# Saturn's rings - why do they exist?

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##### Guest
What force(s) allows the matter in the rings to spread out like that and stay evenly spread out over millions of years.<br /><br />I would think that gravity would pull 2 rocks together, then the gravity of that clump would draw other rocks - until you had one big clump circling the planet. Why is this not the case.<br /><br />Intuitively, it seems that an even band of matter is the least likely outcome. <br /><br />Pretty, though.

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#### vogon13

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Check out the Planetary Rings chapter in the excellent book The New Solar System.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

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#### Saiph

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It's got to do with what's called the "Roche Limit." <br /><br />The basics: A large object can only get so close to a planet before the planets tidal forces overcome the objects structural integrity and rip it apart. <br /><br />The tidal forces are due to the gravitational <i>gradient</i> of an object, i.e. how fast the gravity gets stronger as you approach.<br /><br />What this means is that the closer to an object you get, the faster gravity strengthens, and the stronger the tidal forces get.<br /><br />The effect of the tidal force is that gravity of the planet pulls on the far side of an object less than the near side. Get a big enough difference and it rips the object into pieces.<br /><br />So, if an asteroid orbits within a planets Roche Limit for any significant period of time, it will be torn to pieces. These same tidal forces also work to prevent the debris from reforming into anything coherent, and their own random motions and interactions cause them to spread out in the orbit, forming a ring. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### vandivx

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"What force(s) allows the matter in the rings to spread out like that and stay evenly spread out over millions of years."<br />-----<br /><br />do we actually have some data on the age of those rings?<br /><br />of course the force at work must be gravitation, that is given, question is only how does it do it (gravitational tidal forces as explained above by Saiph)<br /><br />I would suppose the rings spread around the planet due to differing orbital speeds - objects farther out orbit slower than those closer in - but not sure if that would make such even distribution, I would very much suspect other effects are at work here<br /><br />vanDivX <div class="Discussion_UserSignature"> </div>

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##### Guest
An episode of Universe said the rings are 100's million years old.<br /><br />The show also said there is no agreement on why the rings are evenly distributed.<br /><br />It's a mystery.

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#### vandivx

##### Guest
interesting<br /><br />intuitively one would expect such distribution after a long time of settling because it minimizes the energy of the system - when you have a moon in orbit it wobbles the planet and if it would fall appart and distribute evenly on its orbital line you would get steady system without that wobble and that would be the systems minimal interaction energy I would think<br /><br />eliptical orbits are settling into circular ones by the same mechanism I think and distribution (where possible) evenly along the orbit would seem just a continuation of that same process<br /><br />that's just off the top of my head<br /><br />vanDivX <div class="Discussion_UserSignature"> </div>

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#### kyle_baron

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<font color="yellow"><br />An episode of Universe said the rings are 100's million years old. </font>img src="/images/icons/cool.gif" /><br /><br />I saw that same episode also. They said that the rings have two small moons embedded with in them. One on each opposite side of the rings. I guess those moons keep the rings some what stable.<br /> <div class="Discussion_UserSignature"> <p><font size="4"><strong></strong></font></p> </div>

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#### MeteorWayne

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Actually, there are many moons and many rings. It's not a ring, it's a ring system! <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

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#### brellis

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<font color="yellow">it's a ring system!</font><br /><br />It seems that on every orbit of Cassini we're learning more about how the rings are fed and consumed by various moons in varying cycles involving tides, cryovolcanoes and geysers. <img src="/images/icons/cool.gif" /> <div class="Discussion_UserSignature"> <p><font size="2" color="#ff0000"><em><strong>I'm a recovering optimist - things could be better.</strong></em></font> </p> </div>

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#### CalliArcale

##### Guest
No one really knows how old the rings are, but 100 million years is probably a decent guess. That's actually fairly young, compared to things like moons, which are billions of years old.<br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>I saw that same episode also. They said that the rings have two small moons embedded with in them. One on each opposite side of the rings. I guess those moons keep the rings some what stable. <p><hr /></p></p></blockquote><br /><br />These are called shepherd moons, and it turns out that there are more than just the first two ever discovered. The first two discovered (and the ones to which the show was probably referring) are Pandora and Prometheus, two moons which orbit on either side of Saturn's thin F ring. Atlas is another, shepherding the A ring.<br /><br />Many moons orbit within the rings, probably more than we know about, although at some point it does get to a semantic question of what's a tiny moon and what's a big ring particle. The structure of the ring system seems to be profoundly influenced by gravitational interactions with various moons -- in a very real sense, Saturn is a real-world laboratory in celestial mechanics, and observing the ring system can yield hints as to how the solar system formed.<br /><br />Pan, orbiting within the Encke gap, may maintain the gap. It also causes strange compression waves to pass through the rings. Mimas is believed to be responsible for the Cassini division, the most readily observable feature of the rings, even though it is nowhere near it; the division occurs at the point of a particular orbital resonance, which means that tidal interactions with Mimas will tend to push material away from this space. This is similar to the effect that Jupiter has on the asteroid belt, but on a larger scale. The whole system is extremely complex, and astronomers do not understand it well yet. It is of great interest <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

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##### Guest
"The whole system is extremely complex, and astronomers do not understand it well yet."<br /><br />I don't want to minimize the complexity of the problem, but I've seen animations showing what would happen when one galaxy collides with another galaxy - the resultant motion and state of the stars in the 2 galaxies.<br /><br />So, why can't you run the same simulations showing what happens when two big object collide over Saturn - that seems to be the concensus of how the rings started. Two objects collided and scattered particles in orbit that eventually became even rings.

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#### MeteorWayne

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That is not correct.<br />The most likely scenario is that an object's (moon) orbit wound up inside the Roche limit and was torn asunder.<br /><br />Not that a collision couldn't happen, but it's not the most likely scenario. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

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