[X] My Assistant

[Guest_AlexBlackwell_*]

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

[tasp]

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.

[The Messenger]

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).

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

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.

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

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.

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