Wing Ip just had an interesting Iapetus-related paper published in GRL.

okay, now a ring around Iaptetus is an interesting proposal.

Here's an interesting passage from the concluding paragraph:

QUOTE

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.

Does the paper address why the equatorial ridge doesn't go all the way around Iapetus?

Michael

And why would the decay of such a ring produce a ridge rather than a chain of impact craters?

Would this process also explain the albedo assymetry on Iapetus?

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

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.

I doubt it.

I don't think this is 100% confirmed. We don't have enough coverage yet to for sure.

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.

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.

There was a brief blurb about this paper yesterday in EurekAlert.

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.

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 . . . .

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

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 . . .

I think I understand the idea to be one of a low-velocity deposition of debris onto Iapetus' surface. But even if the impact of a ring or ring fragment deposited a line of material, why wouldn't it slump to the level of the surrounding "plain", either from Iapetus' own gravity, or from the repeated impacts which have visibly struck the region? We have some pretty good side views of the ridge, and it seems to me that it's both hard and well bonded to the surface of Iapetus; impact craters that overlap it, instead of simply splatting a soft material down to the level of the original surface, appear to me to be tilted along the slope. Is this really possible for a feature created by a rain of debris measuring in metres rather than kilometres?

Under the scenario proposed I would expect volatiles within the descending ring material to produce a temporary atmosphere around Japetus. Could this have provided sufficient drag to melt some of the infalling ices so that they fell as torrents of freezing rain or slush, solidifying pretty quickly on contact with the ground? I imagine this would produce a pretty hard ridge - a pile of 'cryolavas' rather than loose 'cryoregolith' - that would respond to subsequent (post-atmosphere) cratering similarly to the rest of the moon.

My guess is that due to Iapetus' weak gravity and thus low orbital velocity and also the fact it was probably spinning much more rapidly in the past, the impact velocity would be quite low as others have suggested. Probably too low for impact melting. If impact heating would be low, I'd guess the gentler drag through an atmosphere would heat up the particles even less, giving them time to cool. So once again, no significant melting. The stuff would fall down as it entered the "atmosphere", as dusty material.

I know these are just qualitative speculations with no hard numbers to back them up but I'm not convinced by this no-melting argument. There is no way of knowing the temperature or thickness of any temporary atmsphere formed and sustained by a catastrophic process like this. I would expect an atmosphere formed in this way to be hottest at the top and coolest at the surface (with possibly a very strong temperature gradient: note that Titan's upper atmosphere is remarkably warm even now). It would certainly have had a huge scale height due to the low gravity, probably extending out to the inner edge of the ring. Much would depend on the rate of infall of material. However it would surely have been significantly warmer than the current temperature of Japetus, conceivably warm enough to melt at least some volatiles even without the additional heat from friction. Add frictional heating and it's not hard to imagine a slushy equatorial blizzard growing the bulge layer upon layer, sort of stalagmite-fashion but by freezing rather than mineral precipitation.

The point I was trying to make is the greatest temperature rise an impactor will get is a sudden surface impact so all kinetic energy is instantly converted into heat, with no time to radiate that heat away. A prolonged drag through an atmosphere will leave an object more time to lose the heat and thus remain cooler. I was implying no impact melting --> no atmospheric friction melting.
Wikipedia suggests Iapetus' orbital velocity is 430 m/s. I'm really not an expert on whether or not that is enought to vaporize/melt ice at cryogenic temperatures.

Yes, but it would lose it's heat to the atmosphere which is warmed up thereby.




A very simplistic calculation (using half m v squared and the room-temperature specific heat of water) suggests not. However we are not talking about pure water ice but probably a complex mixture/solution/clathrate material. You don't have to melt every grain to fluidise the material, only the more volatile constituents. Also we can't assume that a catastrophically created atmosphere would necessarily be at 'cryogenic temperatures'.

One other point - the 'catastrophic atmosphere' would be very dusty and therefore opaque (even if quite tenuous) thus greatly reducing radiative heat loss, so almost all the kinetic energy released by the aerobraking of the infalling material would be trapped as heat in the upper atmosphere.

Invoking an atmosphere above a certain very low density will create havoc with the orderly linear progression of the emplacement.

You would wind up with a debris belt all the way around the equator if atmospheric drag forces exceed the magnitude of the differential dynamic spreading affect in the ring materials themselves.


Additionally, due to the enormous volume of space a proto Iapetus would have had to sweep out to accrete itself (and of course its' relatively low orbital speed, too) I think we can infer that heating effects and subsequent melting of Iapetus was uniquely low for a significant moon in our solar system. The 'lumpy' limb of Iapetus seen in the Cassini images also infers a great bearing strength for the Iapetan crust.

Due to its' distance from Saturn, Iapetus would also have experienced a uniquely low rate of tidal heating as it de-spun to tide lock with Saturn. Iapetus had a uniquely ridgid and sturdy crust very early in the game, allowing us the chance to observe some of the most apparently ancient surfaces features yet seen.

I also think we can assume the ring entirely deposited itself onto the Iapetan surface. While the ring spreading effect would tend to 'loft' some material at the high side of the ring system through the Roche limit were it may have had the opprotunity to 'clump up' as we see in the outer reaches of the Saturnian rings, we must also consider drag effects that would have acted on the entire ring system.

Poynting-Robertson effects would have sapped orbital energy from the smaller particles across the ring system, and drag forces from the solar wind and perhaps even the Saturnian magnetotail would have provided a resistive medium for the ring system.


We also note the steepness of the sides of the resulting ridge structure on Iapetus. While I am not an expert in anything, it seems the sides of the ridge are plausibly at the angle of repose for materials deposited from above.


I also cautiously and with all due respect note some of the test footage shown on NASA TV of ice impacts on wing structures during the Columbia accident investigation. Ice was fired at the test samples in a speed range not too far short of the possible touch down speeds of ring materials onto Iapetus. To my untrained eye, (even though the tests appeared to be conducted at room temp and not at -300 F) it appeared the ice did not appreciably wet the surfaces it contacted. Rather, it just tended to pulverize into 'snow'. (in fact, one could see the pulverization occured at the instant of contact, the speed of sound (and fracturing) in the ice being so much higher than the impact speed).

Cryogenically frozen water ice (a plausible ring material) smacking a cryogenically frozen surface in a speed range not exceeding 1500 kph just isn't going to melt much (or vaporize) at impact.



Note, some gas/particle spray liberated at the point of contact will interact with the materials still orbiting above that point. Any material passing through that area of 'spray' will not complete another orbit of Iapetus and will land downrange along the ground track. This is why the main ridge (and the 2 attendants too) slope down away from the high end.

And this, consider the oblique impactor that may have lofted all the ring forming materials originally. Just as we note with the impact that apparently formed earth's moon, the ring materials of Iapetus would have been significantly reduced in volatiles by the impact. Water ice, silicate dust and other (cryo)refractory materials would be all that's left to make the rings.

I'm happy with very low density, but also very tall, opaque, comparatively warm and saturated with volatiles in vapour form. Result: Dirty freezing rain on the equator.

Note the following sentence already quoted by AlexBlackwell from the original paper:

"...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."

I don't think I'm too much out on a limb here in saying that this boundary layer would be accompanied by a tenuous atmosphere that would presumably surround Japetus, though thickest in the ring plane, and that this atmosphere would reduce the rate of heat loss from the system. If the remaining segments of the equatorial ridge have behaved like the rest of the moon over the aeons since the ridge formed then they cannot be loose sandpiles resting at the 'angle of repose'. Precisely directed liquid precipitation can leave deposits whose sides are more than steep enough - witness the process of stalagmite formation - and the result is a nice hard material.

I am having trouble seeing how the precipitating linear stalagmite idea could generate 2 perfectly (less subsequent cratering damage) symmetrical diverging attendant ridges . . . .

Having an equatorial ring 'synced' up to the highest pre-existing spot along the Iapetan equator for its' emplacement generates a structure very similar to what we see. Having the highest spot along the equator either penetrating the ring plane twice per revolution, or having an inclined feature in the ring encountering that same high spot while it sweeps out a ground track +/- a few degrees either side of the equator explains the attendant ridges nicely.

Watch the ground tack of the ISS on NASA TV. Imagine dropping something overboard (neglecting orbital velocity effects to make the point) every time the ISS is directly over a fixed point in the equator. When the ISS is passing N to S across the equator, object always lands along the ground track S of the equator, for the passes S to N across the equator, the item lands along the ground track to the N. You get 2 piles diverging (at a 55 degree angle to the equator in this example, rather than the few degrees as we see on Iapetus) away from the fixed point on the equator.

The 'off ramps' are the 'smoking gun' for the emplaced ring material idea, its' the only way to make such a structure without a bulldozer.


--<--------------


(extend the < arms with your minds eye if you will, I have very limited CG skills )

You mention "attendant ridges". This was my first impression of the "belly band" too, that it consisted of more than one parallel ridge.

However, looking at some of the New Year's 2005 images again, I think my eyes might have deceived me; that, in fact, breaks in the ridge near the moon's edge tricked me into thinking there were multiple ridges.

Are there any images of Iapetus that unambiguously show a multiple ridge structure?

On multiple ridges - perhaps the emplacement of the ridge gradually unbalanced the moon causing small step changes to new, more stable rotation axes.

On the depletion of volatiles presumed to have been caused by the collision event that originally lofted the ring material - depletion is not the same as complete elimination and only a tiny fraction of volatiles is required to form the temporary atmosphere.

On the origin of the ring - how do we know it was 'lofted by collision'? Japetus might have formed as a double moon. The smaller body's orbit would probably be quite unstable and one of the end scenarios for it would be early break-up at the roche limit to form a ring. In that case both bodies might be expected to have been a bit warmer before the ring-forming event due to tidal heating, but there would be no massive loss of volatiles when the rings formed.

I admit a certain favoring of an oblique impact knocking ring forming material into Iapetan orbit. Invoking a similar scenario to that which formed earth's moon (and plausibly Charon too) didn't seem too unlikely.

That a possible Iapetan ring system would be formed from the tidal disruption of a sub-satellite is quite feasible.

Early tidal interactions between Iapetus and a possible sub-satellite {hope the IAU doesn't zorch me for invoking apparently unapproved terminology } are interesting to think about.

I suspect any such object forming above the synchronous rotation altitude for Iapetus at the time would experience an accelaration force (like our moon today does) and would have been eventually 'lofted' into the void (or at least out of Iapetus' Hill sphere). {Perhaps Hyperion is such an object cast adrift by Titan, also the Dionean, Tethysian, and Enceledosian Trojans, too}

Sub-satellites of Iapetus below that altitude would experience a drag force and be subject to disruption inside the Roche limit. Additionally, such close in bodies maybe at further risk of disruption by the 'late accreters' still forming Iapetus. {perhaps the batttered surface of Hyperion records such an epoch from it's possible near Titan 'spawning' grounds} Iapetus having to sweep out such a large volume of space to acrete itself relative to the other Saturnian moons, I think Iapetus took longest to form.

{Note, synchronous rotation about Iapetus today is not possible, it's Hill sphere does not extend out to the ~80 day orbit altitude, but in the distant past, prior to Saturnian tide lock, Iapetus would have had a 'Clarke' orbit at a reasonable altitude.}

I think the great distance of Iapetus from Titan and Saturn would have provided the possibility of unusally stable and long lasting orbits. I note the difficulty of spacecraft orbiting the earth's moon at low altitudes without station keeping ability and their rapid demise smacking the lunar surface, such as the Apollo subsatellite did. Iapetus would have a 'smooth' and clean gravitational field up close (less any masscon effects) for possible sub-satellites.

This discussion has become really interesting. I'm a complete interloper in this thread and I haven't even studied the Iapetus images properly but I was intuitively uneasy about the accumulating sandpile idea and just wanted to see how far we could get with an alternative. I hope some of the others join in too. Where for example is Richard Trigaux? I'm sure he would have something to say about this. In the meantime can I repeat Rob Pinnegar's request for a quick link to the best pictures showing multiple ring ridges and/or diverging chevrons - anybody got them handy?

What you do not see, even in the high speed film clips, is how much ice is immediately vaporized - perhaps some of it quickly recrystalizing. It is a substantial amount (I will try to find a quantity). During these and similar test, quite heavy steal supporting brackets were deflected and bent.


I have to wonder if this is true. When F-16's collide with the desert at similar velocities, they expect the remains of the pilot to weight 18-25 lbs - if there is no cockpit fire. Virtually all of the liquids - water, uncontained oils and fuel - are immediately vaporized.


I like your analysis of the 'bulldozer effect', but I don't see water at any temperature as the source of this ridge deposit - if so, it should look more like drifting snow than Paul Bunyan and his plow.

Why? Are you assuming that environmental parameters at the time and place of deposition were pretty much as they are there today? If so that's a pretty drastic assumption - and just a bit too convenient in my view - given that we are talking about rather a large mass of material falling from space over an unknown period of time. I imagine something a lot more chaotic and violent, involving a wide range of temperatures and matter in all three states. At the very end of the process, yes, there was probably a fine hail of ice particles falling through near-vacuum onto a deep-frozen surface, but I think things would have been a lot more messy as the bulk of the material was coming down.

In such a situation there is too wide a field of possibilities and too much room for contingency, for example in the pattern of sizes, collisions and perturbations among the larger ring fragments, for a 'tidy' explanation that claims to characterise the whole process from start to finish to be convincing, IMHO.

One thing everyone seems to like (myself included) is the idea that the Iapetan ridge is indeed the remains of a fallen ring. Until this discussion I had not realised the dynamical implications of the remoteness of Iapetus from Saturn - that it could sustain a ring, and perhaps previously a satellite, of its own.

Just found John Rehling's rather nice diagram illustrating the remoteness of Iapetus:

http://www.cs.indiana.edu/~hfoundal/ast/planmoon.htm

Hi,
I've just been back from a 1-week-vacation - and the first I read is this new thread....
I'm really upset about this theory on the origin of the equatorial ridge of Iapetus...

I must check the last 33 posts here in detail now...

Bye.

a) Yuck.

Those liquids are already liquid. How are they "fleeing" the scene? It's not necessary that they be raised to the boiling point, is it? If a water balloon were struck by a bullet, much of the water would "depart" the vicinity, but I don't think any of it (or almost none) would actually boil.

Would ice have to be made gas to vanish from a microimpact on Iapetus, or merely liquid? That's a different phase transition, in the first place, and one that must take place, while the jet impact case may not require any at all. The heat of vaporization of water is much higher than the heat of fusion, but overall, it's a lot more energy to boil H2O that starts at saturnian temperatures than at body temperature.

I think this needs a lot more thought to be a serious comparison.

Yes, Aside from assuming that the belly band started as some kind of ring or dissintegrating moon structure.

There are as many potential pitfalls in working backward to find a cause, as there is to working forward to predict an effect. Cassini is doing everything it was designed to do, but we seem to be finding more questions than answers. That's not bad, but it leaves us two options: Make some assumptions and try to prove them wrong on based on the evidence that is here, or wait for another mission.

We need to find better solutions than 'dark stuff' and water ice. We need to narrow down the list of materials by taking a hard look at the physical properties and eliminating what does not fit. What defines the surface besides color? Why are the surfaces of Titan and Iapetus so different from Enceladus and Hyperion?

There is another option: to try to sketch out the whole space of possibilities opened up by the observations rather than placing bets on a single winner prematurely. A future mission will have a lot more than one hypothesis to check out.

On the scale of entire worlds and their histories nature is profligate with ways and means, not economical. We cannot just proceed as we would in a laboratory experiment by looking for the single most 'elegant' or 'economical' explanation.

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?

Therefore a Iapetus ring model must be developed to show how such a ring is behaving over ages.
Iapetus - as most distant large moon of any large planet - should be able to retain a ring for a while.

For mass assumption the ridge mass should be adopted.
There is still the question whether the ring was built up by one major event,
e.g. a large impact like the 'snowman' or the huge southern bassin,
or did Iapetus act like a sort of 'vacuum cleaner' in Saturn's outer vicinity
to collect all kind of interplanetary debris?

Bye.

The ridge is ancient which means the ring is also ancient. Any material left in orbit would probably be perturbed and dispersed away over a long course of time. I wonder how solar light pressure would evolve the ring particles, gently pushing on them. The ring system was probably very unstable, given Iapetus' weak gravity and probably pretty significant perturbations by Saturn (It's conceivable that after the impact that created the rings, Iapetus' rotational axis wasn't perpendicular to the orbital plane as it is today so Saturn's perturbations might have played a role. This along with the increasing "bulginess" of Iapetus itself would make stable orbits pretty hard to achieve.) I wonder if the impactor would melt Iapetus completely. If so, would it wind up undifferentiated now as it's suggested?
There's also the question of meteoroid bombardment on the rings and scattering them away. Of course, all this is arm-waving without any real numbers to back it up.

Personally, though, this ring idea seems too far-fetched for me.

BTW, Cassini did a pretty entensive search for moonlets during a recent fairly close approach. AFAK, no objects or ring remnants were seen.

Highly unlikely. The ring episode must have been very early, and possibly quite brief in geological terms, as there has been much heavy bombardment since. I would be very surprised (though delighted!) to find anything still in orbit around Iapetus.

A comment on followup missions: Whatever future missions head to Titan, Enceladus, or Saturn are going to have to cross Iapetus's orbit once on the way in. In some cases, mission design would allow an opportunistic single pass by Iapetus, perhaps quite closely, while costing essentially nil in terms of the rest of the mission.

I have little doubt that a good optical survey of Iapetus's various longitudes would give us some definitive evidence regarding its mysteries. Cassini has one more look in store: It will be sufficiently close-up, but may (or may not) show us some of the less-interesting longitudes. If this one-look turns out to be uninformative, we will likely wait until some Enceladus/ring mission gives us one-look somewhere else. As has been mentioned elsewhere, it might be possible to get Cassini to perform another close encounter of Iapetus, but the cost might be rather extreme, and I doubt it will happen. We'll get the answer next year or not anytime soon.

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.

This is the simplest case, but I hope you get the picture. Am I grossly in error here?

Not to me! In fact it seems obvious now, and I'm kicking myself for not thinking of it first. In the chaotic early days of the solar system it's quite likely that almost every globe that wasn't too close to something else would have hosted transient rings from time to time. That includes all the major planets and perhaps quite a few of the remoter moons, perhaps also KBOs. The question is which worlds would be likely to preserve the evidence? Clearly there must be no resufacing, no erosion, and the body must be very rigid. That makes Iapetus the best candidate among the worlds we've seen close up, (though it might be worth revisiting the available images of Callisto with this in mind). The Kuiper Belt is a whole new ball game however. With so many binaries and multiple systems out there ring formation must have occurred sometimes, and the resulting equatorial ridges should be well preserved. How fortunate that we'll get our first look in just a few years' time . . .

M'kay... now that's interesting...

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 . .

Links to that place are not a good idea. Can people please find alternate images so I delete those links.

Doug

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:
Click to view attachment

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.

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. 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.