It has long been known that water (H2O) ice is the primary constituent of the rings of Saturn. Various mechanisms should produce a tenuous atmosphere near the rings that reflects this composition. The primary constituents of such an atmosphere were expected to be hydrogen (H or H2) and hydroxyl (OH). The most direct way to measure such an atmosphere would be with instruments like those on the Cassini Orbiter designed to sense this very thin atmosphere as the spacecraft moved through it. Indeed, that is exactly what happened during the Saturn Orbit Insertion time period on July 1, 2004 (see Section 10.3)
The most extensive pre-Cassini observations of a ring atmosphere were by the ultraviolet spectrometers aboard Voyagers 1 and 2 . The only component measured was Lyman-alpha radiation from atomic hydrogen (H). Because the solar system is full of hydrogen, the radiation from Saturn's rings was a small fraction of the total hydrogen observed, and the estimate of about 360 Rayleighs from Saturn's rings was more or less independent of whether the observation was of the illuminated rings or the unlit face of the rings. It was also independent of the tilt of the rings to the Sun, which was about 3.6° for Voyager 1 and 8° for Voyager 2. The hydrogen emission intensity dropped off very slowly with altitude above the rings, decreasing by a factor of 2.72 over about 6,000 km and was not observed at distances from Saturn greater than the outer edge of the A ring. The source of the hydrogen may be high-velocity meteoroid impacts on the ring particles , the only as-yet-proposed mechanism with high enough temperatures to produce—from the rings themselves—the amounts of hydrogen observed.
A source external to the rings may also be a possible source for the atomic hydrogen. It has been suggested that molecular hydrogen (H2) from the outer portions of Saturn's atmosphere might be dissociated by electrons in Saturn's radiation belts into hydrogen atoms of sufficiently high energy to escape the planet's gravity . The escaping hydrogen atoms would then lose energy as they encountered the cold ring particles and remain trapped near the rings. However, it is certainly conceivable that the observed hydrogen is simply part of the extended hydrogen atmosphere of Saturn that surrounds the planet and its rings, and that these early measurements mistakenly identified it as being a part of a ring atmosphere or affected by the rings.
A more complete understanding of the ring atmosphere will require Cassini data, both the in-situ data mentioned above and additional remote sensing, and supporting theoretical analyses, most of which are still unavailable as this book goes to press.
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