On a ring origin of the equatorial ridge of Iapetus Options

[tasp]

Just noticed I did not address moonlets below synchronous altitude and above the Roche limit. With moons above the synchronous altitude experiencing an acceleration in their orbit due to tidal effects, it seems the moons below synchronous altitude would experience a deceleration from those same forces.

(although realize since the moonlet winds up lower, it still winds upgoing faster. Such is the weirdness of orbital mechanics)

I suspect tidal interactions among such moons and moonlets would be tough to calculate 'post mortem' considering. The degree of moltenidity of the primary and the rigidity of the moonlet would all affect how fast the orbital energy would dissipate.

I suspect bodies such as moonlets might be rather porous, rocky, dusty and void filled. I suspect such bodies might be especially efficient at being susceptible to tidal interactions.

[ngunn]

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

Me too - and keep at it, you're doing a great job! The answer to the question 'Would such objects have been more common in the early solar system than they are today?' is pretty obvious. Just like potential impactors that have not yet been 'swept up' they would be much more numerous. The solar system is inherently chaotic (since it consists of more than two bodies) so every kind of object has a 'half-life'. Science is about imagining scenarios as well as doing the maths, and we can help here.

Any sceptics left? or are they all too busy straining their necks to see into a big hole in the ground . . .

[tasp]

I am thinking we aren't going to see an Iapetan style ridge structure on Pluto or Charon, unfortunately.

Wafts of the Plutonian atmosphere up to the top of it's Roche limit will probably disrupt ring formation. If that isn't enough, Pluto and Charon, due to their mass and proximity, will mutually disrupt each others ability to form an orderly ring system.

Originating materials for a subsequent ring system will be perturbed enough to preclude their collapse to the Lapacian plane. Those materials will contact the surfaces of Pluto and Charon at random locations as their orbital eccenticities are bumped up by the mutual tidal effects of Pluto and Charon.

We see such effects in the vicinity of earth's moon, and apparently around Venus. Mass ratios and seperations of Pluto and Charon are inhospitable to ring formation.

Still plenty of interesting things for New Horizons to see though . . . . .

[tasp]

Speaking of New Horizons, how far out can it productively study a KBO? I understand 2 additional flybys after Pluto are possible, if we get real lucky and have a larger choice of follow on targets, how far out can we go?

It seems the larger objects would be preferable to the smaller ones, but if 2 objects are situated fairly close do we take the surer bet and visit them, or would we gamble on a much larger object right at the limits of New Horizons power supply and camera light levels?

With the fascination of the Iapetan ridge system, and only spherical (ie. large) KBOs having even a tiny chance of having a comparable ridge system, can we 'nudge' the acceptable maximum range of NH a tad?

(lower data rate back to earth, using the NH attitude control system to track the dimmer objects for longer exposures, etc. ? Voyager II learned new tricks on the way to Neptune, can NH do the same out to 200 AUs?)

[tasp]

Would this process also explain the albedo assymetry on Iapetus?

The dark staining on portions of Iapetus might be attributed to gaseous materials containing perhaps hydrogen, nitrogen, and carbon being introduced into the Iapetan vicinity at a fixed segment of it's orbit about Saturn.

Let's assume for the moment, that atmospheric leakage from Titan only encounters Iapetus when Iapetus encounters the Saturnian magnetotail as it traverses the far side of Saturn.

What would we see?

We might expect a very tenuous cloud of N2 and CH4 to persist around Iapetus for a while before it dissipates into the void.

What does the Titanian atmospheric constituents do when exposed to solar radiation of sufficient intensity?

They apparently tholinize.

Where do the necessary conditions for this occur on Iapetus?

The leading hemisphere, and within 40 to 50 degrees of the equator. Also, north and south of of that zone, crater bowls with appropriate slopes on their poleward sides will also collect and intensify the distant suns flux, causing dark spots outside the main 'blotch'.


Is this what we see?


I will suggest we do.

Aparently in the roughly 40 days till Iapetus passes between the sun and Saturn, the gas dissipates completely, or is tholinized completely (actually both processes probably occur together) so as Iapetus continues on around Saturn back around to the far side, that hemisphere now sunward doesn't darken.



Would we expect this wafting gas in the Saturnian system to evince itself anywhere else?

Perhaps the dark crater bottoms of Hyperion are dark for the same reason as Cassini Regio? The gas ponds in the craters, the distant sun is roughly focused by the crater walls and makes a warm enough spot for the gas to tholinize.

Why do all the crater bottoms of Hyperion seem to be dark? Hyperions' chaotic rotation eventually aims all its' craters sunward at one time or another, so they all darken.

BTW, there is one largish crater, seen clearly by Cassini, with a broken rim. It did not form much of a dark spot at all, perhaps because that crater cannot pond gas to tholinize as it runs out downslope . . . .

[tasp]

Regarding perturbations of a possible Iapetan ring system by distant Saturn and perhaps Titan, what would be the effect?

The ring system will self-correct any out of plane displacements fairly rapidly. The ultimate result of out of plane deflections would be a slightly increased ring thickness overall, and a bit more pulverization of the already pretty well fragmented ring chunks themselves.

I am not sure we need to worry about the out of plane deflections too much at all.


What about deflections in plane?

Well, the individual ring particles each have their own orbit about Iapetus, increasing their orbital eccentricities a bit will mostly enhance the already existing dynamical ring spreading process which transfers momentum across the ring plane via the individual ring particles bumping into each other. If they bump a little harder, or a little more often, to what effect? Ultimately, we might expect the emplacement rate on the Iapetan surface to be somewhat higher than in an identical system without the in plane perturbations.

I had calculated at one time that a 1 cubic meter per second deposition rate will make quite a heap in ~350 years, and even in the actual perturbed Iapetan system, I doubt we get the ring system down that fast anyhow.

It would be hard to see how the ridge system would be vastly different had it emplaced in 200 years or 2000.


So, I am not to sure that tidal effects from Saturn, Titan, or even the distant sun are going to have much impact on the evolution of a possible Iapetan ring system . . . . .



(hopefully my 'no math' approach (OK, a little math seeps in once in a while) to all this isn't driving the rest of you to distraction too badly)

[Michael Capobian...]

Well, I'm still a bit skeptical as well. For one thing, a spreading ridge of upwelling material often produces a multiple ridge like we're seeing in the close-ups. I would really love to see those detailed shots of the ridge superimposed in context on the map of Iapetus.

So, with the ring-emplacement, we have an interesting theory, but we don't have enough high-resolution images to really test it. Here are some observations after staring at these images far too long:

* The ridge extends from about 50 degrees to 210 degrees, only a little more than half way around Iapetus.

* It's visible only in Cassini Regio. There's no sign of it in Roncevaux Terra, some of which the Saturnshine images show at high resolution.

* Although at large scales there appears to be an equatorial marking at the Eastern edge of Cassini Regio, it doesn't show much topography, is primarily an albedo feature, and may be coincidental.

* The ridge is discontinuous in Western CR, which is not consistent with ring-emplacement. There is a large crater superimposed on it there, but would a large impact create the pattern we see?

* In the area of Cassini Regio west of the landslide basin, the ridge seems to peter out and stops entirely well before reaching the presumed ejecta blanket of the basin.


I think the 2007 encounter will resolve many of the issues here, especially the nature of the ridge in Western CR. If it actually is discontinuous west of the superimposed crater, that will be a very important data point. Not to mention the nature of the white mountains and how the dark material is distributed there. I still believe that a flyby for close-up imaging of the Snowman/Moat would be extremely valuable to understand the dark/light interface, and the likelihood of Iapetan internal activity. If there is a remnant of the ridge there, perhaps wiped out by the Snowman impact, it might help understand the ridge as well.

Michael

[tasp]

Of course, the most interesting bit of the ridge system (the highest parts) are right at the limb of the existing Cassini images. And there is a tremendous amount of subsequent cratering damage all along the ridge structure.

(oh, to have seen it shiny and new . . . . )


That the highest end is discontinuous is a concern. More cratering damage? Perhaps subsidence of the crust beneath, or collapse of the steeper walls occured.

More pictures are needed. And hopefully we get them in 2007.


Additionally, imagine with the minds eye if you will, the lowest edge of the ring, contacting the very highest spot along the Iapetan equator. As the chunks and particles impact the pinnacle, there will be a spray of pulverized ring material released at the contact point. Any material directed upward will interact (ie. decelerate) any material still orbiting in the ring system above the contact point at that instant.

Some of that material will fall to the surface, downrange of the contact point. Some of that material, even though very slightly decelerated, will still manage to complete one more orbit and return to the contact point.

Some of the material, though, will be decelerated into an orbit that is now eliptical. Depending on how close to the surface such doomed ring particles get, they may intercept other high spots along the ground track and accumulate. The lowest objects may get snagged quite aways around Iapetus, where as a particle in a slightly higher orbit might clear that obstruction, and nail another further along.

I think the ring system can emplace at a main primary point along the equator, but there could be a small percentage of materials that may accumulate almost all the way around Iapetus at more than one other secondary location.

Note, the majority of the materials decelerated by the spray above the conact point will fall to the surface along the ground track up to 90 degrees around from the highest spot along the ridge, and this is why the ridge smoothly slope downward from the high spot. Even the 2 nonparallel diverging ridge attendent structures do so.


If one could look down upon either pole of Iapetus and see the silhouette of the equator, the highest heights would all approximately describe an ellipse of elevations around Iapetus.

We also would not expect to see ring material deposits in large deep basins (such as the famous landside basin photographed by Cassini earlier in the mission), there being no eliptical path from the contact pinnacle to the floor of the basin that does not intersect an obstruction along the way. Where the ridge structure is seen, and where it isn't visible, all tell us how the material was organized as it emplaced.

An internal geological process could not be expected to generate structures with all these characteristics. A lack of discontinuites, fissures, or ridge like structures in the deep landslide basin aligned with the equator is also telling us much about the origin of this amazing ridge structure.

[ngunn]

So much to reply to (no, it isn't driving me to distraction - far from it) - but only a few minutes available. On no ridge rings on Pluto or Charon - I agree, unfortunately, but there's always an outside chance since we don't really know their histories. Anyhow Pluto will get one when Charon disintigrates.

On the intimate details of ring emplacement - very good, I'm almost buying it. However I don't think the basic thesis would be under serious threat even if your details turn out to be wrong.

I'm going to go away and extend my d cubed times m over M table to some more objects, like Ceres and the moons of Uranus. Will post anything interesting I find.

[tasp]

On the intimate details of ring emplacement - very good, I'm almost buying it. However I don't think the basic thesis would be under serious threat even if your details turn out to be wrong.

I don't know how thoroughly I have cosidered this topic (I don't know what I don't know, after all) but I have gone over quite a few variations of all this.

Before I encountered and understood the differential ring spreading process as outlined in the New Solar System book, I labored trying to figure out a mechanism to get the ring down on the surface of Iapetus. I considered Iapetus' remoteness from perturbing bodies to imply an extremely long lived ring system. Cool if we want to see one in this epoch, but it doesn't help explain the pictures we have now. I put a great deal of effort into establishing that Iapetus is not to far off the mark from Triton and Pluto to have had an extremely tenuous atmosphere eons ago when the sun was somewhat dimmer. The atmospheric drag emplaced the ring. Unfortunately, that generates a feature that doesn't look like the ridge system on Iapetus.

I have also spent time gyrating the center of mass of Iapetus during the ring emplacement. I get thinking of precessing the axis of a gyroscope as analogous to Iapetus unbalancing during emplacement. The ring system just isn't massive enough, and/or emplaced quickly enough to do anything bizarre to the Iapetan spin axis. While interesting to think about, it doesn't really help us understand how the ring descended and emplaced the ridge structure.

I do have some technical trades backgrounds, and frequently thought of lathes, specifically, machining the inside of a cylindrical piece of metal. Interesting analogy, but tough to explain on a messageboard.

The diverging attendent ridges I consider to be the 'brass ring' here. Explain them, and you have the whole thing nailed down air tight. Realizing that inclined structures in the ring could emplace just like that, and alternatively, that Iapetus could have it's spin axis deflected by a few degrees during emplacement (perhaps by a big impactor) and generate the exact same feature again was fun.

The ring idea seems to tell all actually.

The ring particles actually orbited in the same direction as the ridge tapers downward. (Assuming the pics aren't inverted and I understand the orientation of everything here, as seen on the map earlier in this thread, from left to right)

It would have been possible, and perfectly safe, to stand directly beneath the ring during emplacement in what I call landslide basin. One could have actually approached the ring fairly closely from the sides too, but having meter sized boulders whizzing past your face at just under 1000 MPH would have been a bit scary. Even discerning motion in the ring system would have been difficult from far enough away for the unaided human eye.

I am thinking the rings were white, or light colored. And the fine grooving we see in Saturn's rings would have been greatly diminished here, or absent entirely. Having the inner edge of the ring descending so low to the surface (no corresponding dusky ring analog here) would have presented a very different appearance from Saturn's rings. (and to my aesthetics, less visually appealing, they would not seem to be floating above the moon, but rather resting upon it).


Due to the pretty intense 'meat grinder' effect of lofting the materials in the first place, then collapsing to the Laplacian plane, followed by the incessant bump and grind of the dynamical ring spreading process, I suspect the ring particles to have been pretty uniform size wise, although I don't know if we are talking walnut sized chunks or Volkswagen sized chunks.

The upper edge of the ring, I feel, was rather constrained at or near the Roche limit. Drag forces would have always been sapping energy from the ring system, and the ring spreading process would have worked primarily on the low side of the ring, not the top. We wouldn't have seen any little moons spun off the high side. And as it follows, the entire ring system would have eventually emplacedonto the Iapetan surface.


The absolute fastest the ring system could have emplaced, would, I feel, be in the hundreds of years range. There are some formulas for figuring ring spreading speed, but I will let others chew numbers for this. Heck, an upper limit for ring emplacement time might be up to many thousands of years, but not millions. That some 'straggler' chunks in the ring plane might have persisted for longer periods isn't too important, this all happened billions of years ago (no I am not going to do crater counts to calculate surface ages, either) anyhow, and are nowhere near as interesting as the main phase of the emplacement process.


What a fun moon this has turned out to be. Hope Allen gets lucky and photographs some especially weird and bizarre Kuiper belt objects!

[ngunn]

IThe absolute fastest the ring system could have emplaced, would, I feel, be in the hundreds of years range.

How do you know this? I still think its possible that is was fast and messy in the early stages, forming the main bulk of the ridge out of darker, hard material. The very narrow and beautifully straight lines on top would still have formed cold and dry in the manner you suggest as the highest parts of the ring spiralled in more slowly - and of course they are lighter coloured. You've spent much longer on this than me and I agree that your hypothesis handles the longitude gaps better IF you're right about the topography. Lets hope we get some clinching evidence before the end of the mission.

[JRehling]

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.

Just occurred to me: Suppose Iapetus anciently had a significant rate of rotation in the past. If it rotated in ten hours, that would give the equatorial surface a speed 29% of orbital velocity. So the impacts of ring particles would be about 70% what we've previously supposed, thus with only half the kinetic energy and half the heat energy.

Not that we'd ever dotted the "i"s and crossed the "t"s yet, but that's a possible factor regarding feasibility.

[tasp]

Precisely.

Iapetus, due to its' distance from Saturn, was tidally braked into its' ~80 day rotation very slowly. It's initial rapid rotation at the cessation of the accretion phase would have persisted far longer than the moons closer to Saturn managed.

I haven't bothered to calculate heat of fusion, kinetic energy, etc. for water ice at -300F and 1000 mph, but a little vaporization isn't going to screw up the emplacemnet. And liquid water introduced into the frigid Iapetan environment is going to freeze pretty quickly also.

The farther back in time the emplacement occured, the slower the relative touchdown speed of the emplacing ring material.

**************************

My estimate for the minimum time to emplace the ridge was to calculate the volume of a ridge 10 km high with sides sloping 45 degrees that was, I forget the exact number, so many kilometers long. ( I just tried to average out everything to make the math easier )

Worked it down to cubic meters, and divided out 3600 (secs/hour) / 24 (hours/day) / 365 (days/year) and got 350.

So a 1 cubic meter per second emplacement rate gets you an Iapetan sized ridge in 350 years.

I don't think it emplaced anywhere near that fast, but at least I had a number of some kind for the minimum time to do it that I could ponder.

The ridge is going to look the same whether it formed in 350 years or 350,000 years after all . . . .


I know once the ring system thins out a bit, the 'bump' process slows down, but solar wind drag, Saturn magnetotail drag, and Poynting/ Robertson drag effects are always sapping energy from the remaining orbiting particles, so an equilibrium will be reached with the 'bump' process, drag effects, and emplacement always running at about the same rate till all the orbiting materials are depleted.

The whole ring system winds up on the surface of Iapetus, and the process looks like it will run at a fairly constant rate (although I don't know the exact value of that rate) till all the orbiting stuff is used up.



It would be interesting to compare the volume of the ridge to the volume excavated from that big elongated crater in the southern hemisphere on the edge of Cassini Regio . . . .

[tasp]

Determining elevations on Iapetus is going to be tricky. We have a moon that is kinda squished and irregular, even a little bit lumpy. I also suspect the apparently ancient crust is quite rigid to great depths, and has been so for a very long time. With that kind of crust, equivalents of our own moons' masscons is quite likely.

What kind of accuracy can we expect in the Cassini data? Granted the 20 km height of the high end of the ridge is impressive, but is the region of Iapetus around there generally a lower or higher area than the average?

Also, I am fascinated with the 'landside' basin. But is that area generally higher or lower? Maybe the basin isn't really all that deep (radius to the center of Iapetus speaking) or perhaps the surrounding area is already way below the average making the bottom of the basin perhaps having a different composition (like upper mantle equivalent).

Hope some of this gets pinned down with the close flyby in a year . . .

[tasp]

Found some numbers I crunched a while back on the ridge system.

The total volume is (very roughly) around 10,000 cubic kilometers.

If all in one chunk, it would make a sphere ~27 kilometers across.

This 27 km sphere would scarcely have surface gravity. ( similar to Phobos or Deimos perhaps, less density, more volume, probably pretty close)

If spread in a ring system around Iapetus, from skimming the surface, to the top of the Roche limit, I am having trouble imagining this much material generating much in the way of self-gravitational effects. The predominant effect on the ring system will be the gravitational field of Iapetus.

Tidal effects of Saturn (and the sun, too) are, IIRC, proportional to the difference in the gravitational field from the close to the far side of the object of interest. How much will Saturn's gravity field strength fall off across the diameter of the Iapetan Roche limit ( <5000 km) at the 3,000,000 km distance from Saturn? Not much, especially compared to the Satellites interior to Titan.