On a ring origin of the equatorial ridge of Iapetus Options

[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.

[ngunn]

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

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

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

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

[The Messenger]

Why? Are you assuming that environmental parameters at the time and place of deposition were pretty much as they are there today?

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

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

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

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

[ngunn]

we seem to be finding more questions than answers. That's not bad, but it leaves us two options: Make some assumptions and try to prove them wrong on based on the evidence that is here, or wait for another mission.

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

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

[JRehling]

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.

[ugordan]

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.

True, but that intersection isn't likely to fit the arrival hyperbola so careful arrival timing (on the order of years!) is likely to be required.

I'm not an expert in orbital dynamics, but is Iapetus' orbit ascending node fixed w/ respect to the stars? A probe entering the Saturnian system at a hyperbolic trajectory will probably have a more-or-less fixed point (arrival angle w/ respect to the sun and with a given injection energy) where closest approach is made.
Take a simplification: the arrival plane is Saturn's equatorial plane. The intersection, c/a possible points are then Iapetus' ascending and descending node. Then, you have to wait until the Saturn's revolution around the Sun rotates one of the nodes to the point where the approach trajectory interects the Iapetus' orbital radius. Only then is the arrival geometry right.
This would constrain the possible arrival times to the Saturn system to two fairly short periods each Saturn orbit, each half an orbit apart. That'd be a long delay between launch windows.

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

[mchan]

True, but that intersection isn't likely to fit the arrival hyperbola so careful arrival timing (on the order of years!) is likely to be required.

I'm not an expert in orbital dynamics, but is Iapetus' orbit ascending node fixed w/ respect to the stars? A probe entering the Saturnian system at a hyperbolic trajectory will probably have a more-or-less fixed point (arrival angle w/ respect to the sun and with a given injection energy) where closest approach is made.
Take a simplification: the arrival plane is Saturn's equatorial plane. The intersection, c/a possible points are then Iapetus' ascending and descending node. Then, you have to wait until the Saturn's revolution around the Sun rotates one of the nodes to the point where the approach trajectory interects the Iapetus' orbital radius. Only then is the arrival geometry right.
This would constrain the possible arrival times to the Saturn system to two fairly short periods each Saturn orbit, each half an orbit apart. That'd be a long delay between launch windows.

I do not quite follow this. Why does the arrival plane (containing the hyperbolic trajectory) need to be Saturn's equatorial plane to encouter Iapetus? Cassini arrived out of the equatorial plane and got a close approach to Phoebe inbound.

I don't know astrodynamics either. It seems to me the arrival plane can be at any inclination between 0 and Iapetus orbit inclination to Saturn's equatorial plane, and have a chance of intersecting Iapetus orbit. The problem is to make sure that Iapetus is at the point of intersection when the probe gets there.

Oh, and a probe would not want to arrive with 0 inclination to the equatorial plane to avoid traversing the rings.

[ugordan]

I do not quite follow this. Why does the arrival plane (containing the hyperbolic trajectory) need to be Saturn's equatorial plane to encouter Iapetus?

As I was saying, it was merely a simplification so I can more easily put into words what I mean. A real arrival trajectory will almost never be coplanar to Saturn's equator. By using an equatorial plane, the two obvious intersection points would be the ascending and descending nodes. If you increase the inclination, the intersection points move much, but OTOH, there's still maximally two of them.