科研之友
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摘要
Saturn's F ring is subject to dynamic structural changes over short periods. Among the observed phenomena are diffuse extended bright clumps (ECs) similar to 3-40 degrees in longitudinal extent. These ECs appear, evolve, and disappear over a span of days to months. ECs have been seen by the two Voyager spacecraft, the Cassini Orbiter, and various ground- and space-based telescopes. Showalter (Showalter, M.R. [2004]. Icarus 171, 356-371) analyzed all Voyager images of the F ring and found that there were 2-3 major and 20-40 minor ECs present in the ring at any given time. We expand upon these results by comparing the ECs seen by Voyager to those seen by Cassini in 2004-2010. We find that the number of minor ECs has stayed roughly constant and the ECs have similar distributions of angular width, absolute brightness, and semi-major axis. However, the common exceptionally bright ECs seen by Voyager are now exceedingly rare, with only two instances seen by Cassini in the 6 years, and they are now also much dimmer relative to the mean ring background. We hypothesize that these bright ECs are caused by the repeated impacts of small moonlets with the F ring core, and that these moonlets have decreased in number in the 25 years between missions.
- 出版日期2014-10
