Saturn's F-Ring Has Quieted Down
Comparing measurements over 25 years, planetary scientists have noted a drop in bright clumps in Saturn’s tenuous F-ring.
Cassini scientists have now had ten years to observe Saturn’s rings in detail since the spacecraft went into orbit in 2004. They also have observations from the much briefer flybys of Voyagers 1 and 2 in 1981. This gives them an observational period of 33 years, with a large time gap of 23 years in between (1981 to 2004). Saturn’s F-ring is of particular interest, since it is thin and delicate, showing clumping and twisting under the gravitational influence of the shepherd moons Prometheus and Pandora, as well as from other bodies. Has anything changed in the interim? A new paper in Icarus says yes. Combining observations from the Voyagers and Cassini from 2004 to 2010, they noted differences in the bright extended clumps they call EC’s:
We find that the number of minor ECs has stayed roughly constant and the ECs have similar distributions of angular width, absolute brightness, and semimajor 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.
This change over time made it into 3 of the list of 5 highlights of the paper:
- Voyager saw significantly more very bright clumps than Cassini.
- We hypothesize that bright clumps are caused by repeated impacts of small moonlets.
- The number of small moonlets may have decreased from Voyager to Cassini.
We will readily admit that 33 years is far too short a time to draw firm conclusions. Even a young-earth creationist could not draw definitive conclusions from half of one percent of his timeline. There is always the possibility that the recent 33 years is an anomalous period over the lifetime of the planet. It should be noted, however, that since Saturn’s orbit is 29 years, the two spacecraft should have encountered similar orbital characteristics at Saturn. The Voyager mission provided two encounters nine months apart, so they did not observe exactly the same events. Cassini’s data set is much richer.
We will suggest a couple of thoughts about this paper’s findings, however. One is that it was a surprise to scientists who would have expected everything about Saturn to be in a steady state after 4.5 billion years. A second thought is that destructive processes appear to dominate, apparently causing rapid change. Scientists looking for evidence that ring material clumps into larger bodies that become small moons will be disappointed with this paper’s discovery. The clumps are not growing; they are breaking down under the onslaught of small moonlets battering the ring.
And so we ask once again, could this process continue for 4.5 billion years? Are we just lucky to see this tenuous ring when there are observers to catch it? Combined with all the other evidence of Saturn’s youth we have reported over the years (q.v. in the Search bar on Saturn, Titan, Enceladus, Iapetus etc.), this discovery fits better, so far, with a young Saturn than with an old one. If scientists believe in following evidence where it leads, a young Saturn would seem to be the best working hypothesis.