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On a ring origin of the equatorial ridge of Iapetus
ngunn
post Sep 6 2006, 02:08 PM
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I did ask the other day for someone to re-post an appropriate image link for this discussion but nobody did, so I went browsing. I liked this one because it's easier to 'read' a landscape when down is at the bottom; also I found the chocolate brown quite appealing . .
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djellison
post Sep 6 2006, 02:13 PM
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Links to that place are not a good idea. Can people please find alternate images so I delete those links.

Doug
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ugordan
post Sep 6 2006, 02:15 PM
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QUOTE (ngunn @ Sep 6 2006, 03:08 PM) *
I did ask the other day for someone to re-post an appropriate image link for this discussion but nobody did, so I went browsing. I liked this one because it's easier to 'read' a landscape when down is at the bottom; also I found the chocolate brown quite appealing . .

I'll try to dig up the presentation later. If anything, I'll get the image off the PDS. In the meantime, here's what I think is a more likely natural color of Iapetus:
Attached Image


Yes, it's a dark image. Mainly due to bright ice being overexposed in the northern latitudes if I increase the brightness any more. The brightness difference is notable. I believe the south polar ice (not seen here) is even brighter compared to this view.


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ngunn
post Sep 6 2006, 02:31 PM
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QUOTE (djellison @ Sep 6 2006, 03:13 PM) *
Links to that place are not a good idea. Can people please find alternate images so I delete those links.

Doug


Sorry to have caused a problem. If you refer to posts 28 and 31 you'll see why I posted the image I did. I know nothing about the site it came from except that at first glance the surrounding text is plainly batty. Please go ahead and delete.
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Themisto
post Sep 6 2006, 03:06 PM
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QUOTE (ngunn @ Sep 6 2006, 04:31 PM) *
Sorry to have caused a problem. If you refer to posts 28 and 31 you'll see why I posted the image I did. I know nothing about the site it came from except that at first glance the surrounding text is plainly batty. Please go ahead and delete.


This text is really funny, you can laugh all the time. laugh.gif Unfortunately, it also consumes a lot of time to read.

For more Iapetus images, this page might be a potential source.

There was another question about the motion of the nodes of Iapetus' orbit. I believe it has a precession rate of ~3000 years.
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djellison
post Sep 6 2006, 03:10 PM
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That's better smile.gif

We used to worry about mentioning words that would raise this places googlability.

However - I've since looked at how google works - and it's by links. The more links TO a particular site that exist, the more popular it is considered by google and the higher up the list it goes. Hence I really don't want to link to 'over there' because it's a source of missinformation and lies and not something I want to see climb up the google ladder.

Astonishingly - this place rates very very highly with google...clearly lots of people like us wink.gif

Doug
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ngunn
post Sep 6 2006, 03:45 PM
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OK let's see if this works..

http://www.aaw-darmstadt.de/bilder/japetus...%20cr%20enh.jpg

Yes! - though I prefer it rotated 90 degrees clockwise. It clearly shows three almost parallel ridges over part of the length, possibly due to a combination of constructive and destructive processes plus small adjustments to the rotation axis during ring-fall. I wish I'd been there to see it (but not too close).
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tasp
post Sep 6 2006, 06:47 PM
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3 intersecting ridges, all describing segments of great circles about Iapetus.

Not to belabor the point, but there is not an internal geological process that gives a whit about great circles configured this way. The feature 'shouts' external orbital causation. Consider precession of the rotation axis of Iapetus (or inclined structures in orbit above), syncronization of the deposition process as the highest spot on the equator passes through the ring plane twice per rotation. The ground track per the relevant orientations of the ring, equator, highest spot along the equator, and precession (or inclined structure) is where the ring emplaces.

{Sorry for the crappy diagram}

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mchan
post Sep 6 2006, 11:15 PM
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QUOTE (ugordan @ Sep 6 2006, 05:29 AM) *
True, but that intersection isn't likely to fit the arrival hyperbola so careful arrival timing (on the order of years!) is likely to be required.

I'm not an expert in orbital dynamics, but is Iapetus' orbit ascending node fixed w/ respect to the stars? A probe entering the Saturnian system at a hyperbolic trajectory will probably have a more-or-less fixed point (arrival angle w/ respect to the sun and with a given injection energy) where closest approach is made.
Take a simplification: the arrival plane is Saturn's equatorial plane. The intersection, c/a possible points are then Iapetus' ascending and descending node. Then, you have to wait until the Saturn's revolution around the Sun rotates one of the nodes to the point where the approach trajectory interects the Iapetus' orbital radius. Only then is the arrival geometry right.
This would constrain the possible arrival times to the Saturn system to two fairly short periods each Saturn orbit, each half an orbit apart. That'd be a long delay between launch windows.

I do not quite follow this. Why does the arrival plane (containing the hyperbolic trajectory) need to be Saturn's equatorial plane to encouter Iapetus? Cassini arrived out of the equatorial plane and got a close approach to Phoebe inbound.

I don't know astrodynamics either. It seems to me the arrival plane can be at any inclination between 0 and Iapetus orbit inclination to Saturn's equatorial plane, and have a chance of intersecting Iapetus orbit. The problem is to make sure that Iapetus is at the point of intersection when the probe gets there.

Oh, and a probe would not want to arrive with 0 inclination to the equatorial plane to avoid traversing the rings. smile.gif
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ugordan
post Sep 7 2006, 06:58 AM
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QUOTE (mchan @ Sep 7 2006, 12:15 AM) *
I do not quite follow this. Why does the arrival plane (containing the hyperbolic trajectory) need to be Saturn's equatorial plane to encouter Iapetus?

As I was saying, it was merely a simplification so I can more easily put into words what I mean. A real arrival trajectory will almost never be coplanar to Saturn's equator. By using an equatorial plane, the two obvious intersection points would be the ascending and descending nodes. If you increase the inclination, the intersection points move much, but OTOH, there's still maximally two of them.


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ngunn
post Sep 7 2006, 08:51 AM
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QUOTE (tasp @ Sep 6 2006, 07:47 PM) *
The feature 'shouts' external orbital causation.


Absolutely, and this is the central wonderful realisation which should set us looking for similar structures elsewhere. The detailed argument beyond that point though: it's plausible but do you really think it's conclusive, case closed? I think that's claiming too much.

Incidentally on the search for similar structures - I don't just mean ridges. More generally we should look out for distinctive equatorial topography of any kind that might have had a ring origin. The specific type of formation will depend on circumstantial details of the process but great circles will be the giveaway.
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tasp
post Sep 7 2006, 02:58 PM
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Some of the criteria that seem to be needed for us to see such a feature are

*ancient surface

*object needs to approximate a sphere

*object may need to be remote from other objects (our own moon does not seem to have low circular orbits stable enough for there to be enough time for a ring system to form and emplace. Materials in randomly inclined lunar orbits will contact the lunar surface prior to colapse to the Laplacian plane)

*object needs to have a surface sufficiently solid and rigid for the materials to emplace on

*object needs a very low density (prefer none) atmosphere or materials will drop all around the equator.
(we may see such structures someday, but they won't look like the Iapetan ridge formation).

*object needs to be somewhere an appropriate glancing impact can loft materials is likely to occur. Iapetus may have encountered an 'outie' satellite of Saturn or a displaced Saturnian Trojan object
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tasp
post Sep 7 2006, 03:03 PM
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We may find Iapetan like ridge structures on a percentage of the larger KBOs.

Not sure NASA wold mass produce New Horizon clones and start launching them en masse to the outer solar system ( [laugh] ), but it would be interesting to have a few more examples to study.
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ngunn
post Sep 7 2006, 03:45 PM
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QUOTE (tasp @ Sep 7 2006, 03:58 PM) *
Some of the criteria that seem to be needed for us to see such a feature are

*ancient surface

*object needs to approximate a sphere

*object may need to be remote from other objects (our own moon does not seem to have low circular orbits stable enough for there to be enough time for a ring system to form and emplace. Materials in randomly inclined lunar orbits will contact the lunar surface prior to coplace to the Laplacian plane)

*object needs to have a surface sufficiently solid and rigid surface for the materials to emplace on

*object needs a very low density (prefer none) atmosphere or materials will drop all around the equator.
(we may see such structures someday, but they won't look like the Iapetan ridge formation).

*object needs to be somewhere an appropriate glancing impact can loft materials is likely to occur. Iapetus may have encountered an 'outie' satellite of Saturn or a displaced Saturnian Trojan object


I've been having similar thoughts about the criteria. The only one of yours I would dispute is the last. I think an equally likely source for ring material is a sub-satellite that fragments at the Roche limit - no collision required. On 'ancient surface' I would prefer to say something like 'primitive' surface since in principle at least a ring-forming event does not have to be ancient (after all we see rings round the giant planets now that are probably not ancient, at least in their present form).

On 'remote from other objects' I have been doing some rough calculations of a parameter that I think should relate to the capacity of a body to retain independent satellites or rings. It uses 3 quantities: mass of the world in question, m, mass of the most significant perturbing body, M, and the distance between them, d.
As the strength of tidal forces falls off as the cube of distance so the formula becomes: d cubed multiplied by m over M. Using this formula all the major planets, including Mercury, come out orders of magintude ahead of all the solar system moons - no surprise there, though interestingly Pluto scores almost as highly as Jupiter. Of the moons, Luna and Titan score highest. Our moon could not have retained a ring at an early stage if it was much closer to the Earth then, but it could now! Titan could well have had subsatellites or rings early on but there would be no evidence left. Next comes Japetus, then Callisto. I haven't done the calculations for KBOs besides Pluto but they should all come out MUCH higher than Japetus, so definitely good places to look for ring ridges.

On the contingent circumstances that would determine the type of great circle feature formed - there's the presence of any atmosphere certainly, but also the nature and quantity of the ring materials, the size distribution of the ring 'particles', the degree of tidal perturbation of the ring, and the rate of infall of material and the temperatures reached in the process, to mention just the most obvious ones.
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TritonAntares
post Sep 7 2006, 03:50 PM
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Hi,
before we should keep on speculating whether the equatorial ridge was built by an ancient Iapetian ring,
we should take a closer look at the ridge, e.g. if there are older structures below it - what could answer its built up...

Here again what I wrote half a year ago - and an interesting pic:
QUOTE (TritonAntares @ Apr 6 2006, 08:54 PM) *
One question to answer should be:
What is older, the equatorial ridge or its surrunding/overlaying area, speak the craters?

Therefore I compared these two shots of the 'belly band':
Attached Image

Visible left is a large bassin (large pink circle) in the western part of CR cut by the part of the ridge with the 'white peaks'.
Thus the crater must be older than them, even if its central peak (small pink circle) doesn't coincide with the 'white peak' NW of it
and it is also lower. So you could guess the ridge there is fairly young, maybe eruptive....

In the right image another part of the ridge in central CR is shown. It seems to be quite old.
Lots of craters (red circles) crashed into it and disturbed its line.
The craters in the blue circles look somehow tilted, probably raised up by the ridge.
But due to bad resolution this is difficult to discern...

And don't forget there is no evidence for the ridge east of CR, only some uncertain albedo features.

Is the equatorial ridge powered by some longitudinal subsurfaced source or is there a gravitational cause from one direction?
The belts different heights and ages then could be explained by a longitudinal shift over a longterm period.
So, if there is really an older feature below a ring-built belt we could eventually be in trouble with the ring's age....

For me speaking,
I'm more convinced of an internal origin of the belt - probably connected with a large impact and/or some internal processes.

This ring theory sounds too strange and unlikely... blink.gif huh.gif
But planets around a neutron star did that as well ... wink.gif

Bye.
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