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On a ring origin of the equatorial ridge of Iapetus
ngunn
post Sep 8 2006, 10:44 AM
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QUOTE (TritonAntares @ Sep 7 2006, 04:50 PM) *
Hi,
This ring theory sounds too strange and unlikely... Bye.


That makes you the second poster here, along with Ugordan, to be sceptical of the whole idea. Any particular reason for this, or just a general hunch? As an undergraduate in 1970 I remember arguing with my geology professor about the origin of lunar craters. His line was "Why invoke an exotic external cause when there's a perfectly natural geological explanation?"

With hindsight it seems surprising that fossil ring structures were not predicted ahead of the Iapetus discovery, given the ubiquity of rings when things break up in the vicinity of other things, plus the fact that most ring material will inevitably spiral inward over time and has to end up somewhere.

So, A QUESTION: Has anybody here come across a prior prediction of fossil ring structures, either in the scientific literature, in fiction, or in informal communications?
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ugordan
post Sep 8 2006, 11:05 AM
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I have absolutely no idea on which internal process could create the ridge, but I am skeptical of the ring hypothesis mainly because I don't see it very likely that smallish Iapetus could have enough gravity to align the orbiting debris along its equatorial plane. Isn't that possible only due to the rotational bulge at the equator? Iapetus has a small radius so a 16 hour rotation wouldn't produce centrifugal forces as big as on a body twice the radius. The bulge would be lower. Therefore, orbiting debris would be unlikely to align into an quatorial ring that easily. An equatorial ring maintained long enough, even due to Saturn's perturbations? I also don't like the explanation on why the ridge isn't complete around the equator. And how does it all fit with the dark stuff? Why is the color of the dark stuff practically identical to Hyperion's color, only differing in albedo?

There's just too much magic fairy work here, IMHO.


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ynyralmaen
post Sep 8 2006, 11:34 AM
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QUOTE (ngunn @ Sep 8 2006, 12:44 PM) *
So, A QUESTION: Has anybody here come across a prior prediction of fossil ring structures, either in the scientific literature, in fiction, or in informal communications?


SPOILER ahead for those who haven't read Kim Stanley Robinson's "Red Mars"...


Does the collapse of the space elevator in the abovementioned novel count? smile.gif
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ngunn
post Sep 8 2006, 12:04 PM
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QUOTE (ugordan @ Sep 8 2006, 12:05 PM) *
I have absolutely no idea on which internal process could create the ridge, but I am skeptical of the ring hypothesis mainly because I don't see it very likely that smallish Iapetus could have enough gravity to align the orbiting debris along its equatorial plane. Isn't that possible only due to the rotational bulge at the equator? Iapetus has a small radius so a 16 hour rotation wouldn't produce centrifugal forces as big as on a body twice the radius. The bulge would be lower. Therefore, orbiting debris would be unlikely to align into an quatorial ring that easily. An equatorial ring maintained long enough, even due to Saturn's perturbations? I also don't like the explanation on why the ridge isn't complete around the equator. And how does it all fit with the dark stuff? Why is the color of the dark stuff practically identical to Hyperion's color, only differing in albedo?

There's just too much magic fairy work here, IMHO.


Very useful to have specific doubts expressed, thanks. On getting the ring to be flat and thin - an equatorial bulge helps but I don't think it's essential. I admit I have no idea about the timescale required for flattening the ring or for it's destabilisation by Saturn. We will have to see if a proper scientific rebuttal along these lines emerges in due course. I would just point out that if the ring material came from a former subsatellite rather than from a randomly oriented collision it would have been pretty near the equatorial plane to start with. If in addition the former satellite was made of rather loosely bound material the process of smearing it out at the Roche limit could have been quite orderly. It would simply have 'poured' itself (as it were) into a flat sheet. On the unusual albedo features - I think they are much younger than the underlying solid structure of Iapetus.
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ngunn
post Sep 8 2006, 12:12 PM
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QUOTE (ynyralmaen @ Sep 8 2006, 12:34 PM) *
Does the collapse of the space elevator in the abovementioned novel count? smile.gif


Not unless you want to join the discussions elsewhere about the ridge on Japetus being artificial. wink.gif
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ugordan
post Sep 8 2006, 12:25 PM
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QUOTE (ngunn @ Sep 8 2006, 01:04 PM) *
On getting the ring to be flat and thin - an equatorial bulge helps but I don't think it's essential.

It actually might be essential. A perfectly spherical object will act as a point gravity source (barring mascons) and it won't exert any torques that change the orbital plane of an object. At least I think so.

QUOTE (ngunn @ Sep 8 2006, 01:04 PM) *
If in addition the former satellite was made of rather loosely bound material the process of smearing it out at the Roche limit could have been quite orderly. It would simply have 'poured' itself (as it were) into a flat sheet. On the unusual albedo features - I think they are much younger than the underlying solid structure of Iapetus.

Again, I don't know how strong Iapetus' tidal force could be to do this sort of thing and overcome disruptive perturbations by Saturn. I'd be much more comfortable with the ring idea if there was a simulation showing it's dynamically possible. Beyond that, all this seems just arm-waving. Or my gut feelings vs. your gut feelings. smile.gif


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ngunn
post Sep 8 2006, 12:45 PM
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QUOTE (ugordan @ Sep 8 2006, 01:25 PM) *
It actually might be essential. A perfectly spherical object will act as a point gravity source (barring mascons) and it won't exert any torques that change the orbital plane of an object. At least I think so.
Again, I don't know how strong Iapetus' tidal force could be to do this sort of thing and overcome disruptive perturbations by Saturn. I'd be much more comfortable with the ring idea if there was a simulation showing it's dynamically possible. Beyond that, all this seems just arm-waving. Or my gut feelings vs. your gut feelings. smile.gif


Just arm waving certainly! I don't claim to be doing anything else, but it's fun anyway. Ring flattening - I thought that a ring would eventually flatten itself due to mutual interactions between the particles???
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ugordan
post Sep 8 2006, 12:50 PM
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Yeah, but why would it flatten itself along the equatorial plane if the particles were ejected on a random inclination? They don't know / don't particularly care about the equator.


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Bill Harris
post Sep 8 2006, 12:56 PM
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This has been a fascinating discussion. Before Cassini arrived at Saturn and we got the first decent-resolution views of Iapetus there was the underlying and unspoken fear of finding a Monolith in the middle of things. And guess what? Mother Nature upped the ante and trumped us with this equatorial ring... blink.gif

--Bill






PS-- equatorial ring AND ridge


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ngunn
post Sep 8 2006, 01:18 PM
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Hello Bill - nice to know it's not just the 4 or 5 of us here exchanging hand signals! Ugordan I agree with you about randomly ejected material - that's why I prefer a disintegrating moon in an equatorial orbit for the source of the material.
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tasp
post Sep 8 2006, 01:33 PM
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QUOTE (ngunn @ Sep 8 2006, 05:44 AM) *
So, A QUESTION: Has anybody here come across a prior prediction of fossil ring structures, either in the scientific literature, in fiction, or in informal communications?



Yes. Rings, anyhow.

I recall prior to the Voyager 1 flyby of Saturn, the JPL team was cautioned to "expect the unexpected" and rings around moons were specifically mentioned as something that might be seen.

Sorry I don't have a link for this, but I seem to recall this was shown in the PBS Nova episode about the Voyager Saturn flybys. IIRC, the episode title was 'Resolution on Saturn', and I am quite sure I haven't seen that episode since it's original broadcast.


Also, I think there is art work (at least) showing the earth's moon forming after the Orpheus impact. The debris field is invariably portrayed as a ring like structure. That the entire structure wouldn't be incorporated into the nascent moon seems obvious, but the eventual disposition of such material seems to be unremarked upon.


As for a specific prediction of the appearance of an emplaced ring structure on a small airless body made prior to Cassini, I haven't found anything yet.
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tasp
post Sep 8 2006, 01:40 PM
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QUOTE (ngunn @ Sep 8 2006, 08:18 AM) *
Hello Bill - nice to know it's not just the 4 or 5 of us here exchanging hand signals! Ugordan I agree with you about randomly ejected material - that's why I prefer a disintegrating moon in an equatorial orbit for the source of the material.



Please check out the Planetary Rings chapter in the New Solar System book. It made me a believer.

That a sufficient quantity of materials originally in, let's say a 45 degree inclined orbit, could all wind up in the equatorial plane seems counterintuitive.

But it can.

Oblateness of the primary causes the materials to spread out around the primary. Once this happens, the materials start colliding amongst themselves, twice per orbit. Half the collisions occur as materials pass north to south over the equator, and half occur as the materials pass south to north, 180 degrees around.

Result is to pulverize the chunks, and put them into an equatorial orbit.

Pierre Laplace came up with the idea a very long time ago.
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Rob Pinnegar
post Sep 8 2006, 01:56 PM
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Two things today:

QUOTE (ugordan @ Sep 8 2006, 05:05 AM) *
I have absolutely no idea on which internal process could create the ridge, but I am skeptical of the ring hypothesis mainly because I don't see it very likely that smallish Iapetus could have enough gravity to align the orbiting debris along its equatorial plane.


(1) Hmmm. You may just have come up with a device to explain the inward spiralling of the rings.

Saturn's ring system is pretty much flatter than a board. Presumably it gets hit by objects from time to time (comets, meteroroids and the like) that would be able to scatter ring particles into inclined orbits. However, any such particles will end up in a ring-crossing orbit, and will end up hitting other ring particles and getting re-absorbed into the rings. Each impact will of course add net momentum (and angular momentum) to the ring system, but over time these should average to zero -- and even if they didn't, tidal effects from Saturn and the moons ought to be more than capable of keeping things in order.

A ring around Iapetus, though, would experience perturbations from Saturn that would warp the ring. It probably wouldn't warp it like an old vinyl LP record left out in the sun -- instead it would tend to "smudge" the ring perpendicular to Iapetus' equatorial plane. The ring would constantly be trying to flatten itself out through the mechanism described above, i.e. ring particles would constantly be hitting each other from "above" and "below". Any component of a collision perpendicular to the ring plane would take net energy away from the ring system, and though I haven't done the math, I'm betting that this would tend to cause the ring to spiral in towards Iapetus over time.

Wild hypothesizing: If you look at the distribution of dust in the asteroid belt, there are concentrations about 9 degrees above and below the ecliptic. This is thought to be due to particles from a recent major asteroid-asteroid collision. Due to their inclined orbits, the dust particles spend a disproportionate amount of time away from the ecliptic, and tend to "collect" at +- 9 degees altitude.

Perhaps the perturbations from Saturn could have been responsible for an analogous "three-layered" structure in the proto-Iapetan ring, that is reflected in the mass distribution of the ridge structure? (I have to be honest -- I don't believe this for a minute -- and am just tossing it out here as an idea.)

(2) The recently posted image that shows the "triple structure" of the ridge is in fact the one I referred to the other week. Great. Now, are we absolutely certain that those three parallel lines are three parallel ridges and not some other type of linear structure within a single ridge? This was my reason for asking about it in the first place.

Perhaps it would be possible to produce a stereo pair to get some altitude information? There were quite a few images taken of this region during the New Year's distant flyby. I have no idea how to do that sort of thing, unfortunately.

[Edit: I was typing this post when Tasp put up his above message -- which also covers the topic of "ring flattening". Neat coincidence, that.]
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tasp
post Sep 8 2006, 02:27 PM
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Some where here at UMSF is a nice map of Iapetus (help finding it would be appreciated).

The two shorter segments of the ridges are not parallel to the main center structure.

The two 'attendant' ridges diverge smoothly and symetrically from the main equatorial ridge.

They both are exactly the same length and height (not counting subsequent random cratering damage) and both follow segments of great circle paths about Iapetus.


The 'great circle' paths are the key to understanding the orbital emplacement of materials upon the Iapetan surface.
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tasp
post Sep 8 2006, 02:30 PM
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I will note that Iapetus is subject to the smallest tidal effects from Saturn of any of Saturns' moons. The tidal effects are also lower that for any of the 4 Galilean satellites of Jupiter, Triton, and I suspect most or all of the major moons of Uranus.
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