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On a ring origin of the equatorial ridge of Iapetus
Guest_AlexBlackwell_*
post Aug 29 2006, 06:18 PM
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Wing Ip just had an interesting Iapetus-related paper published in GRL.
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volcanopele
post Aug 29 2006, 06:20 PM
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okay, now a ring around Iaptetus is an interesting proposal.


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Guest_AlexBlackwell_*
post Aug 29 2006, 06:25 PM
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Here's an interesting passage from the concluding paragraph:

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An additional important strength of this model has to do with the equatorial location of the ridge system if it is indeed related to a ring remnant. The discovery of satellites around some of the largest Kuiper belt objects might indeed be used as supporting evidence of this new idea [Brown et al., 2006]. We venture to propose this scenario because it could potentially throw new light on the origin of Iapetus as well as satellite formation in general. For example, the ring formation might have been related to the inclined orbit of Iapetus (with i = 7°) against the local Laplacean plane which is very different from those of all other regular satellites (with i ~ 0°) of Saturn. Could this unique feature have originated from a heavy collision event leading to the formation of an accretion disc? We don't really know.
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Michael Capobian...
post Aug 29 2006, 06:40 PM
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Does the paper address why the equatorial ridge doesn't go all the way around Iapetus?

Michael
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David
post Aug 29 2006, 07:05 PM
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QUOTE (Michael Capobianco @ Aug 29 2006, 06:40 PM) *
Does the paper address why the equatorial ridge doesn't go all the way around Iapetus?


And why would the decay of such a ring produce a ridge rather than a chain of impact craters?
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jsheff
post Aug 29 2006, 07:06 PM
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Would this process also explain the albedo assymetry on Iapetus?
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Guest_AlexBlackwell_*
post Aug 29 2006, 07:44 PM
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QUOTE (Michael Capobianco @ Aug 29 2006, 08:40 AM) *
Does the paper address why the equatorial ridge doesn't go all the way around Iapetus?


QUOTE (David @ Aug 29 2006, 09:05 AM) *
And why would the decay of such a ring produce a ridge rather than a chain of impact craters?

Given that I'm quoting liberally from the paper, I wish it were available biggrin.gif

QUOTE
As described in the classical diffusive accretion model [Pringle, 1981], collisional interaction among the ring particles will lead to the inward and outward radial dispersal of the ring material. If Iapetus possessed a thick atmosphere at that time, the ring particles would drift inward systematically because of gaseous drag. What kind of ring mass is required to build the ridge system? While the height of the ridge reaches as much as 20 km at some locations [Denk et al., 2000; Porco et al., 2005], there are also peaks only a few km high or less [Denk et al., 2005b]. Just for the sake of estimate, the maximum ring mass can be computed to be Δm = 2πRIΔw Δh ρ ~ 4.4 × 1021 g for Δw ~ 50 km, Δh ~ 20 km and ρ ~ 1 g cm−3 for water ice composition. This mass is equivalent to an object of 74 km-radius with a mass of 0.1% of that of Iapetus. A better inventory would require more complete information on the height distribution of the ridge system around Iapetus. The important thing here is that the impact site of the ring particles must be defined by the intersection of the ring plane and the satellite surface which is the equator. A possible consequence of the surface impact is simply that regions with prior ring mass injection would tend to intercept more material – at grazing angle - because of their greater heights. This effect might help to partially explain the non-uniform height distribution of the ridge system as mentioned above. On the other hand, local geological process plus cratering events could also contribute to the disruption of the equatorial ridge (T. Denk, private communication, 2005).

...The surface landing mechanism [of the ring particles] might be assisted by the formation of a boundary layer between the satellite surface and the inner edge of the ring system. The viscous heating in slowing down the ring particles would lead to the pulverization and even partial liquification of the infalling material. The bulk of the ridge system might then be built up bit by bit as a sort of sandpile but in a grand scale It is required that the surface of Iapetus should be solidified already at this stage. Otherwise, no trace of the ring remnant would be able to remain. In the final phase, the residual ring system would gradually disappear because of destructive bombardment and erosion by the interplanetary stray bodies.


QUOTE (jsheff @ Aug 29 2006, 09:06 AM) *
Would this process also explain the albedo assymetry on Iapetus?

I doubt it.
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Decepticon
post Aug 29 2006, 11:56 PM
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QUOTE (Michael Capobianco @ Aug 29 2006, 02:40 PM) *
Does the paper address why the equatorial ridge doesn't go all the way around Iapetus?

Michael




I don't think this is 100% confirmed. We don't have enough coverage yet to for sure.
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JRehling
post Aug 30 2006, 01:07 AM
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Roughly speaking, I guess the fact that the ridge would be built up instead of torn down comes down to secondary impacts, and the fact that smaller impacts don't produce secondaries moving at orbital velocity.

Note carefully (!!!): Iapetus ring particles would not be coming in at interplanetary speeds. They would be coming in at Iapetus orbital speeds, and that's way less than even lunar orbital speeds.

I wonder if, even if the ridge and albedo blotch are not identical in their origin, if they might be similar. Let's say a lot of smaller debris rained down on the equator, and then thereafter one big "moon" hit where the Snowman is and rained dark stuff downrange (east to west). The basic mechanism would be stuff in Iapetus orbit decaying until it hits surface.
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tasp
post Aug 30 2006, 05:11 AM
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An object skimming the surface of Iapetus will have a velocity of ~1500 km/hr. (under 1000 mph)

For cryogenic water ice, (~-300 F) I suspect vaporization upon impact would be nil. There may be a 'blast of particles though.



Consider a ring around Iapetus (glossing over how it got there, for the moment).

What happens to the lowest orbiting particle?

It strikes the highest point along the Iapetan equator.

What happens to the second lowest orbiting particle?

It hits the first one.


You originally start to form a pile. At some point in time, you literally run out of room in the pile area (insufficient kinetic energy of the oncoming particles) and the pile starts to accrete only into the direction of the oncoming particles. (analogy-you plow snow from all over your driveway into one pile, eventually, your pickup is not powerful enough to cram the pile any higher, and if you keep shoving snow into the pile, always from the same direction, you will start to form an elongated pile, and the pile will lengthen into the path of the oncoming snowplow)

The orbitally decaying ring system will form a ridge. Ridge 'grows' into the oncoming stream of material. Process concludes when ring material is depleted.

Iapetus had enough ring material to make a ridge 90 to 120 degrees long.


An atmosphere is not necessary for the process to occur. Differential ring spreading (transference of angular momentum radially across a ring system is well understood. Check out the Planetary Rings chapter in The New Solar System. That chapter also outlines clearly how a cloud of debris in virtually any orbital inclination will eventually settle into an equatorial ring system.


That the ridge is observed to be non-continuous may be due to the ridge suffering subsequent cratering damage, or perhaps settlement due to crustal overloading.


The best Cassini images also show perfectly (less subsequent crating damage) symetrical diverging 'attendent' ridges (the off ramps). I cannot imagine any internal geological process that could make such elegant matched attendents. They are the 'smoking gun' for declaring an external process made the ridge.

(the attendants can be explained by a percentage of the ring system being inclined to the equator. Twice each orbit for the inclined material, it will cross the equator. Once, from north to south, and once from south to north. The high spot on the ridge 'synchronizes' the simultaneous equal deposition of material into 2 matched divering stuctures.)


(I suspect at the contact point with the high spot on the surface for the inclined material, a small debris cloud forms, and material orbiting above that spot at the instant of the lower materials contact will be subjected to a drag force and accumulate down range.)

-or-

As the material deposits into the ridge structure, a point is reached where the spin axis of Iapetus shifts a few degrees (like unbalancing a gyro with a small weight) and Iapetus starts preceesing underneath the still descending ring system, the high spot still penetrates the ring plane twice per rotation, and the symmetrical attendant ramp form as before.

-or-

A large (unrelated) impact occurs somewhere on Iapetus and it knocks the spin access off a few degrees. Effect is still the same as above.



Why don't we see ridges any where else?

Iapetus is the most remote from it's primary, satellite we have seen so far. Tidal effects (causing the ring system to not form in the first place on other moons) are uniquely low at Iapetus (also one of the reasons we will not see a ridge system on either Pluto or Charon).

Also, Iapetus plods along at 5000 kph (or is it mph?, doesn't matter) in it's orbit around Saturn. Iapetus probably accreted very slowly (compared to the other satellites) and perhaps had a solid crust far earlier in its' history than the other moons, the impact heat being reduced by the lower incoming velocities and longer periods to radiate the heat away. Also, Al 27 heating in the accretable materials of Iapetus had longer to radiate away.

Iapetus is the closest we get to a 'cold formed' moon considering its' size.


We may see some equatorial ridge structures on the larger KBO's, if there are enough of them that are spherical (think about it), remote enough, and had an originating oblique impact to orbit a ring forming debris cloud.

This post has been edited by tasp: Aug 30 2006, 05:15 AM
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Guest_AlexBlackwell_*
post Aug 30 2006, 07:53 PM
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There was a brief blurb about this paper yesterday in EurekAlert.
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Rob Pinnegar
post Aug 30 2006, 09:41 PM
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I guess this idea probably originated with those images of Pan from a while back. If this proposed ring actually existed, it must have been very long-lived to last beyond the main era of heavy bombardment (either that or it was impact generated towards the end of said era).

The only thing that bothers me a bit about this idea is that, looking back at some of the images from the New Year's encounter a year and a half ago, there seem to be places in the ridge where the structure shows multiple parallel linear features -- which seems a bit more complex than this theory would seem to allow. However, given that we can't really see what those features are, they can't rule out the idea.
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tasp
post Aug 30 2006, 11:29 PM
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QUOTE (Rob Pinnegar @ Aug 30 2006, 04:41 PM) *
The only thing that bothers me a bit about this idea is that, looking back at some of the images from the New Year's encounter a year and a half ago, there seem to be places in the ridge where the structure shows multiple parallel linear features -- which seems a bit more complex than this theory would seem to allow. However, given that we can't really see what those features are, they can't rule out the idea.



It is the identical diverging attendant ridges that to me fairly shout externally caused.

Not counting the subsequent random cratering damage, the 'off ramps' are perfectly matched in slope, length, start and end points, and angle (north and south) to the main equatorial ridge structure.

Additionally, the 2 diverging attendants (btw, they are not parallel) describe segments of a great circle about Iapetus. Extend them all the way around Iapetus with your minds eye, and they cross the equator 180 degrees around and return to their starting point at the high end of the ridge. How could an internal geological process do something like that? It can't. All orbiting bodies ground tracks follow either the equator, or if inclined, great circle paths. We have a feature that shows three ground tracks that can only originate from an orbiting causation.


Handy way to get an idea of the scale of the ridge is to observe the very top of a vigorous thunderstorm anvil cloud in the US midwest. Such features can sometimes hit 20 km in height, similar to the 20 km height of the high end of the equatorial ridge.

An amazing feature.

Good place to send another pancam equipped rover . . . .
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dvandorn
post Aug 31 2006, 12:04 AM
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Does Iapetus' ridge have to have been created by a ring orbiting Iapetus? Could Iapetus have traversed a ring strand (or series of ring strands) ejected from Saturn's early ring system while the moons settled into their various stable resonances?

-the other Doug


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tasp
post Aug 31 2006, 03:02 AM
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Maintaining focus (or collimation) of the strand through its travels through the Saturnian system would be a very difficult feat to achieve. Any plausible force acting to accelerate a strand or filament would disperse the materials.

Additionally, trifurcating the filament symetrically, and having it by chance emplace it self aligned to the Iapetan equator multiplies tiny probabilities.

The amazing collimation of a ring around Iapetus is assured by the tendency of chunky materials in randomly inclined orbits about Iapetus (lofted most likely by an oblique impactor) to collapse to the Laplacian plane in fairly short time spans. Deposition of the ring materials can result from the natural process of momentum transfer across the ring system once it has collapsed to the equatorial plane. (a better description of these two processes is in the Planetary Rings chapter in the excellent book The New Solar System.

Further enhancements to sharp, well defined rings around Iapetus is its' remoteness to other perturbing bodies. Distant Titan and Saturn would produce relatively small tidal affects across the diameter of an Iapetan ring system.

Orbital periods for materials just prior to contact with the Iapetan surface would be slightly short of 3 hours. I am not sure of the orbital period at the altitude of the Iapetan Roche limit.

I figured a while back that if the ring system depostied itself at 1 cubic meter per second, you could get a ridge system similar to what's seen (45 degree slopes, I forget the length and sloping heights I used) in around 350 years. I suspect the process was slower than that, but it gives some numbers to play with.


In the past, Iapetus would also have been the last significant moon of Saturn to achieve tidal lock with Saturn. It is possible the ring system emplaced onto an Iapetus rotating considerably faster than the once every ~80 days currently seen. Any increase in rotation rate for Iapetus slows the touchdown speed (<1500kph) for the emplacing materials.



We should be looking for a largish elongated crater on Iapetus, too. It being the possible oblique impactor crater that lofted the material that formed the rings.

There is a largish oval crater (axis about 45 degrees to the equator) with an interesting elongated central peak complex in the southern hemisphere on the eastern edge of Cassini Regio. Might be a good place to start calculating volumes . . .
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