Why Saturn’s rings are so sharp ?!

[MavericK]

Why Saturn’s rings are so sharp


Here’s a conundrum for you: why do Saturn’s rings have such sharp edges?
It’s question that has puzzled planetary scientists for many years. Various ideas have been put forward but none adequately explain the structure we see today.
To understand just how sharp the edges are consider this: pictures from Cassini show that the density of particles at the edge of the outer B ring, for example, drops by an order of magnitude over a distance of only 10 metres or so.
That’s extraordinary given that the ring is 25580 kilometres wide.

A few back of the envelope calculations show how strange this state of affairs is. The ring contains particles ranging in size from 10 metres to less than 1 cm, with distances between them of only a few metres.
You’d expect these particles to be colliding at a fair old rate and just a hundred collisions per orbit should create enough debris to blur any sharp edges between rings within a few orbits.
And yet Saturn’s rings are some 10^12 orbits old.
Now Dima Shepelyansky at the Université de Toulouse in France et amis think they have worked out what’s going on. They say that the moon Mimas exerts a gravitational force that “kicks” the rings every time it orbits causing the motions of the particles to become synchronised with Mimas and each other.
The result is a dramatic reduction in the number of collisions between particles, far less debris than expected and hence a much lower tendency for the rings to become blurred.
They’ve even simulated the sharp edge effect with a computer model of 1000 particles.
As for physical evidence in favour of this idea, the team points out that the outer B ring has an epicyclic frequency that appears to be in resonance with the frequency of Mimas’ orbit.
That’s exactly what you’d expect but they’ll need more data from other rings to make certain of their claim.
That should be forthcoming from Cassini data. So we should know one way or another soon whether Shepelyansky’s ideas hold water.
Ref: arxiv.org/abs/0812.4372: Synchronization Mechanism of Sharp Edges in Rings of Saturn


EDIT: nice video !

[MavericK]

J'ai quelques questions.

You’d expect these particles to be colliding at a fair old rate and just a hundred collisions per orbit should create enough debris to blur any sharp edges between rings within a few orbits.

Est-ce que c'est un fait etabli ca ? que disent les simulations ?

They say that the moon Mimas exerts a gravitational force that “kicks” the rings every time it orbits causing the motions of the particles to become synchronised with Mimas and each other.
The result is a dramatic reduction in the number of collisions between particles, far less debris than expected and hence a much lower tendency for the rings to become blurred.

Est-ce qu'il serait impossible d'expliquer le phenomene par la dynamique meme des anneaux (anneaux+saturne, sans lunes) ?

[darwish]

Salaam Maverick,

Est-ce qu'il serait impossible d'expliquer le phenomene par la dynamique meme des anneaux (anneaux+saturne, sans lunes) ?

So far, neither the detailed mechanisms of formation of planetary rings nor their complex spacial and dynamic behavior, originating from inelastic particle collisions resonances, gravitational perturbations and others, are completely understood.

It seems that interactions between rings of colliding particles and external or embedded satellites play a major role in the formation of heterogeneous discs with sharp edges, there is an exchange of angular momentum between inner or outer satellites, relatively to the discs, thus leading to many (inelastic) collisions inside the rings. The physics involving many colliding particles sets the study of rings dynamics apart from classic celestial mechanics, in which few or no collisions are assumed. We are quite sure about one fact, narrow rings and sharp edges require the presence of a confinement or a repulsion mechanism in order to halt the radial spreading.

Another important ingredient in planetary rings dynamics, as compared to many other exapmles of granular matter, is the presence of a volume force: mean orbital speed depends on the planetocentric distance. This systematic velocity shear provides an energy source which balances the collisional dissipation, leading to a local steady-state in short time scales. The steady-state properties, e.g. ring thikness, depend on the amount of dissipation.

Est-ce que c'est un fait etabli ca ? que disent les simulations ?

Personnaly, I don't know if this is a well established fact. You are aware that numerical simulations are very useful in this very case since we cannot use analytical methods, however, some difficulties still remain. We can emphasize the fact that collisions between celestial bodies can be catastrophic leading to the destruction (fragmentation) of the bodies involved, thus leading to an additional complexity (Molecules are not destroyed in a molecular gas). Different simulations can only give different or complementary informations, there is absolutely no simulation that explains many facts at the same time (Many parameters to tweak). I'm sure about one thing, numerical simulations are like short skirts, they show a lot but hide the essential.

I hope this help.

[MavericK]

Salaam Darwish, and thanks for the answer.

It seems that interactions between rings of colliding particles and external or embedded satellites play a major role in the formation of heterogeneous discs with sharp edges, there is an exchange of angular momentum between inner or outer satellites, relatively to the discs, thus leading to many (inelastic) collisions inside the rings

Do you really mean leading to and not suppressing ?
If yes, why would not such collisions blur the edges ?


Darwish, do you think we need another space mission (after CASSINI) or a deeper understanding of granular matter dynamics ?

[darwish]

Let's do it qualitatively

If yes, why would not such collisions blur the edges ?

In an isolated disc made of a gravitating system of colliding particles, the total angular momentum is conserved but the total energy decreases. Inelastic collisions spread out the particles and extend the disc inwards and outwards. The system broadens under the combined effects of differential rotation and of collisions (equivalent to "friction" for a gas --> viscosity). The energy which is continuously lost as a consequence of inelastic collisions is obtained at the expense of bodies moving inwards and outwards. Taking into account the conservation of angular momentum, the energy lost by the particles moving inwards is larger than the energy gained by the particles moving outwards: in spreading, the disc loses a small amount of its total energy. The time scale for this process is very long: it can be longer than the age of the Universe for rings around Saturn without nearby satellites. This image of a slowly spreading disc is in good agreement with a homogeneous ring system with smooth edges, but observed rings present considerable radial structure. The discovery of narrow rings and sharp edges indicate either that rings are young or that confinement mechanisms are at work. During a collision, part of the relative velocity is transformed into vertical and radial motion. After some time, equilibrium is established between the radial and the vertical velocities. The thickness of the disc depends on the coefficient of restitution. For very inelastic collisions, the system is completely flattened. For intermediate collisions, the thickness of the disc is of the order of a few times the size of the particles for a system of equal sized particles. For almost elastic collisions, the thickness increases and it becomes three-dimensional.

Darwish, do you think we need another space mission (after CASSINI) or a deeper understanding of granular matter dynamics ?

Actually, my concerns are exclusively relative to granular flows, which occur in laboratory experiments (powders, sand, friction, micro-gravity flight experiments) or in the geophysical context such as rock avalanches or sand dunes (not those of Arrakis). In these systems, the mean density is much larger than the values found in astrophysical context, so we cannot approach different dynamics on an equal footing. But from a fundamental point of view, we have a lot to do in the comprehension of the forces governing the collision of particles. For e.g., we don't know how the restitution coefficient is correlated to the relative impact velocity, how plasticity and deformation at the microscopic level affect the collision, and so on. During the last decade, some leading physicists in the field of granular matter have explained by the mean of state of the art simulations the formation, the growth and the migration of giant sand dunes in the planet Mars. From their studies, it is for now obvious that universal behaviour and scaling laws are very common in granular physics. They have even shown how sand spreads poly disperse rocks over the surface of Mars in a special and curious fashion. So space missions can helps us understand the dynamics of granular matter only to the extent that it could bring us precious data of extraterrestrial systems, i.e., systems under different atmospheric conditions (Mars dunes), huge granular collisions (Saturn rings and arcs), ... etc.

[MavericK]

Salaam Darwish,

I've got the point. Thanks a lot for the answers .
It's a very interesting field !

[darwish]

You are welcome.

N.B. 1000 messages. Should I stay or should I go?

[MavericK]

N.B. 1000 messages. Should I stay or should I go?

Hahaha.. congratz my dear !!
You should STAY of course ! for several reasons, for ex :
- To stay in touch with your nervous systems.
- To keep a group (at least 3 persons) of physicists in this big forum and contribute to this section
- To have fun reading some freaky analysis
- To use unknowns regions of your brain trying to decipher some comments
- To wonder how observer-independence is a principle in physics
- To keep contact with the Algerian-attitude !
- To witness violation of causality at the macroscopic level
- ...... etc

[Sidi Noun]

In an isolated disc made of a gravitating system of colliding particles, the total angular momentum is conserved but the total energy decreases. Inelastic collisions spread out the particles and extend the disc inwards and outwards. The system broadens under the combined effects of differential rotation and of collisions (equivalent to "friction" for a gas --> viscosity). The energy which is continuously lost as a consequence of inelastic collisions is obtained at the expense of bodies moving inwards and outwards. Taking into account the conservation of angular momentum, the energy lost by the particles moving inwards is larger than the energy gained by the particles moving outwards: in spreading, the disc loses a small amount of its total energy. The time scale for this process is very long: it can be longer than the age of the Universe for rings around Saturn without nearby satellites. This image of a slowly spreading disc is in good agreement with a homogeneous ring system with smooth edges, but observed rings present considerable radial structure. The discovery of narrow rings and sharp edges indicate either that rings are young or that confinement mechanisms are at work. During a collision, part of the relative velocity is transformed into vertical and radial motion. After some time, equilibrium is established between the radial and the vertical velocities. The thickness of the disc depends on the coefficient of restitution. For very inelastic collisions, the system is completely flattened. For intermediate collisions, the thickness of the disc is of the order of a few times the size of the particles for a system of equal sized particles. For almost elastic collisions, the thickness increases and it becomes three-dimensional.

C'est malhonnête d'utiliser le travail des autres sans les nommer...

In an isolated disc made of a gravitating system of colliding particles, the
total angular momentum is conserved but the total energy decreases. Inelastic
collisions spread out the particles and extend the disc inwards and outwards.
The system broadens under the combined e.ect of di.erential rotation and
of collisions (equivalent to “friction” for a gas). This phenomenon (Brahic,
1975, 1977) is analogous to that described by various authors for an accretion
disc around a compact object (Prendergast and Burbidge, 1968; Lynden-Bell
and Pringle, 1974). The energy which is continually lost as a consequence
of inelastic collisions is obtained at the expense of bodies moving inwards
and outwards. Taking into account the conservation of angular momentum,
the energy lost by the particles moving inwards is larger than the energy
gained by the particles moving outwards: in spreading, the disc loses a small
amount of its total energy. The time scale for this process is very long: it
can be longer than the age of the Universe for rings around Saturn without
nearby satellites (Brahic, 1977). This image of a slowly spreading disc is in
good agreement with a homogeneous ring system with smooth edges, but
observed rings present considerable radial structure. The discovery of narrow
rings and sharp edges indicate either that rings are young or that con.nement
mechanisms are at work.

- André Brahic, Dynamical Evolution of Viscous Discs.

[absent]

Noun

Qui te dit que Darwich n'est pas André Brahic Hein ?

En tout cas il a l'aire de s'y connaitre...
Tu bosse sur la matière granulaire Darwich ?

[Zakia]

Ce que j'ai compris :

Que les anneaux de Saturne n'ont pas une épaisseur uniforme.

Que les mini jupes montrent beaucoup mais cachent l'essentiel .

Le reste, je suppose qu'il faut tout de même un petit minimum que je suis loin d'avoir

J'ai quand même quelques questions :

pourquoi Saturne a des anneaux et comment se sont-ils formés? De quelles particules (ou de quoi, si je pose mal la question) sont composés ces anneaux? Pourquoi Saturne a autant de lunes ? Et puis tant j'y suis : de quoi est composée Saturne? C'est une planète solide ou gazeuse (ça j'ai su mais j'ai oublié )?

Mille excuses si les questions vous semblent simplistes.

[Sidi Noun]

Far_solitaire:

Noun

Qui te dit que Darwich n'est pas André Brahic Hein ?

En tout cas il a l'aire de s'y connaitre...

vous avez l'air d'approuver le plagiat... en tout les cas, Brahic a le sens de l'humour... enjoy!:

[Zakia]

Je doute que Darwish ait quelque leçon que ce soit à prendre en matière d'honnêteté intellectuelle...
La discussion était nettement entre 2 physiciens et certainement pas à la portée du premier venu. Je suppose donc que citer la source dans ce cas était superflu. Autrement dit, je vois mal un scientifique bourrer le mou à un autre ayant la même formation...

La courtoisie élémentaire aurait été de poser la question plutôt que d'intervenir de manière aussi abrupte et faire un procès d'intention...

Mais il est vrai qu'il est tellement plus facile de faire la leçon aux autres n'est-ce pas...aussi facile de zapper la poutre qu'on a dans l'oeil pour tilter sur la brindille dans l'oeil du voisin...

[Sidi Noun]

Zakia6060,

Je doute que Darwish ait quelque leçon que ce soit à prendre en matière d'honnêteté intellectuelle...
La discussion était nettement entre 2 physiciens et certainement pas à la portée du premier venu. Je suppose donc que citer la source dans ce cas était superflu. Autrement dit, je vois mal un scientifique bourrer le mou à un autre ayant la même formation...

Alors pourquoi ce même André Brahic, ne s'adressant pas non plus au premier venu, mais à d'autres physiciens, cite-t-il ses sources?

La courtoisie élémentaire aurait été de poser la question plutôt que d'intervenir de manière aussi abrupte et faire un procès d'intention...

je vous dirais aussi que la bienséance élémentaire aurait été de laisser le concerné s’expliquer lui-même. Vous m'avez tout l'air d'une groupie amourachée, et ça pourrait le gêner...
Pour le procès d'intention, je vous demanderais de considérer les faits:
copie textuelle, partielle ou intégrale, sans donner crédit à l'auteur original, constitue plagiat... suffit de voir les écrits plus haut, pas besoin de deviner l'intention... si vous avez une autre définition du plagiat, allez-y.

Mais il est vrai qu'il est tellement plus facile de faire la leçon aux autres n'est-ce pas...aussi facile de zapper la poutre qu'on a dans l'oeil pour tilter sur la brindille dans l'oeil du voisin...

montrez-moi la poutre dans mes yeux et je corrigerai...
quant à donner des leçons, vous n'avez rien à m'envier.

[Zakia]

bof...et rebof...

Avant toute chose, je te présente mes excuses MavericK, ce n'est plus de la pollution sur ton topic, c'est carrément de l'exécution:22:

Je vais faire une réponse globale. Je ne vais pas m'amuser à démonter point par point vos commentaires Sidi Noun. L'homme a mis des millions d'années à acquérir la station verticale, je me sens la flemme de faire le chemin inverse en ce dimanche midi. Surtout que ça pourrait en plus causer une hernie, vu qu'il faut descendre très bas...

Il se trouve ici, que même si c'est sur un forum, la discussion s'apparentait plutôt à un échange privé entre 2 personnes connaissant le sujet. L'absence de citation de source n'est pas relevante. Rien ne vous dit non plus que la discussion n'a pas été continuée en dehors du forum de manière plus pointue, le domaine étant plutôt spécialisé...
Si l'intéressé avait été là, il ne se serait même pas donné la peine de relever je crois. Il est plus malin que moi. Il se trouve que, par principe, je n'aime pas voir attaquées des personnes que j'apprécie. Et si celles-ci sont absentes,l'adage leur donnant tort de ce simple fait, j'interviens. Et si ça fait de moi une groupie amourachée, eh bien j'assume Pour le procès d'intention, je maintiens. Il suffisait de consulter le profil de l'intéressé pour se rendre compte de son honnêteté intellectuelle...ce qui vous aurait par la même occasion, donné un aperçu de son humour...

Quant aux règles de bienséance, poutre et compagnie, je vous retourne la réflexion. Vous auriez pu vous souvenir de tout cela sur un autre post, il me semble. Mes réponses sur le dit post me valant probablement la diatribe ci-dessus...
Mais je suppose que ce qui est bon pour les autres ne l'est pas pour vous? Faites ce que je dis, pas ce que je fais...