The collision of Prometheus and PandoraPrometheus
were characters in a Greek myth, but now they are moons of Saturn. They both orbit close to Saturn's F ring
, zipping around this planet once every 15 hours or so.
Here you can see Prometheus carving strange slots in the F ring! This ring is made of ice boulders, maybe up to 3 meters across. Sometimes these chunks of ice form temporary clumps up to 10 kilometers in size. At other times, these clumps get pulled apart. Prometheus steals boulders from the F ring with its gravitational pull. And each time it comes as close as possible to Saturn, it carves a new slot in the F ring.
Why does this happen? It's complicated, and people keep learning more about it. I'm certainly no expert!
People used to call Prometheus and Pandora shepherd moons
. The idea was that they help stabilize the F ring. It's a cool idea. The singer Enya even made an album with this title.
But more recent work casts doubt on this theory. Last month Emily Lakdawalla of the Planetary Society wrote:The most surprising thing I've learned: You know how Prometheus and Pandora are the F ring shepherds? Prometheus on the inside, and Pandora on the outside, herding the billions of tiny particles that make up the ring into place? It's not true. Pandora is not involved in controlling the F ring's tight shape.The first paper I looked at was written by Jeff Cuzzi and seven coauthors: "Saturn's F Ring core: Calm in the midst of chaos." (Let's pause for a moment to appreciate the quality of that paper title, which is both interesting and accurate, not boring or silly.) The paper seeks to explain why the central core of Saturn's F ring is so consistently shaped, even though various things are constantly acting to perturb it. In particular, Prometheus periodically plunges into the F ring, drawing out dramatic streamers and fans. In fact, Prometheus and Pandora, far from behaving as shepherds, actually act to stir up the motions of particles in most of the region near the F ring. Furthermore, there are other bodies that Cassini has spotted in the F ring region whose behavior is so chaotic that it's been hard to follow them; these things have "violent collisional interactions with the F ring core," so, all in all, it's really difficult to explain why the core of the F ring generally looks the same as it has ever since the Voyagers passed by.
According to her account of some recent papers, the key is a kind of resonance. Resonant frequencies shape Saturn's rings in many ways, but here the key is something called a 'Lindblad resonance'.
The orbit of Prometheus precesses
. In other words, its point of closest approach to Saturn keeps slowly moving around. So, the period with which this moon orbits Saturn is slightly different
than the period with which it moves in and out from Saturn. A Lindblad resonance
happens when a chunk of ice goes around Saturn exactly once each time Prometheus goes in and out! Lakdawalla writes:So: consider a moon and a ring particle orbiting Saturn. We don't care (for the moment) what the orbital periods of the moon and ring particles are; what we do care about is the "in-and-out" period of the ring particle in its orbit. You have a Lindblad resonance if, every time the moon passes by the ring particle, the ring particle happens to be on the same position in its in-and-out motion.
The full story is even more complicated than that - obviously, since it has to explain all the weird patterns in the picture here. The F ring consists of several strands, and these even braid around each other
. But I'll let you read her blog for more:http://www.planetary.org/blogs/emily-lakdawalla/2014/07010001-ringmoons-shepherds.html
What I really
want to tell you is some other news: how the F ring was formed in the first place!
It's in an interesting place. Any moon too close to Saturn would be broken up by tidal forces unless it was held together by forces stronger than gravity. The Roche limit
says how close is too close: it's 147,000 kilometers from the center of Saturn. The F ring is 140,180 kilometers from the center of Saturn. So it's just within the Roche limit.
That could be a clue. But how did the F ring actually form? A new paper says it was created by a collision between Prometheus and Pandora! The authors write:Saturn’s F ring is a narrow ring of icy particles, located 3,400 km beyond the outer edge of the main ring system. Enigmatically, the F ring is accompanied on either side by two small satellites, Prometheus and Pandora, which are called shepherd satellites. The inner regular satellites of giant planets are thought to form by the accretion of particles from an ancient massive ring and subsequent outward migration. However, the origin of a system consisting of a narrow ring and shepherd satellites remains poorly understood. Here we present N-body numerical simulations to show that a collision of two of the small satellites that are thought to accumulate near the main ring’s outer edge can produce a system similar to the F ring and its shepherd satellites. We find that if the two rubble-pile satellites have denser cores, such an impact results in only partial disruption of the satellites and the formation of a narrow ring of particles between two remnant satellites. Our simulations suggest that the seemingly unusual F ring system is a natural outcome at the final stage of the formation process of the ring–satellite system of giant planets.
If so, the F ring and these moons have been engaged in a drama for millions of years, starting with the very formation of Saturn's rings. We missed the beginning of the show.
The paper is here, but it ain't free:
• Ryuki Hyodo and Keiji Ohtsuki , Saturn’s F ring and shepherd satellites a natural outcome of satellite system formation, Nature Geoscience
The other paper I mentioned is free:
• J. N. Cuzzi, A. D. Whizin, R. C. Hogan, A. R. Dobrovolskis, L. Dones, M. R. Showalter, J. E. Colwell and J. D. Scargle, Saturn’s F Ring core: Calm in the midst of chaos, http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140011651.pdfAbstract: The long-term stability of the narrow F Ring core has been hard to understand. Instead of acting as 'shepherds', Prometheus and Pandora together stir the vast preponderance of the region into a chaotic state, consistent with the orbits of newly discovered objects like S/2004 S 6. We show how a comb of very narrow radial locations of high stability in semimajor axis is embedded within this otherwise chaotic region. The stability of these semimajor axes relies fundamentally on the unusual combination of rapid apse precession and long synodic period which characterizes the region. This situation allows stable 'antiresonances' to fall on or very close to traditional Lindblad resonances which, under more common circumstances, are destabilizing. We present numerical integrations of tens of thousands of test particles over tens of thousands of Prometheus orbits that map out the effect. The stable antiresonance zones are most stable in a subset of the region where Prometheus first-order resonances are least cluttered by Pandora resonances. This region of optimum stability is paradoxically closer to Prometheus than a location more representative of 'torque balance', helping explain a longstanding paradox. One stable zone corresponds closely to the currently observed semimajor axis of the F Ring core. While the model helps explain the stability of the narrow F Ring core, it does not explain why the F Ring material all shares a common apse longitude; we speculate that collisional damping at the preferred semimajor axis (not included in the current simulations) may provide that final step. Essentially, we find that the F Ring core is not confined by a combination of Prometheus and Pandora, but a combination of Prometheus and precession.
Whew - complicated! S/2004 S 6
is a weird little thing they've discovered in the F ring. Nobody even knows if it's solid or just a clump of dust. You can see it here:https://en.wikipedia.org/wiki/S/2004_S_6 #spnetwork #saturn #prometheus #pandora #rings