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On a ring origin of the equatorial ridge of Iapetus
tasp
post Sep 8 2006, 02:36 PM
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I also point out that the New Solar System books' chapter on Planetary Rings describes the 'bump' process that occurs in ring systems and naturally causes them to spread, without any outside influences at all.

(I suspect outside influences did play a role in the evolution of this putative ring system, most notably drag forces from Poynting Robertson effects, and solar wind/Saturn magnetotail interactions, that resulted in the entire ring system emplacing on the surface, and not also concurrently generating some small satellites just outside the Iapetan Roche limit)
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ugordan
post Sep 8 2006, 02:38 PM
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However, the surface gravity at Iapetus is also vastly lower than at any of the Galileans so tidal force alone isn't the only measure here.
Tasp, nobody is questioning the mechanism by which the planets flatten out the rings. I'm questioning whether Iapetus is oblate enough so its weak gravity can flatten the rings and counter Saturn's perturbations as well.


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tasp
post Sep 8 2006, 02:52 PM
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Virtually all solar system objects are believed to have formed with rotation rates around 10 hours, IIRC. Iapetus, uniquely distant from its' primary compared to the other major moons of our solar sytem, would have taken longest of all to spin down to tide lock with its' primary (Saturn). Ample time existed for the original congenital oblateness of Iapetus to have 'frozen' in place upon the completion of Iapetus' accretion.

I consider Iapetus to be sufficiently oblate from theoretical grounds alone to accomplish the needed collapse to the Laplacian Plane of the inclined debris cloud. (Hopefully Cassini will shore up the wobbly limb I am on here)
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ngunn
post Sep 8 2006, 02:54 PM
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Is this the one? http://photojournal.jpl.nasa.gov/jpegMod/PIA07778_modest.jpg
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tasp
post Sep 8 2006, 02:59 PM
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QUOTE (ngunn @ Sep 8 2006, 09:54 AM) *



Thanx.

It seems to be. (I couldn't get it to expand with my rickety PC though which makes the details harder to see, is there a version that allows magnification to many times screen size? Or do I have my computer settings messed up again?)


blink.gif
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ugordan
post Sep 8 2006, 03:04 PM
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Iapetus is undeniably oblate, but the question is is it enough. I suspect another factor determining how fast and efficient the flattening of the rings would be is the orbital period and speed. I calculate an exactly 3 hour orbital period for the lowest possible orbit. Is the tendency to flatten the rings greater than the tendency to disperse them by various factors, such as Saturn perturbations and light pressure effects, etc. That seems to be the principal question.

Tasp, try this.


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hendric
post Sep 8 2006, 05:54 PM
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I still don't see how the two divergent ridges get created by a decaying ring. The orbital speed of the ring at low altitude would not be the same as the rotational velocity of a proto-Iapetus. So, how do the angled ridges get created?? If the ring was at an angle to Iapetus, as it descended it would not stay above a single location.

I favor a tectonic explaination. We have symmetrical ridges here on Earth due to seafloor spreading. Seems like a reasonable explaination to me. The question would be why would there be a single crack along a great circle? Maybe Iapetus had a Europa-like episode with a shallow ocean, and as it slowly froze it expanded/contracted enough to crack open. Without nearby moons and tides, the crack went around the planet symmetrically.


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TritonAntares
post Sep 8 2006, 08:29 PM
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QUOTE
37th DPS Meeting, 4-9 September 2005
Session 47 Icy Satellites
[47.08] The topography of Iapetus' leading side

B. Giese (DLR-Institute of Planetary Research, Berlin, Germany), T. Denk, G. Neukum (Institut fur Geologische Wissenschaften, Freie Univ. Berlin, Germany), C. C. Porco (Cassini Imaging Central Laboratory for Operations, Space Science Institute, Boulder, CO, USA), T. Roatsch, R. Wagner (DLR-Institute of Planetary Research, Berlin, Germany)
We have used Cassini-stereo images to derive a topographic model of Iapetus' leading side. The model reveals that Iapetus has substantial topography with heights (referenced to a 747 x 744 x 713 km ellipsoid (Thomas et al., in preparation)) in the range of -9 km to +15 km...

Hhm, this strange ellipsoid figure could be a hint for some internal force once having driven a probable tectonic mechanism and causing the ridge... huh.gif
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tasp
post Sep 9 2006, 05:10 AM
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QUOTE (hendric @ Sep 8 2006, 12:54 PM) *
I still don't see how the two divergent ridges get created by a decaying ring. The orbital speed of the ring at low altitude would not be the same as the rotational velocity of a proto-Iapetus. So, how do the angled ridges get created?? If the ring was at an angle to Iapetus, as it descended it would not stay above a single location.



The single location is the highest spot along the Iapetan equator. That is what synchronizes the emplacement of the diverging features.

For every rotation of Iapetus, the highest spot along the Iapetan equator penetrates the ring plane twice. Once as it crosses from north to south, and again 180 degrees later when the high spot penetrates the ring plane crossing from south to north.

The lowest edge of the ring descends (via the 'bump' or dynamical ring spreading process) during the interval the high spot is not in the ring plane. As the highest spot passes through the ring plane, the chunks comprising the low edge of the ring impact the high spot, making it higher. The 'spray' from the impact interacts with materials still in orbit immediately above the high spot, decelerating them, and causing them to contact the surface downrange of the highspot, along the ground track of the ring at that time. This also explains why all three ridges taper downward from the high end.

That's why the 2 diverging attendent ridges are identical (less subsequent cratering damage), and are 'keyed' to the highest spot along the equator.


Note: inclined features in the ring or a small shift in the spin axis of Iapetus during emplacement will produce the same result, 2 diverging attendent ridges. I am not sure how to distinguish between shifting the axis of Iapetus a few degrees and inclined features in the ring. I think the 2 causes will produce identical features.

I cannot imagine an internal process that can create perfectly matched attendant ridges to the main ridge structure as elegantly as the emplacing ring with either an inclined element, or a small shift of the Iapetan spin axis during the emplacement.



I also note, it appears to my inexperience eye that the high end of the ridge structure is antipodal to the sub-Saturn point of Iapetus. Is the ridge structure massive enough to be the deciding factor in aligning Iapetus to Saturn? (as mass distribution in earth's moon does to earth?)



Does anyone with more computer graphics savy than me (that would be everyone else) want to take a stab at modeling this process for all to see?
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tasp
post Sep 9 2006, 05:28 AM
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QUOTE (TritonAntares @ Sep 8 2006, 03:29 PM) *
Hhm, this strange ellipsoid figure could be a hint for some internal force once having driven a probable tectonic mechanism and causing the ridge... huh.gif



Due to Iapetus large distance from Saturn, it's low orbital speed would have caused Iapetus to be the slowest formed satellite of the major Saturnian moons.

This would allow longer time for impact heat to radiate away during accretion, and the accreted materials would be more depleted of Al26 (or is it 27? I forget) and subsequent radiogenic heating of Iapetus would be lower.

Also, due to the great distance to Saturn, tidal heating of Iapetus would be uniquely low, Iapetus having been very leisurely braked into tide lock with Saturn.

The effect of all this is that Iapetus would have experienced much lower temperatures, and have been far more rigid crustally to preserve the 'congenital lumpiness' of its' later stages of accretion.
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tasp
post Sep 9 2006, 05:31 AM
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QUOTE (ugordan @ Sep 8 2006, 10:04 AM) *
Iapetus is undeniably oblate, but the question is is it enough. I suspect another factor determining how fast and efficient the flattening of the rings would be is the orbital period and speed. I calculate an exactly 3 hour orbital period for the lowest possible orbit. Is the tendency to flatten the rings greater than the tendency to disperse them by various factors, such as Saturn perturbations and light pressure effects, etc. That seems to be the principal question.

Tasp, try this.


Thanx, that's it!

Much appreciated.
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tasp
post Sep 9 2006, 05:35 AM
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Also, check out the big elongated crater at 0 to 30 degrees longitude, 50 degrees south latitude.


Interesting crater to compute an excaveted volume for and compare to the ridge volume . . . .



(it might be the source crater for the ring materials blink.gif )
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ngunn
post Sep 9 2006, 08:59 AM
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QUOTE (TritonAntares @ Sep 8 2006, 09:29 PM) *
Hhm, this strange ellipsoid figure could be a hint for some internal force once having driven a probable tectonic mechanism and causing the ridge... huh.gif


. . or the fossil tidal bulge produced by a close-orbiting subsatellite. I notice nobody else fancies this origin for the ring material - is it ridiculous for some reason that I have failed to notice?

On a philosophical note: Assuming the sceptics are wrong (and I leave that one to tasp) I think we are responding here to a real eureka moment in planetary science, perhaps on a par with Vine and Matthews. When we study the planets we have become accustomed to looking for evidence of mantle convection or crustal plate movement. Now, for bodies with primitive surfaces at least, we will always have to ask the question 'Is there any evidence of a former ring?'
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tasp
post Sep 9 2006, 01:56 PM
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QUOTE (ngunn @ Sep 9 2006, 03:59 AM) *
. . or the fossil tidal bulge produced by a close-orbiting subsatellite. I notice nobody else fancies this origin for the ring material - is it ridiculous for some reason that I have failed to notice?



I have spent much time contemplating binary satellites and small moons of moons.

(keeping in mind I am not smart enough to do the math for any of the stuff I think up)

Perhaps moons do form (sometimes) attendent satellites as they accrete. What happens to the little guys? We do not see them in this epoch, after all.

(I hasten to add, I expect the outer asteroidal satellites, especially the retrograde orbiting ones, to be binary in similar percentages to the presumed source populations for such bodies)


I suspect small attendent moons that might form are attrited by several processes. Incoming impactors headed for the primary moon might collide and disrupt such objects on their way in. Gasses released during the impacts might also form temporary atmospheric drag effects to lower the little moons enough to contact the surface of the primary moon, this will also delete them.

Tidal effects of the primary moon as it accretes will intensify, and small moonlets around the Roche limit may be disrupted, also keep in mind, moonlets will not form inside the Roche limit anyhow.

Other tidal effects may effect moonlets above the synchronous rotation altitude. As we see today, earth's moon experiences a tidal effect that is slowly lofting the moon away from earth. In the past, with the earth's moon circling at a lower altitude, this effect was stronger. For a moonlet just above the synchronous altitude, such forces would be as strong as they are going to get. (here's where my math impairment really hurts). Would such forces loft the moonlet out of the moons Hill sphere within the age of the solar system? Much less than the age of the solar system?


I dunno.



Do I suspect such bodies are still around waiting, to be recognized as such?

Yes.


I feel Hyperion was a former moonlet of Titan. Its' amazingly battered surface recording the flux of accretors heading towards Titan as it formed we see today. Titan was able to 'spin off' Hyperion out of it's Hill sphere, but Hyperion remained in Titan's vicinity and eventually wound up in a 4:3 resonance with its' former host.


Such objects as Methone, Calypso and the rest of the small moon Trojans may also be such former moonlets. They having been 'spun off' long ago, their low speed as they emerged from the moons Hill sphere having allowed them to be captured into a Trojan relationship with their former hosts.



( I am aware Occam's Razor would indicate that since we don't see moonlets today, the moonlets probably never existed, but we do have some interesting bodies about, (Hyperion, Methone, Calypso, et al) and perhaps an open attitude as to where they came from might be warranted. )



So back to your question, it is possible moon formation never or very, very rarely generates a subsidiary body, but it is also possible that such objects do form in significant numbers, but they are subject to forces that either destroy them, or moves them around sufficiently that we do not percieve their origins . . . .
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tasp
post Sep 9 2006, 02:32 PM
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QUOTE (TritonAntares @ Sep 6 2006, 03:51 AM) *
Hi,
let us anticipate the equatorial ridge was built up by an ancient ring orbiting Iapetus.

How likely is it now for CASSINI to detect any remnants of this former ring structure?
Or put it the other way - is it possible for a ring to vanish completely without leaving any dust particles in orbit?

Bye.



I really don't expect much (any) material left in the Iapetan equatorial plane. Effects such as Poynting Robertson drag, drag from the solar wind and also Saturns' magnetotail will all disrupt orbital stability of materials in that location.

Wouldn't hurt to look of course, though.

A recent outer retrograde satellite might have thwacked Iapetus and there might be some disorganized debris in the Iapetan Hill sphere loitering about . . . .


Note, I strongly suspect Iapetus is about as dead geologically interiorly as a satellite can get. Any Enceladosian style interior heating would spherize our little lumpy friend in short order. Probably not a good thing for long term retention of an equatorial ridge system up to 20 km high. . . . .
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