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On a ring origin of the equatorial ridge of Iapetus
David
post Aug 31 2006, 07:23 AM
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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?
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ngunn
post Aug 31 2006, 10:00 AM
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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.
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ugordan
post Aug 31 2006, 11:18 AM
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QUOTE (ngunn @ Aug 31 2006, 11:00 AM) *
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?

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.


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ngunn
post Aug 31 2006, 11:50 AM
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QUOTE (ugordan @ Aug 31 2006, 12:18 PM) *
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.
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ugordan
post Aug 31 2006, 12:08 PM
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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.


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ngunn
post Aug 31 2006, 12:38 PM
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QUOTE (ugordan @ Aug 31 2006, 01:08 PM) *
A prolonged drag through an atmosphere will leave an object more time to lose the heat


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


QUOTE (ugordan @ Aug 31 2006, 01:08 PM) *
I'm really not an expert on whether or not that is enought to vaporize/melt ice at cryogenic temperatures.


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'.
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ngunn
post Aug 31 2006, 01:06 PM
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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.
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tasp
post Aug 31 2006, 01:27 PM
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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.
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tasp
post Aug 31 2006, 01:33 PM
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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.
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ngunn
post Aug 31 2006, 02:01 PM
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QUOTE (tasp @ Aug 31 2006, 02:27 PM) *
Invoking an atmosphere above a certain very low density will create havoc with the orderly linear progression of the emplacement.


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.
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ngunn
post Aug 31 2006, 02:52 PM
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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.
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tasp
post Aug 31 2006, 03:56 PM
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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 biggrin.gif )
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Rob Pinnegar
post Aug 31 2006, 05:20 PM
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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?
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ngunn
post Sep 1 2006, 11:26 AM
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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.
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tasp
post Sep 1 2006, 01:45 PM
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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 blink.gif } 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.
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