Why do rings form around the equator?

[benjam]

I was wondering what forces cause the particles in the rings around a planet to congregate on the plane of rotation (the equator), even when that plane is not even close to the plane of the ecliptic like Uranus?<br /><br />And second, <br />Why can some small (assume captured) moons of Jupiter have highly irregular orbits, while the small particles of the rings (assume caused by collisions, which would (I'm assuming) begin as a highly irregular halo of particles) do not?

 

[MeteorWayne]

Great questions, welcome to SDC!<br /><br />In general, rings are caused by the disruption of a moon of the planet, either by an impact, or the moon approaching too close to the planet and being torn apart by gravity.<br />Since major moons form from a disk much like the disk that formed the solar system, they orbit in the same plane as the equator. So when disrupted, they will tend to stay in that same plane. There are other effects that force the ring particles gently in that direction even if they don't start out perfectly aligned.<br /><br />Uranus was apparantly tilted on it's side early in it's existence. Those same gentle forces also tend to move moons into that plane. (It has to do with angular momentum, a rather long explanation, which I'd have to research to answer accurately). Rings are short lived features (relative to the age of the solar system) so by the time Uranus' rings were created, the plane of the moons was aligned with the equator, so the rings tend to say there.<br /><br />Ring particles are not captured from elsewhere, like the irregular moons of the outer planets, but rather created in place from an object already in that plane.<br /><br />Hope this helps!<br /><br />MW <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>

 

[vogon13]

Imagine you are orbiting Saturn, inclined to the equator 45 degrees, and in a circular orbit 50,000 miles above the cloud tops.<br /><br />What happens?<br /><br />Twice every time you orbit Saturn, you pass through the equatorial plane and through the rings. You will experience a rather drastic 'splat' every time you pass through the rings, and your orbit will be drastically modified till you are orbiting in the same plane as the rings.<br /><br />How do you get the material in the equatorpal plane in the first place, though?<br /><br />Imagine a great many objects orbiting Saturn (or any object you want) all in randomly inclined orbits. Collisions will happen frequently, and due to the oblateness of the object you are revolving about (btw, oblateness is quite common for rotating objects), the orbits of all these objects will tend to 'slide' around the object (referenced to the fixed stars). There is a unique plane, coincident with the equator of the object where all these materials will eventually 'settle' towards.<br /><br />This effect will occur even if most of the materials start off in even a drastically inclined orbit.<br /><br /><br /><br />{note, materials in an orbital inclination of exactly 90 degrees are a special (and very unusual) case, and are beyond the scope of this thread.}<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>

 

[benjam]

Thank you for your reply, but I am still a little confused on the 'slide' that you are referring to.<br /><br />And I have a physics degree, so please don't spare the details.

 

[vogon13]

There is a much better discussion of this ( and a very good diagram too) in the book "The New Solar System" in the planetary rings chapter.<br /><br />Due to copyright concerns, I have always been a little leary of copying that sort of thing on to an on-line website like this.<br /><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>

 

[vogon13]

There is a much better discussion of this ( and a very good diagram too) in the book "The New Solar System" in the planetary rings chapter.<br /><br />Due to copyright concerns, I have always been a little leary of copying that sort of thing on to an on-line website like this.<br /><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>

 

[vogon13]

Maybe this helps:<br /><br />For a perfectly symmetrically spherical object, orbits would be perfectly repeatable. Since most rotating objects aren't uniformally dense and perfectly spherical, the tilted orbits orientation to the fixed stars will tend to move around the object. When you have a large number of objects in these orbits, they all won't 'scooch' around in perfect synchronization.<br /><br />This inevitably leads to collisions if there are sufficient objects and time.<br /><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>

 

[willpittenger]

I think the others have danced around this, but the best explaination has to do with gravity and tidal forces. As you may remember, objects in LEO, due to the lower orbit, orbit faster than the Earth spins. This causes them to drop in orbit because the tidal forces act as a brake. Conversely, the Moon's orbit is moving away from Earth because the tidal forces are now speeding it up.<br /><br />I am not sure I can explain how that relates to what moves the plane of the rings around or not, but I will give it a whack. Let's assume a satellite in a 20° inclination orbit. The tidal forces are trying to hurl everything in orbit outward. This happens regardless of where our satellite is in its orbit. This has the effect of attempting to raise the orbit. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

 

[benjam]

So it's basically a precession type thing that decreases the inclination of the orbit over time?

 

[vogon13]

The orbits precess around the primary, the collisions collapse the vertical extent of the materials to the equatorial plane.<br /><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>

 

[willpittenger]

In the case of the Moon, its inclination to the ecliptic is not relevant. You want the inclination relative to the Earth's equator.<br /><br />Also, how does the Earth's inclination (the ecliptic) relate to the Sun's rotation?<br /><br />On a bigger scale, does the solar system take a repeatable orbit around the Milky Way that you could state an inclination for? <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

 

[benjam]

With regards to the 'Why aren't the planets all skewed out of the ecliptic' statement.<br /><br />I would imagine that because the planets (and most of the larger moons) were formed at the same time from the dust disk of the sun, that that is the major reason that all those are in the same plane, and because there really isn't much out there to knock a planet the size of Jupiter or Saturn out of it's plane, that even after '4 giga years' they would continue to remain within the stable orbits they currently occupy.<br /><br />And all of the 'official' planets have orbits within a reasonable angle of the ecliptic (which is based on the Earth-Sun system, and may not be the average, which would cause some of the planet's ecliptic angles to be even smaller).<br /><br />I also think that the rings travel along the plane because that's where all the moons are, and they would pull the ring particles toward them as they passed. But I like the oblateness explanation, even though I may not understand it fully.

 

[willpittenger]

<blockquote><font class="small">In reply to:</font><hr /><p>It's not clear what keeps the planets in a very tight plane within a few degrees of the plane of the ecliptic.<p><hr /></p></p></blockquote><br />How about the same force that works on the rings? Like I speculated earlier, the gravitational tidal forces act like a centrifuge. The effect on an object (A) moving slower than the object (B) that A orbits would be magnified if the orbit is slower than that of the B's rotation.<br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>The movement of the solar system in orbitting around the galaxy probably doesn't have much to do with the plane of the ecliptic or rings of Saturn being where they are.<p><hr /></p></p></blockquote><br />I never said they were related. However, the solar system's orbital inclination might be related to the rotation of the galaxy as a whole or that of the super black hole at the Milky Way's core. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

 

[willpittenger]

What computer modeling has been done on the subject? I know they did a lot regarding collision of galaxies. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

 

[shinosa]

Sorry to jump into this chat late, but I stumbled across it on Google because my wife and I were debating this topic just the other day. Unfortunately, I'm a complete layman on astronomy, so much of this conversation has gone over my head. But I just wanted to ask, is it true that most moons and rings form along the plane of a planet's equator, with our moon being an exception?<br /><br />And somewhat related, are the rings of Uranus always depicted in images as vertical because of the planet's tilt from the ecliptic plane?<br /><br />Thanks for your insights. This has been a fascinating conversation to catch up on.

 

[MeteorWayne]

Welcome to SDC!!<br /><br />Yes most "regular" moons form along a planet's equator since that's the plane of the disk that they were formed from. The irregulars (tiny, half retrograde, highly inclined moons) are not so constrained since they appear to be captured asteroids and Kuiper belt objects.<br /><br />And Yes #2, Uranus' rings are most likely generally depicted face on because of the near 90 degree inclination of it's equator to the plane of the solar system.<br />As a side point, that is not currently the case, since Uranus' equator and rings are nearly edge on to us right now. It's equinox is December 2007. For the next few years we can see the moons and moonshadows on the surface of the disk. Not that that's easy <img src="/images/icons/smile.gif" /><br />Next equinox is in 42 years. <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>

 

[benjam]

-- esp. since Saturn's axis of rotation is almost 90 deg. perpendicular to its orbital plane. --<br /><br />No, Saturn's tilt is only 26.73° (http://en.wikipedia.org/wiki/Saturn) so it's not that unreasonable to assume that the rings started out very close to the plane they are now (if they were shattered moons).<br /><br />But Uranus' rings cause quite a bit more trouble with a tilt of 97.77°<br /><br /><br />-- There is a rather large movement off a plane extending out from the earth's equator of that 18-28 degrees or so amount. --<br /><br />But is that measured from the ecliptic, or the equatorial plane? Because if it were the ecliptic, thats a difference of only +- 5 degrees from the equatorial plane (which is roughly 23 degrees from the ecliptic)

 

[shinosa]

OK, since I got such a clear answer (and thanks!), I'll press my luck and go for two.<br /><br />When a moon passes the Roche limit and breaks apart, why don't the pieces simply continue to crash toward the planet instead of drifting into rings? Is it just hitting some sort of equilibrium that stops the moon's parts from getting closer to the planet? <br /><br />I understand how the rings form, in the sense that interior particles start moving faster than the exterior parts...I just don't understand why the moon's pieces don't stay on course to smack into the planet.

 

[MeteorWayne]

You're just full of good questions, aren't you?<br />I don't know the answer, so I'll ponder for a bit and see if someone who knows can enlighten us. <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>

 

[vogon13]

Pierre Laplace worked this all out a couple of centuries ago without a computer . . . .<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>

 

[willpittenger]

The typos make you hard to follow, but I think I understand. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

 

[willpittenger]

In one word, Time. It takes a long time for objects to go from Saturn's Roche limit area to the planet's atmosphere. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

 

[willpittenger]

The Io flux tube is probably the source of most of that line noise. You should leave the Jupiter area before you get microwaved by your location. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>