Wing Ip just had an interesting Iapetus-related http://www.agu.org/pubs/crossref/2006/2005GL025386.shtml published in GRL.
okay, now a ring around Iaptetus is an interesting proposal.
Here's an interesting passage from the concluding paragraph:
QUOTEAn 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
Would this process also explain the albedo assymetry on Iapetus?
QUOTEAs 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.
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 http://www.eurekalert.org/pub_releases/2006-08/agu-ajh082906.php 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.
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.
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.
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.
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?
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.
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.
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.
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
That's better
We used to worry about mentioning words that would raise this places googlability.
However - I've since looked at how google works - and it's by links. The more links TO a particular site that exist, the more popular it is considered by google and the higher up the list it goes. Hence I really don't want to link to 'over there' because it's a source of missinformation and lies and not something I want to see climb up the google ladder.
Astonishingly - this place rates very very highly with google...clearly lots of people like us
Doug
OK let's see if this works..
http://www.aaw-darmstadt.de/bilder/japetus%20N00026247%20cr%20enh.jpg
Yes! - though I prefer it rotated 90 degrees clockwise. It clearly shows three almost parallel ridges over part of the length, possibly due to a combination of constructive and destructive processes plus small adjustments to the rotation axis during ring-fall. I wish I'd been there to see it (but not too close).
3 intersecting ridges, all describing segments of great circles about Iapetus.
Not to belabor the point, but there is not an internal geological process that gives a whit about great circles configured this way. The feature 'shouts' external orbital causation. Consider precession of the rotation axis of Iapetus (or inclined structures in orbit above), syncronization of the deposition process as the highest spot on the equator passes through the ring plane twice per rotation. The ground track per the relevant orientations of the ring, equator, highest spot along the equator, and precession (or inclined structure) is where the ring emplaces.
{Sorry for the crappy diagram}
---<-----
Some of the criteria that seem to be needed for us to see such a feature are
*ancient surface
*object needs to approximate a sphere
*object may need to be remote from other objects (our own moon does not seem to have low circular orbits stable enough for there to be enough time for a ring system to form and emplace. Materials in randomly inclined lunar orbits will contact the lunar surface prior to colapse to the Laplacian plane)
*object needs to have a surface sufficiently solid and rigid for the materials to emplace on
*object needs a very low density (prefer none) atmosphere or materials will drop all around the equator.
(we may see such structures someday, but they won't look like the Iapetan ridge formation).
*object needs to be somewhere an appropriate glancing impact can loft materials is likely to occur. Iapetus may have encountered an 'outie' satellite of Saturn or a displaced Saturnian Trojan object
We may find Iapetan like ridge structures on a percentage of the larger KBOs.
Not sure NASA wold mass produce New Horizon clones and start launching them en masse to the outer solar system ( [laugh] ), but it would be interesting to have a few more examples to study.
Hi,
before we should keep on speculating whether the equatorial ridge was built by an ancient Iapetian ring,
we should take a closer look at the ridge, e.g. if there are older structures below it - what could answer its built up...
Here again what I wrote half a year ago - and an interesting pic:
I have absolutely no idea on which internal process could create the ridge, but I am skeptical of the ring hypothesis mainly because I don't see it very likely that smallish Iapetus could have enough gravity to align the orbiting debris along its equatorial plane. Isn't that possible only due to the rotational bulge at the equator? Iapetus has a small radius so a 16 hour rotation wouldn't produce centrifugal forces as big as on a body twice the radius. The bulge would be lower. Therefore, orbiting debris would be unlikely to align into an quatorial ring that easily. An equatorial ring maintained long enough, even due to Saturn's perturbations? I also don't like the explanation on why the ridge isn't complete around the equator. And how does it all fit with the dark stuff? Why is the color of the dark stuff practically identical to Hyperion's color, only differing in albedo?
There's just too much magic fairy work here, IMHO.
Yeah, but why would it flatten itself along the equatorial plane if the particles were ejected on a random inclination? They don't know / don't particularly care about the equator.
This has been a fascinating discussion. Before Cassini arrived at Saturn and we got the first decent-resolution views of Iapetus there was the underlying and unspoken fear of finding a Monolith in the middle of things. And guess what? Mother Nature upped the ante and trumped us with this equatorial ring...
--Bill
PS-- equatorial ring AND ridge
Hello Bill - nice to know it's not just the 4 or 5 of us here exchanging hand signals! Ugordan I agree with you about randomly ejected material - that's why I prefer a disintegrating moon in an equatorial orbit for the source of the material.
Two things today:
Some where here at UMSF is a nice map of Iapetus (help finding it would be appreciated).
The two shorter segments of the ridges are not parallel to the main center structure.
The two 'attendant' ridges diverge smoothly and symetrically from the main equatorial ridge.
They both are exactly the same length and height (not counting subsequent random cratering damage) and both follow segments of great circle paths about Iapetus.
The 'great circle' paths are the key to understanding the orbital emplacement of materials upon the Iapetan surface.
I will note that Iapetus is subject to the smallest tidal effects from Saturn of any of Saturns' moons. The tidal effects are also lower that for any of the 4 Galilean satellites of Jupiter, Triton, and I suspect most or all of the major moons of Uranus.
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)
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.
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)
Is this the one? http://photojournal.jpl.nasa.gov/jpegMod/PIA07778_modest.jpg
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 http://photojournal.jpl.nasa.gov/jpeg/PIA07778.jpg.
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.
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 )
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.
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 . . . . .
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?)
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)
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
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.
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.
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 . . . .
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 . . .
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.
Found some rough estimates I did (my fault if I slipped a decimal along the way) of what the27 km sphere would shape out as a disk.
A circular disk 112 km across, and 1 km thick, also, an 80 km disk 2 km thick.
Compare the disk sizes to crater volumes. Seems like there is no shortage of craters on Iapetus that plausibly could have been the possible 'source' crater for a ring system.
Keep in mind I have totally neglected the efficiency of an impactor in placing materials into orbit about Iapetus. Not sure how to proceed with that.
Would an oblique impact produce a spray of debris with a gaussian distribution of exit velocities? I dunno.
If it did, and the curve of velocities was from zero meters per second to perhaps something faster than Iapetan escape velocity, do we get the peak of the curve in the range of velocities needed to orbit Iapetus?
In this favorable scheme of things, maybe ~20% of the materials do what we want?
I have also neglected the fate of the impactor. Materials from the surface of Iapetus are going to be entrained along with the pulverized remains of the original impactor. Of course, the faster the impactor hits, the smaller it can be. As it's incoming speed increases, do we see a change in the velocity distribution of the ejecta? We might be able to come up with a minimum size of the incoming impactor if we look at at enough of the criteria. Far beyond my skills in any regard. Might stick with an impactor of around 27 km maximum size just for starters. Maybe someone can can get an idea of how efficiently such an object coming in at around ~5 degree angle to the surface can loft debris into orbit.
Also, what does the resulting crater look like? Elongated of course, but what kind of length to width ratio? 1:2? 1:3? Do we get an elongated central peak too? Hopefully we can get to the point where we have enough of this pinned down to go a little further than just saying the crater in the southern hemisphere on the eastern edge of Cassini Regio 'looks right'.
What was Iapetus' rotation rate at the time of ring formation?
OK, I don't have a number for this ( a guess would be 10-20 hours) but there might be a way to find out.
The impactor that lofted the ring material, also must have yielded a great many objects that did not achieve orbit (probably a pile of stuff at greater than escape velocity too), and all the slow stuff came back down on Iapetus.
If we can identify a population of secondary impact craters, and define the areal extent of the debris, we might be able to garner some statistics about the impacts. The ones on the shallower trajectories might also be expected to create elongated craters too, and the long axis of the craters will point back to the parent crater.
-but-
The point back angle will vary with distance from the crater. Iapetus might have been rotating on it's axis faster than once every ~80 days in that time period. The materials lofted further downrange will have had the Iapetan axis turn through a larger angle than the closer debris.
For materials destined to impact Iapetus 180 degrees around, the flight time will be just under 1 1/2 hours. If Iapetus rotated in 15 hours in those days, it will turn 36 degrees during that 1 1/2 hour period. This will also skew the footprint of the entire debris field.
If we can identify craters formed secondarily to the ring lofting impact crater, we could potentially know the rotation period of Iapetus in those days. The location and the orientation of that subset of Iapetan craters could yield an amazing piece of hard data.
Should an accurate timescale of the Iapetan tide lock / spin down period be known, (presumably this is a largish number), we might get a fairly precise date for the formation of the equatorial ridge structure.
Would this interesting bit of information be worth another Iapetan flyby during the Cassini extended tour?
Any chance we could discern the appropriate craters on Iapetus for this technique?
Thought of another way to get the symmetrical diverging attendent ridges, too.
{to reiterate, the attendant ridges might have formed as inclined elements in the outer ring system eventually came low enough to contact the existing 'high spot' along the equator. Or, during emplacement of the ring materials, toward the end of the process, Iapetus suffered a major (unrelated) impact that nudged the Iapetan rotation axis off a few degrees}
The new idea; there were two impacts that lofted ring forming materials into Iapetan orbit, seperated in time by virtually any conceivable amout of time consistent with the surface ages as determined by the crater counts on the various aspects of the ridge system. During the interim period of time, the Iapetan axis shifted a few degrees, either from another impact, or possibly long term effects on the Iapetan orbit of solar (or whatever) perturbations.
{it would even be possible in this epoch (send NASA lotsa money!) for us to nudge a smallish asteroid into a grazing collision with Iapetus, reloft some materials, and watch the whole ring forming and emplacement process at our convenience}
We can even run a slighty different kind of experiment and generate a variant type of ridge system.
Find a highly spherical object. Contrive a very tall mountain 5 degrees from the equator on this object. Have the grazing collision take place, and have the ring form. Perturb the ring to orbit inclined to the equator 10 degrees.
You now get a structure that does not have the central ridge, and the two diverging ridges that form will have unequal lengths, in proportion to the number of degrees off of the 180/180 degree equatorial sweep the current system shows.
If you contrive the mountain at 10 degrees, you again only get one ridge, but it will make a beautiful sweep
(~) relative to the equator. (it will still follow a great circle path though)
One aspect of the ring emplacement I have not covered in too much detail is just what happens as the ring particles contact Iapetus.
I will attempt to flesh out some thoughts I have had about this.
As the ring particles orbit Iapetus, various drag forces act upon the ring system (Poynting/Robertson, solar wind effects, Saturnian magnetspheric drag, etc.) and sap energy from it. These forces probably would not be terribly effective upon a solid object 27 kilometers in diameter, but since it is in a disconsolidated form with an enormous surface area, these forces are of sufficient magnitude to dissipate the orbital velocity of the ring mass in considerably less time than the age of the solar system.
As the ring materials collapse to the Laplacian plane, I expect them to be rather pulverized. A precise size might be a little iffy, but somewhere between bowling ball and Volkswagen size is my guess. Seems the physics of all this won't be affected too much by scaling effects. Also, while cryogenic water ice (best guess for the ring composition) is quite sturdy, I don't think the mechanical properties of the material are too different from ordinary materials we are familiar with.
So, what happens as the materials descend towards the surface of Iapetus?
The dynamical ring spreading process occurs, and transfers momentum from the 'low' side of the ring system towards the 'high' side. As a result the materials at the low edge of the ring, slowly descend towards the surface. But as they do this they maintain their circular orbits. (if you watched in time lapse, the paths would be a very, very tight spiral, but we can consider it a circle with out being to far off).
I expect the particles to be in a rough equilibrium with each other in regards to their individual rotation rates. That is, particles in the lowest orbits are going to be rotating individually in about 3 hours as they revolve around Iapetus in 3 hours. Various particles may jostle and bump, but for the most part, we can consider the lumps to be not spinning as they orbit.
How fast do they descend?
I consider an absolute minimum time to emplace the ridge structure at 350 years. This is a 1 cubic meter per second emplacement rate. The actual rate is probably vastly slower, and is regulated by drag effects on the ring system as a whole. 0.1 cu/meters per second implies deposition in 3500 years, and 0.01 cu/meters per second implies 35,000 years.
At the low deposition rates, we must also consider extremely slow descent rates for the ring system. Individual chunks of 1 cu/ meter size might be coming down once every minute and 40 seconds or so.
I had imagined in the higher depostion rates something occuring very much like bowling balls smacking and shattering upon a sturdy cliff edge, with still orbiting ring particles immediately above the 'splat'.
This might be too simplistic. At very low sink rates, the individual ring chunks will graze the highest spot along the Iapetan equator. Imagine, if you will, a 1 meter sized sphere of ice, traveling ~1500 kph intersecting an icy plane with only millimeters of intersection.
What happens?
Even cryogenic ice will experience friction, and the ice chunk, once it contacts the surface, will not be in orbit anymore, it will become a hockey puck. Its' forward velocity will assure it will continue on downrange, and the mounting deceleration forces and frictional contact with the surface will tend to break it up as it skitters along the ground. Additionally, you have cryogenic ice, contacting a cryogenic surface, in a cryogenic environment. Kinetic energy of the incoming lump is going to be dissipated over time and distance, and the materials (already thermo processed in the original impact event that lofted them into orbit in the first place) are going to experience low melting and micro vaporization.
Repeat a few zillion times, and you get a ramp shaped snow bank, dead straight, aligned perfectly with the Laplacian plane (equator), and tapered downwards from the contact spot downrange.
Another object with an equatorial ridge
http://www.news.cornell.edu/stories/Nov06/kw4.arecibo.html
Movies here
http://echo.jpl.nasa.gov/~ostro/kw4/index.html
Hmm...interesting, Alan, thanks.
I think that NH may make or break this conjecture after all. Pluto is probably the closest analog to the "ringed Iapetus scenario" we're likely to see in many ways: it almost certainly had rings at some point(s) after the event(s) that created its satellite system, and may still have them today. The satellite system can be considered a well-preserved dynamical artifact because, of course, there's no Saturn to mess things up. If Pluto has a fossil equatoral ridge, that would provide a strong argument for a formerly ringed (and mooned?) Iapetus.
hmm, "Alpha" looks an awful lot like Pan.
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