher
- Elsevier Science
- Year
- 1998
- Tongue
- English
- Weight
- 264 KB
- Volume
- 136
- Category
- Article
- ISSN
- 0019-1035
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✦ Synopsis
New 4800-7200 A ˚spectra of Saturn's rings, and the icy This is below likely daytime temperatures on Ganymede's surface at low saturnian satellites Enceladus, Tethys, Dione, Rhea, and Ialatitudes: water ice with an implausibly high bolometric albedo of 0.9, and petus, do not show the weak 5773 and 6275 A ˚absorption bands infinite thermal inertia, will have a temperature of 73 K on Ganymede's due to high-density condensed O 2 that are seen, with a maxiequator, and lower albedos and finite thermal inertias will yield much mum depth of 1.8%, on Ganymede's trailing side. The 5773 A ˚higher peak temperatures.
Another puzzle is the scarcity of dense O 2 on Ganymede's leading band depth must be Յ0.6% (and very probably Յ0.3%) on the hemisphere, where the absorption bands are weak or absent (Spencer et rings, Tethys, Dione, Rhea, and the trailing side of Iapetus, and al. 1995). Origin of the O 2 by radiolytic processing of surface ice by low-Յ1.2% on the trailing side of Enceladus. The lack of detectable energy jovian magnetosopheric ions, which were expected to preferenabsorption does not rule out abundant O 2 molecules that are tially impact the trailing side, was proposed by Spencer et al. (1995) and separated too well to produce these absorption bands. The Calvin et al. (1996) as an explanation for this asymmetry. However, observations show that the presence of O 3 , recently detected Ganymede's newly discovered intrinsic magnetic field (Kivelson et al. on Rhea and Dione (Noll et al. 1997b, Nature 388, 45-47), does 1996) is likely to protect the surface from low-energy ions, and higher- not require the presence of detectable quantities of high-density energy ions will impact both leading and trailing hemispheres (see Discus- O 2 . I discuss the implications of the new observations for models sion below). An alternative explanation for the leading/trailing asymme- of O 2 formation on Ganymede.