Saturn Rings: F is for Flamboyant
The Cassini spacecraft just started its extended mission on July 1. Among its many achievements during the four-year prime mission (2004-2008) was the elucidation of complex processes occurring in Saturn’s rings. One ring in particular, the thin outlying F-ring, attracted particular interest.
Voyager scientists from the early 1980s could hardly believe their eyes when separate strands of this narrow ring appeared to braid around each other. Thanks to Cassini, a little more is known now, but the discoveries are no less surprising.
A press release from JPL last month announced the discovery of collisions within the F-ring. These collisions, on the order of 30 meters per second, create some of the flamboyant features seen by the cameras: rapid perturbations, spurs, grooves, gouges and fan-like structures that vary rapidly. “Large scale collisions happen in Saturn’s F ring almost daily,” said one of the authors of the paper in Nature,1 “making it a unique place to study. We can now say that these collisions are responsible for the changing features we observe there.”
Interactions with the two shepherd moons, particularly Prometheus (the larger and nearer one) also perturb the ringlets. Prometheus both incites violence in the embedded moonlets, causing them to collide more frequently, and it gets struck itself during its periodic close passages of the ring.
One detail strangely omitted from the reports is an estimation of how long these processes could continue. Carl Murray said that “Saturn’s F ring is perhaps the most unusual and dynamic ring in the solar system; it has multiple structures with features changing on a variety of timescales from hours to years.” But for a ring undergoing almost daily collisions of its constituent moonlets, could this ephemeral ring last for billions of years? No one in the popular reports was asking the question.
The original paper, however, noted that these processes could not last for long. “It is difficult to understand how the observed ~1 km-wide ring component seen in some of the highest resolution images can survive in such a chaotic environment,” they said. “Nonetheless, the evidence suggests not only that it does, but also that it even maintains enough integrity to precess uniformly; the only obvious mechanisms to prevent its destruction are self-gravity and collisions.” They left to future researchers the need to make progress in understanding how self-gravity might work.
The authors placed an upper limit for the origin of the ring at a million years – a tiny fraction of the assumed 4.5-billion-year age of the solar system (and of Saturn itself). To save the age of the system, they either had to presume a moon broke up in the vicinity of the F-ring a million years ago, or that the F-ring is balanced between forces of accretion and disruption. The latter suggestion, however, does not take into account the brightness of the ephemeral ring (which would have been darkened by dust pollution), nor other disruptive processes, such as external bombardment and sunlight pressure, that would also erode the ring over time.
Space.com printed a review of the surprises Cassini found at Saturn over the last four years.
1. Murray et al, “The determination of the structure of Saturn’s F ring by nearby moonlets,” Nature 453, 739-744 (5 June 2008) | doi:10.1038/nature06999.
Great. Now we can add another evidence that the consensus age of the solar system is vastly overdrawn (see also last month’s admission about Enceladus, 06/19/2008). You will notice that the scientists have to keep adding ad-hoc “rescuing devices” to their paradigm in order to maintain the 4.5-billion-year figure. Multiplying rescuing devices makes a theory look bad. The straightforward interpretation is that the rings are not so old. Thank you, Cassini, for adding impetus toward another scientific revolution. Keeps gurus honest.