Iapetus Theories, Extended Discussions Options

[tasp]

Iapetus is not small enough to be able to dissipate atmosphere instantly to the void.

There is a 'retention half life', poorly defined however. I am of the opinion a given parcel of gas introduced to the Iapetan environment will dissipate. But more like 1/2 of it in mere weeks, another 1/2 in the same interval, and so on till it is essentially 'gone'.


This is, for most small bodies in the solar system, a moot point. However, for Iapetus, perhaps the reality is a bit more complex.

If atmospheric 'blow off' from Titan 'spirals out to Iapetus, then we might expect a very tenuous increase in the Iapetan vicinity as the effluent 'wafts' by.

Alternatively, if Titanian atmospheric 'blow off' is transported primarily via the Saturnian magnetostail, then we might expect an intermittent application of the effluents. Probably mostly when the inclined Iapetan orbit intersects the appropriate arc behind Saturn twice every Saturnian year.


There is never much 'gas' around Iapetus at any given time. But over 3 or 4 billion years, it mounts up.

A color change has been noted in the Cassini Regio 'crud' (yeah, I vote for black on white) as one wends their way around Iapetus. I suspect, as the gas dissipates, the composition changes slightly per the molecular weight of the effluents, most likely methane and N2. Which ever one is heavier becomes more concentrated (even as the absolute amount decreases) and we see the resulting composition change in the 'precipitation' or staining that occurs over time (and time being each Iapetan revolution about Saturn, ~80 days)

We also note a color (and/or saturation of the color) change in some of the stained craters seen in the recent flyby.

My estimation is we are seeing 'ponding' effects of the gas. Whereas on the flattish surface areas forming the 'stain' in a relatively specific arc of the Iapetan revolution about Saturn due to the gas pretty much dissipating locally, the craters retain a 'pond' of the gas longer, and the staining reaction continues for a longer fraction of the Iapetan orbital arc. And we observe the subtle change in the appearence of the 'crud'.

We have also noted that the eastern and western 'extensions' of Cassini Regio staining overlap on the oppposite hemisphere. We need to realize that specific areas of 'staining' occur 180 degrees around (~40) days the Iapetan orbit, and the intersecting patterns are applied alternatingly.


Has anyone found a fresh crater (white splat) in the dark region yet ??

We can get an idea of the 'regeneration' rate of the 'black crud' by noting just how rare white splats are. 'None' is an interesting answer, as it indicates an ongoing process is 'repaving' the 'crud'.

A process that is amenable to study, btw, by disposing of Cassini at mission end in Cassini Regio, and observing the fresh crater with a future mission. Perhaps Steve will make a rover for us.

[tasp]

I suppose my 'no math' approach to all of this is annoying, and my occaisional calls for someone else to do some math even more so.

And in keeping with tradition, is any possible amount of gaseous staining effluent in the Iapetan realm sufficient over 3 or 4 billion years to accumulate sufficiently to make a spot as dark as Cassini Regio, detectable during a specific instrument integration time for anything on the Cassini spacecraft ??


In other words, if 1 micro micron of CH4 and N2 can 'crud out' a foot thick layer in 3 billion years, can Cassini 'see' that much gas (or whatever amount it is to make the math work) during a given observation with the VIMS (or whatever instrument is most appropriate) ??

And, the $64 dollar question, has such an observation been made already ??

[tasp]

Let me take a shot at the picture in post #670 above.


(btw, mentally rotate the picture on it's side, it makes more sense with the 'peak' up)

The black 'lobes' extending down the side are not quite what they seem.

{they are not lobes of 'black stuff' sliding down the peak}

The 'original' lobes of material sliding down the peak are white. So is the entire 'original' peak.

The 'black crud staining' has just happened (by a happy coincidence of insolation angles that trigger the darkening reaction) to have 'contrast enhanced' for us the flows of material, ballistically emplaced at the higher elevations by the 'final elliptically orbiting' ring residue, and that have slid down the side of the peak partially.

This process is commonally observed in the midwest, this time of year, when granular bits of plant matter are 'emplaced' in 'peaks' at grain handling facilities for the farmers.

I will attempt to get some pictures of 'lobate flows' on grain piles as the harvest season progresses in my area.

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[tasp]

{sorry that picture of the grain pile isn't bigger. It is quite relevant to that big pile on Iapetus.}

{Juramike: I tried to send a PM, let me know if it didn't go thru}

[tasp]

Here's a bigger corn pile picture. Most of the 'flow features' have run all the way down the sides. I'll look for better examples.

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[tasp]

Here is a grain pile picture processed in HP Image Zone.

The flow lobes are visible towards the bottom of the pile.

(the blocking at the top is spurious)

Attached thumbnail(s)

 

[tasp]

The 'big pile' on Iapetus is also a bit different from the grain piles because of the north-south spread of the incoming ring particles. The ring emplacement at the highest spot along the equatorial ground track is quite orderly. However, we do get 2 forms of 'contact' here. Particles lowered via the 'bump' process while still in the circular orbiting portion are lowered much less than one average particle diameter per orbit (2hr 55m). This condemns all the particles into grazing contacts with the high spot.

However, for a particle ever so slightly higher than the particle immediately preceeding it, it's fate is to contact the pulverized remains of it's predecessor, and enter a final elliptical orbit about Iapetus. These particles are what accumulates in the Voyager Mountain areas. It is possible for a ring particle destined to reach this area, to be slighly north or south of the exact ring center of the plane. (the collapse to the Laplacian plane cannot 'perfectly' flatten the ring system, we can expect N-S thickness of the ring system of a few average particle diameters. This allows those particles contacting the 'spray' of their immediate predecessors the opprotunity to experience a small deflection north or south out of the ring thickness. We see the effects of this in the N-S spread of the Voyager Mountains. This mechanism preferentially affects those particles destined to impact in the Voyager Mountains, and explains how we get a 'field' of seperate piles there, and yet have the highly organized 'wedge ramp' formed from the majority of materials emplaced via the grazing contact at it's pinnacle.

We also see the effect of a pile growing to 'shadow' other piles downrange. I think we can expect an ellipse, traced onto the Iapetan surface (and modified by local topology) as the touch down area for the materials destined to reach there. That there might be ring materials emplaced in a wide swath around the equator is quite possible, but due to the statistics of the amounts of materials deflected into all the possibilities, we only note where sufficient materials accumulate to make a pile.

We see a 'discontinuity' in the Cassini Regio crud extending westward from the Voyager Mountains, this is from the smaller and smaller amout of materials emplaced in this region. This affects the local slope angles and alters the light/dark pattern of the 'crud' and allows us to note it's presence by the subtle disruption in the otherwise 'random' pattern of the 'crud'.

The Cassini Regio crud is a seperate phenomena from the 'wedge ramp', but these 2 different features of Iapetus do 'interact' with each other.


It is certainly amazing, that a process such as dynamical ring spreading can exquisitely and precisely control the trajectories of ring particles so long ago around Iapetus. The key to understanding the ring emplacement, that the individual ring particles experience a decay of their orbits much less than their average size per orbit, permits (requires actually) that the particles all contact the very highest point along the equator, and to contact it in a grazing fashion. That we can infer motion of particles (most likely) billions of years ago to a matter of apparently just a tiny fraction of a meter is a stunner. And that contact, can then 'sort' the particles into 2 categories, and emplace a highly organized wedge ramp, and a disparate field of piles almost all the way around Iapetus simultaneously!

[tasp]

I found an interesting passage in the Planetary Rings chapter of The New Solar System.

Joseph A. Burns writes:

"Because the strength of each gravitational perturbation (due to planetary oblateness, a satellite, the sun) depends on the distance to the perturber, the mean plane mentioned above is actually a slightly warped surface, looking like a snapped-down hat brim extending out from the planet. The warp in this surface off the equatorial plane is less than a kilometer for known planetary rings since the disturbing satellites lie near their planets equatorial plane; if Neptune were to have a ring, it could be pulled dramatically out of planar configuration by massive Triton, because of this satellite's highly inclined orbit.

Now, allowing for my ancient copy of this book (published prior to confirmation of the Neptunian ring system) and that this effect was not observed at Neptune by Voyager 2, I did not perceive this passages significance to Iapetus till now.

Why this effect is not observed at Neptune is beyond my ken, but it occurs to me (finally) that this is the 'smoking gun' for explaining the symetrical diverging attendent ridges observed during the first Cassini flyby of Iapetus.

Iapetus' inclined orbit to Saturn has produced a ring system with the 'hat brim' effect predicted by Joseph Burns.

The 'warpage' was preserved as the ring system inched it's way down to the surface and 'imprinted' itself into the emplaced wedge ramp structure.


I had speculated that Iapetus had suffered a major impact during the ring emplacement, altering its spin axis a few degrees, and producing the symetrical diverging structures. While that effect could theoretically be possible, a major impact on Iapetus during ring emplacement sufficient to alter the spin axis will disrupt the ring system and terminate emplacement (in the organized wedge ramp) immediately.

Burns has accurately predicted this effect and we have but to gently coax him into doing the calculations to see if the inclination of the warpage is consistent with the divergence angle of the attendent ridges.


Does anyone have contact with Joseph Burns ??

My copy of The New Solar System has the following (dated information):

Associate Professor of Theoretical and Applied Mechanics at Cornell University

PhD 1966

BS in Naval Architecture, Webb Institute

Fellowship Goddard Spaceflight Center


I think he has done something quite amazing with this warpage idea, and we might have actual proof of the concept in hand. I would like to extend a cordial invitation to him to join us at this, our modest message board.

Failing that, I will consider asking Doug for subpoena powers.

{that's a joke}

[The Messenger]

Failing that, I will consider asking Doug for subpoena powers.

{that's a joke}

The Unmanned Star Chamber?

[JRehling]

Iapetus' inclined orbit to Saturn has produced a ring system with the 'hat brim' effect predicted by Joseph Burns.

Interesting idea. I am a bit lost in the details, though.. here's a question:

If there were a ring system with funky warps in it, I would still expect the same shape for the ridge profile (a Gaussian with no kinks or grooves) because the ring particles would be revolving very rapidly around Iapetus. So let's say (arbitrarily) the kink was over longitude 270W at a time when some of the particles were low enough to strike the surface (after a long gradual orbital decay). Only minutes later, however, the "survivors" would be at longitude 90W, so if the "snow" of falling particles were uniform throughout the ring particles' orbit, any kink or warp in the ring would still spread evenly around all longitudes. And every orbit must cross the equator twice, so even a highly inclined ring would leave a ridge profile that centered on the equator.

The only way I can see a kinked/warped ring creating distinct, parallel subridges would be if:

1) A big "stretch" of ring all fell to the surface at the same time, eg, within fewer than 10 minutes.

OR

2) The dynamics of the ring particle orbit would be such that if the warp stuff started snowing down at about latitude 1N, in the vicinity of 270W, then the ring particles that just barely missed impact at 270W on one go-around were nonetheless safe from impacting the surface anywhere else in the orbit because the dynamics of the decayed orbit meant there was a significant enough difference between periapsis and apoapsis that a given ring particle was basically fated to fall at some given longitude.

(1) seems impossible. It's way beyond me to create a model that tests (2), but one problem I see even with that explanation is how the difference between periapsis and apoapsis could be great enough that any particles near enough to their final impact could clear the other portions of the equatorial ridge elsewhere in their orbit. A particle that just missed impact at 270W and had an apoapsis at 90W would have to cross the equator at 0W and 180W and have enough altitude to clear the ridge (which, by the endgame, was at least 20 km tall and perhaps considerably taller if this is what we see after eons of relaxation).

So is an orbit possible that would have a periapsis of about zero, a semimajor axis taking it about 20 km over the mean surface elevation, and an apoapsis therefore of 40 or more km? Well, yes, it's possible, but would a significant number of particles end up in this configuration? And why would there be distinct ridges at two parallels (say, 0N and 1N) but not equal amounts of stuff filling in the gap between?

[tasp]

The highest point on the equatorial ground track is what 'keys' or synchronizes the wedge ramp structures. As the pinnacle penetrates the warped portion of the ring plane (twice per Iapetan day) it triggers emplacement only during that brief moment. The rest of the time, the ring system is not emplacing.

The decay of the orbit s so slow, that emplacement onto the diverging attendent ridges still meets the criteria regarding the descent per orbit is much less than the average particle diameter. In fact the descent rate is so slow, that factoring in the % of time the ring is emplacing per 2 hr 55 min still meets that requirement.

[tasp]

An example:

Imagine dropping something overboard from the ISS, but only when you are exactly over Quito Ecuador. Every so often, you pass directly over Quito, half the time going from the northern hemisphere to the southern and half vice versa. Assuming your dropped items are all decelled the same, you get 2 'piles', one NE of Quito, and one SE of Quito.

Now instead of the a 'point source' ISS, imagine one extending all the way around the world.

Most of the time, it is not over Quito, but when it is, drop something.

See how it works ??

On Iapetus, the very highest pinnacle on the equatorial groundtrack is 'Quito'. And we see 'trails' extending NE annd SE from it.

[tasp]

The Unmanned Star Chamber?

I think Doug would enjoy presiding over a more powerful message board.

[tasp]

{I was a little pressed for time this AM, will flesh it out a bit more}

The planar portion of the Iapetan ring system will have countless particles in virtually circular orbits. Particles in adjacent orbits (for example, a particle currently at an altitude of 500.000 km, and a particle at 500.001 km will scrape (assuming their diameters to be ~.0005km) as the lower faster one passes by the slightly higher slightly faster one. At the instant of the 'bump' a tiny bit of momentum gets transfered from the lower to the higher particle. This causes the lower particle to edge ever so slightly lower, and the higher one to loft ever so slightly higher.

This effect is operative across the entire ring plane assuming a reasonable 'fill factor' of particles.

The net effect is for the lower circular edge of the ring to contract, and the upper edge to expand.

We have an exquisitely slowly contracting circle about an irregularly shaped object. At some point, the lowest edge of the circular ring system contacts the very highest point along the ground track. For an irregular object, such as Iapetus, there can be only a single highest spot, even if it is only a matter of inches higher than the next highest spot.

For a particle in the lowest edge of the ring, the instant it contacts this highest point, it is DOOMED. It decels, breaks up, shatters, and goes thud. The result is the perfectly straight, equator hugging wedge ramp.

Now, we have another possibility that can occur once we have a particle decelling after pinnacle contact. An adjacent ring particle, ever so slightly higher, so as to just barely clear the pinnacle, can now pass through, graze or 'kiss' the pulverized remains of it's predecessor. This particle can be decelled by a random amout depending on the degree of 'overlap' with it's predecessors debris, and how much the debris has deceled in the interval since it's contact with the pinnacle, and its' being overtaken by the following particle.

Additionally, these overtaking particles, due to the slight vertical (north/south) thickness of the ring plane, can themselves be deflected slightly north or south by hitting the predecessor debris off-center. These particles complete one final elliptical orbit beneath the main, still circularly orbiting, ring system. (near the end of the emplacement, this 'gap' is 20 km high) and can emplace along the Iapetan equator, and somewhat north or south of it. We see the 'statistics' of all the possible decels that can occur in the 'piles' that are in the Voyager Mountain area. There are also slope changes evident further west of the VMs, caused by lesser amounts of materials accumulating, that can be discerned in the black/white CR staining. Ring particles probably do drop all around the equator, but we have to have an accumulation sufficient to make a pile big enough for Cassini's camera to see them.

Now, we have a third outcome. Dr. Burns idea regarding a 'warped brim' on the ring system is spot on. The outer portion of the Iapetan ring system was apparently inclined a few degrees to the majority of the planar formed ring system, and was inclined due to the effects of Saturn's gravity and Iapetus' existing orbital inclination.

This inclined portion of the ring system emplaced the symmetrical diverging attendant ridges (or off-ramps as I like to call them) as outlined in the preceeeding posts. This inclined portion of the ring system was able to maintain it's inclination during the descent due to the 'collapse to the Laplacian plane' process no longer being operative once the particles are ~planar.

{The materials 'lofted' across the ring system by the 'bump' process aren't actually 'lofted', by the way. The ring system, for lack of a better term, is 'lossy'. Transient wisps of atmosphere from the occaisional minor impact on Iapetus, Poynting Robertson drag, solar wind drag, etc. are always 'burning off' any trend towards lofting of ring materials. We do not get an 'Atlas' dust pile moon at the top of the Iapetan ring sytem, unfortunately.}


A ring system about Iapetus explains everything amazing about Iapetus' equator, except the dark Cassini Regio 'crud', which is an entirely seperate and unrelated phenomena coincidently located on Iapetus.

Orderly, organized and exquisitely precise emplacement of the main planar ring system formed the main wedge ramp. A subset of particles generated in this phase deposited the Voyager Mountains. And finally, the outer inclined portion of the ring system emplaced the symmetrical divering attendent ridges. The ring particles orbited conventionally, from west to east, and were originally lofted by a large glancing impact on Iapetus, most likely, in my opinion, forming the big elongated crater on the SE edge of Cassini Regio. The lofted impact debris collapsed to the Laplacian plane rather quickly, and in a process related to that believed to have spawned earth's moon and Charon.

We have an orbitally decayed ring system, all layed out for us to study, on Iapetus. An amazing discovery on a very, very unique little moon.

[tasp]

And some things we don't know.

(or I don't think we know yet)


* How do we figure elevations on Iapetus ?? We have reports of the spherical distortion Iapetus exhibits (due apparently to it's frigid rigid crust). I think for detailed ring system study, we need an accurate means of computing surface elevations along the equatorial ground track, relative to the inner circular edge of the ring system at an altitude equivalent to the very highest point along the ground track.

* We need to rough out an idea of the Iapetan surface along the equator, minus the accumulated ring deposits.

* Why is there a 'gap' ~2/3 down the length of the wedge ramp ??

* I suspect the highest Voyager Mountain (if we can figure out which one it is per the above criteria) is displaced from the 'trifurcation' point of the wedge ramp (hard to tell exactly where that is due to crater damage , btw) by the amount Iapetus rotated on it's axis in 2 hrs, 55 minutes. And if that distance is expressed in degrees, it becomes easy to solve for the total length of the Iapetan day in that era. But how does this get 'proved' ??

* I think I unbderstand the 'tiger stripes' (extensional faulting from impact induced mantle displacement), but that isolated chasm in the white hemisphere is a real puzzler. Is it a crater chain, but a crater chain so 'dressed up' with the dreaded 'Iapetan Dalmatian Effect' that it is virtually unrecognizable ??

* The surface insolation angles are drastically critical to the formation of the 'black crud'. Any low temperature chemistry experiments underway to ID this stuff ??