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Posted by: tasp Sep 25 2007, 02:55 AM

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.

Posted by: tasp Sep 25 2007, 03:07 AM

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 ??

Posted by: tasp Sep 25 2007, 03:25 AM

Let me take a shot at the picture in post http://www.unmannedspaceflight.com/index.php?showtopic=4561&view=findpost&p=100478 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.

Posted by: tasp Sep 25 2007, 04:10 AM

{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}

Posted by: tasp Sep 25 2007, 04:14 AM

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.

Posted by: tasp Sep 25 2007, 04:24 AM

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)

Posted by: tasp Sep 25 2007, 01:16 PM

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!

Posted by: tasp Oct 2 2007, 01:34 PM

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}

Posted by: The Messenger Oct 2 2007, 02:12 PM

The Unmanned Star Chamber?

Posted by: JRehling Oct 2 2007, 02:32 PM

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?

Posted by: tasp Oct 2 2007, 02:41 PM

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.

Posted by: tasp Oct 2 2007, 02:49 PM

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.

Posted by: tasp Oct 2 2007, 02:53 PM

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

Posted by: tasp Oct 3 2007, 02:57 AM

{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.

Posted by: tasp Oct 8 2007, 02:39 AM

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 ??

Posted by: JRehling Oct 8 2007, 04:28 AM

There's good cause for general gloominess on the issue of what the non-ice component is on the surfaces of:

Ganymede
Callisto
Iapetus
etc.

We have spectra on resolved discs for the first two from Galileo, Cassini, probably HST, and NH, and the best stab at what the stuff is reads something like "a dark nonice substance".

It's possible, even likely, that the black crud has not undergone significant chemical changes since before it even became part of Iapetus (ditto Ganymede and Callisto). Some of the last stuff to become part of Iapetus's accretion could have been ice with a carbonaceous component (eg, cometstuff). Then the process of thermally-driven darkening would simply be the ice sublimating away, with the dark stuff remaining chemically the same as it was eons ago.

The nucleus of Halley's Comet is predominantly very dark. I think it may be useful to think of its darkening and that of Iapetus as two manifestations of the same basic phenomenon: Start with something bright mixed with something dark, and gradually take the bright away.

Within the broad range of candidate dark stuffs, the question is which compounds are found on each world. And IR spectroscopy, about the best tool going for remote sensing of composition, doesn't tell us. We're probably going to have to plant a lander down, melt the ice out of a sample scoop, and analyze what's left. When will one, much less all, of these worlds be in line for a flagship mission including a lander? Sometime between the 22nd century and never.

Posted by: ngunn Oct 8 2007, 10:37 AM

Excellent comparison JRehling. I think that says it all. Now fast forward a comet like Halley until it has no bright stuff left and let it impact in low latitude Roncevaux Terra. Bingo - you've seeded a new dark patch.

Posted by: As old as Voyager Oct 8 2007, 05:21 PM

Interesting piece on NS Space suggesting dark spots may be due to sublimation triggered by sunlight:

http://space.newscientist.com/article/dn12751

I've always wondered if some sort of interaction with the solar wind is causing the darkening on Iapetus leading hemisphere;after all, it's the part that hits the Sun's particle wind head on.

Posted by: As old as Voyager Oct 8 2007, 05:22 PM

Interesting piece on NS Space suggesting dark spots may be due to sublimation triggered by sunlight:

http://space.newscientist.com/article/dn12751

I've always wondered if some sort of interaction with the solar wind is causing the darkening on Iapetus leading hemisphere;after all, it's the part that hits the Sun's particle wind head on.

Posted by: Jyril Oct 8 2007, 06:56 PM

The dark material is most likely from irregular outer satellites. Not counting possible photochemical processes, the Sun affects the moon's surface only by warming it.

Posted by: Pertinax Oct 8 2007, 08:06 PM

Similar to the NS article is this recently posted Space.com piece.

http://www.space.com/scienceastronomy/071008-cassini-iapetus.html


-- Pertinax

Posted by: David Oct 9 2007, 12:01 AM

Which is in turn remarkably similar to a discussion on this very forum.

Posted by: tasp Oct 14 2007, 05:40 PM

The 'color' dichotomy (reddish to greenish) might be explained more readily by the 'gaseous darkening agent' idea. If the gases (let's assume CH4 and N2 for the moment) are in fact introduced to the Iapetan realm as Iapetus transits the Saturnian magnetotail and then they dissipate to space in less than ~79days, we might surmise a few things:

* The weight of methane is ~18 and the nitrogen ~28 (if doing the math correctly) and we might expect the methane to dissipate faster than the nitrogen. This alters the composition of the gas as it dissipates, enriching it in nitrogen. If we are seeing the dark staining as a polymer of the 2 gases, the stains precise composition might slightly change in response to the changing percentages of the reactants. Perhaps the color shift between the reddish and greenish is resultant fom this change in the composition of the reactants.

* Just posted today, is information on tholinization high in the Titanian atmosphere. It would seem low pressure is not an obstacle to the formation of materials from CH4/N2 gas mixtures at Saturn's distance form the sun. In fact, evidence exists that higher pressures might inhibit the reactions.

* I have found posts back to 11/3/2005 here at UMSF discussing insolation/heating effects catalyzing darkening reactions on Iapetus and Hyperion. Insuperable difficulties to these ideas don't seem overly forthcoming at this juncture; in fact, the data collected in that interval seems to support these concepts.

* What has been referred to as 'ponding effects' (craters in dark areas exhibiting color saturation/tonal differences in their darkening in comparison to darkened adjacent plains) seem to approach 'clincher' status for a gaseous agent over a particulate one. A crater would be expected to retain a pool or 'pond' of gas longer than adjacent open terrain. More reactants and/or reactants persisting longer allowing a greater portion of the methane to dissipate relative to the nitrogen, handily explains why we see craters with different saturation or tone than the surrounding areas. Particles will not 'know' (to anthropomorphize a bit) they are in craters, a gas will be constrained by the topology of the crater and produce the observed effect.


(I wanted to repost that dark crater with dark surroundings picture, but I have lost track of it, anyone has it handy, I would surely appreciate it, thanx)

Posted by: David Oct 14 2007, 10:25 PM

It's still not clear to me how you expect a gaseous cloud from any source other than the satellite itself not to have thoroughly dispersed before it could have any effect on Iapetus. Space is pretty large, and even if by some improbable event all of Titan's atmosphere (say) were to be released into circumSaturnian space at one go, I'm not sure it would have the least effect on the neighboring moons. Solids can travel across millions of kilometers and retain their shape, mass, and other characteristics -- but how can gasses?

Posted by: tasp Oct 15 2007, 01:31 AM

Over the eons, Titan is presumed to have lost thousands and thousands of cubic kilometers of atmosphere (primarily CH4 and N2 and some other goodies).

Where did it go ??

Well, it doesn't instantaneously dissipate to the furthest reaches of our galaxy. Rather it 'wafts' (for lack of a better term) about in the Saturn system in a very low pressure state.

At Titan's distance from Saturn, the Saturnian magnetosphere can be proximate to Titan when it is sunward from Saturn, but Titan is quite deep in it over the night side. I suspect the escaped gases are 'entrained' and/or swept up by the magnetosphere's edge , or even the solar wind as it 'flows' about the Saturnian sphere of influence. Once 'entrained' the gases are swept (transported) 'downstream' from Saturn and out towards the outer solar system.

Once every orbit about Saturn, Iapetus passes through this CH4/N2 enhanced 'plume' trailing anti-sunward away from Saturn. (I note that near as I can tell, Iapetus' orbital plane is aligned rather more closely to the ecliptic, ~20 degrees or so, rather than the expected 28 degrees due to Iapetus inclination about Saturn).

At any given Iapetan magnetotail pass, undoubtedly, only a tiny, tiny amount of gas is deposited in the Iapetan realm, and that gas eventually dissipates to the void, but not before something interesting happens, ie, Cassini Regio.

Over the eons, an appreciable thickness of 'crud' accumulates. Enough for us to percieve as very dark brown, at least the thickness of a sheet of paper. (how thick is opaque mylar, after all ?) The process is very inefficient, most of all the gas lost from Titan wafts past Iapetus, but enough 'sticks' around to 'regenerate' the dark coating where impacts damage Cassini Regio.

Every 80 days we get a new dose of gas, and in less than 80 days, most of the gas dissipates. (some lingers in deep craters like Snowman)

Once the gas is around Iapetus, it can participate in the darkening reaction. As Iapetus traverses the 'plume' imagine the 'local noon' meridian. East of this N/S line, noon has already occured, and the surface is cooling. This area is less likely to experience darkening. West of that N/S line, local noon is approaching, the surface is still warming, and the darkening reaction procedes vigorously.

The gases persist for some weeks, and as the noon meridian travels westward, we put another 'coating' of black stuff on Cassini Regio. As noted by others, as we travel across CR, the color of the deposition changes. I suspect this is due to the ratio of CH4 to N2 changing as the CH4 dissipates roughly twice as fast as the N2. Some topological forms, such as craters, are able to 'pond' the gas somewhat longer than the open terrain. This explains why the west edge of CR continues into the 'Dalmation stipples' of all those dark craters that we see far fewer of on the east edge of CR.

We note another effect, the large southern hemisphere basin with the dramatic cliffs. Those cliffs in the SE quadrant are darkened too. Why?? The topology there, a long slope to the NE experiences a 'local noon' (due to the angle of the surface topology) drastically later then the adjacent plains. For the darkening to occur requires a surface that at some time during the ~80 day Iapetan day is perpendicular to the suns rays, and it must be positioned to be in an area capable of retaining the gas when the sun angle is correct, otherwise we do not see the darkening.

We don't see CR extending all the way about Iapetus because, for the most part, the gas generally is not retained from magnetotail passage to magnetotail passage (~80 days) but only for most of it, about half that long. Some craters are able to retain some gas longer, and we see them for their dark floors. A few craters have 'heart' shaped dark bottoms, they are situated such that twice per Iapetan 'day' they have gas, and are at the correct insolation angle. The 'heart' shape occurs from the sun shining over different sections of the crater rim.


I know this is a tough sell. We are looking at the results of atmospheric (for lack of a better term) effects at astonishingly low pressures. This is so alien, so far beyond (below, actually) any other atmospheric phenomena we have encountered before, that it just isn't intuitive what is happening here. Iapetus just happens to be at a point on the graph where it's gravity is just high enough, and it's temperature is just low enough, where it not only has the ability to hold a bit of gas for a few weeks, but it also happens to be located in just the right place in the solar system to get a new 'fix' of the gas every 80 days. Imagine Eros, tiny and hot, any gas released there is lost all but instantaneously, and Pluto, with a small mass, deep cold, and a thin atmosphere. Now consider Iapetus, bigger than Eros, and cold enough (prior to the darkening) to hold a bit of gas for a few weeks. Yeah, there probably is a region on the graph for that, the trick at Iapetus is, there is a source to replensh the gas, just a bit at a time, from Titan's atmosphere. Every 80 days, for a billion years (assuming the sun finally got hot enough in that epoch) we see a bit more 'endarkening' occuring, and now we have a Cassini Regio to marvel at.

Posted by: tasp Oct 15 2007, 05:41 PM

More on Iapetus:

* Comparing the amount of gas available to be lost from Titan to the amount of dust available to be lost from Phoebe (or some/many/most/all of the 'outie satellites) it seems we would have to give the nod to Titan. Titan has a deep atmosphere (deep enough to concern how closely Cassini can approach it) and it's ability to retain all of it perfectly is suspect.

Posted by: Bill Harris Oct 15 2007, 09:04 PM

In my wildest imaginings I see that gas impinging on the surface of Iapetus and the left-over carbon combining as a fullerene. The dark areas are covered with buckyballs.

--Bill

Posted by: tasp Oct 16 2007, 02:17 AM

I am not sure the relevence of this experiment to Iapetus, but for your consideration:

In the winter, when we used to burn our trash in the backyard, little flecks of charred newsprint would be wafted out of the fire, and some would come to rest on the freshly fallen snow.

Such little bits, melted little holes, roughly equal to their diameter in the snow, and went straight down.

I am picturing such a bit on Iapetus doing pretty much the same thing, and once it melts down into the surface the depth of a few flecks diameters, it won't be capable of much further effect on the albedo of Iapetus.

Is this pertinent to the dusting scenario ??

Posted by: tasp Oct 16 2007, 02:24 AM

Regarding Iapetan surface chemistry, presumably we are pretty sure of water ice and CO2 ice on Iapetus. My not being an expert of any kind in cryochemistry leads me to wonder if perhaps the surface materials of Iapetus can react with the Titanian CH4 and N2. If we are forming tholins, kerogens, 'gunk', whatever, if the surface materials are 'participating' in the reaction, it might help us define what reactions are occuring.

Posted by: The Messenger Oct 16 2007, 02:40 AM

Buckyballs do form in fuel rich environments (we have trouble with them fouling and pitting valves in inert high pressure (40+Atm), high temperature testing systems, but the key words are high pressure and high temperature. Until we find more evidence of lightning on Titan, we have neither.

Posted by: Bill Harris Oct 16 2007, 08:53 AM

I know, 'tis but a silly theory. This an odd corner of the universe.

--Bill

Posted by: The Messenger Oct 16 2007, 02:35 PM

I would think it would be, except for one thing: The holes on Iapetus are holes - craters that predate any flecks of anything landing on the surface. A very high percentage of the dark spots correlate directly with the sol-facing side of craters and ridgelines. The scenarios that require black stuff to land in craters and somehow end up in the bottoms is inconsistent with which parts of the craters are darkened as one moves away from the equator on the light side of the moon.

Posted by: tasp Oct 16 2007, 05:28 PM

Are we talking at cross purposes ??

My intended gist for citing the specks on snow in my backyard was to highlight a difficulty I suspect with the dust scenario.

Am I correct that you concur with the dust scenario having that problem, and an additional problem in somehow getting the dust in the 'correct' places on Iapetus ??


{We've all had to absorb a great deal of information since the September Iapetus flyby, and it helps if we are mutually acurately comprehending each other's posts. Thanx}

Posted by: The Messenger Oct 17 2007, 02:59 AM

Yes.

Seeding may have initialized the process on the leading side, but the bottoms of the craters, after the white stuff has melted/sublimed/absorbed, is dark.

Posted by: David Oct 17 2007, 07:12 AM

I don't think that the possibility should be ruled out that in certain places ice may simply have failed ever to form, or was naturally lost without any "seeding" when landforms were created that maximized local insolation.

Posted by: ngunn Oct 17 2007, 11:14 AM

Before Cassini, Iapetus was widely assumed to be an icy ball with a dark covering of dust from somewhere else on one side. Now we have seen the evidence of the runaway albedo/thermal segregation process in all it's fascinating detail, including all that pristine white freshly-frosted terrain which looks dusty NOWHERE. Yet the dust refuses to go away. It is hanging around like phlogiston after the discovery of oxygen. It is invoked as a trigger at the start of the process, and as a top-coat colour tinting agent at the end of it. It is 'sliding' to the bottoms of craters which are in fact, as David points out, favoured sites for sublimation - possibly without seeding.

I'm not saying the exogenous dust isn't there or that local concentrations of it couldn't have triggered the formation of SOME of the dark areas. Just that If you were to start from what we know now you wouldn't necessarily infer its presence in significant amounts on the surface of Iapetus today. It is just one possible candidate for an asymmetric triggering process that took place a very long time ago.

I am interested in what effects traces of Titan's leaky atmosphere might have on Saturn's other moons and read posts on this subject avidly. I feel that any gases present in the vicinity of Iapetus would be so tenuous that the molecules would be in independent ballistic trajectories with no possibility of significant 'ponding' in craters. However the mechanism for gravitational fallout of suspended solids from Titan's atmosphere onto another moon would need detailed analysis which is way beyond me - I hesitate even to try to imagine it without some more solid facts and figures. I think this needs to be done ab initio without deciding in advance which Iapetan peculiarity (if any) it might account for.

Posted by: tasp Oct 17 2007, 03:44 PM

A reddish enhanced darkened crater in a greenish enhanced darkened plain (or vice versa, I forget which way it parsed out) is tough to explain without invoking compositional changes in a gaseous discolorant that lingers (ponds) over time in the crater. And these 'discrepantly' colored craters have to be explained.

Titan has lots of tenuously held gas, Iapetus might be getting a new 'dosage' every ~80 days for a billion years. Seems like if dust transport from outies is deemed plausible, so to might be gas from Titan. Titan has much more gas than the outies have dust, after all.

And a mechanism that liberates the dust in sufficient quantities from the outies seems difficult to fathom. At Titan we have the Saturnian magnetosphere pulsating in Titan's vicinity, and a solar wind 'entraining' the effluents and restraining their dispersal, and preferentially lofting them anti-sunward to 'paint' Iapetus every ~80 days.

Posted by: ngunn Oct 17 2007, 04:24 PM

It's an interesting idea and I am trying to make it work. I think for significant ponding in Iapetan craters you'd need a gas with a very high molecular weight - a contradiction in terms if you're talking about organics. There may be other possibilities. Radon for example. But that would be native to Iapetus, emanating over time from the dark fraction of its surface materials that is so conveniently concentrated and exposed by the sublimation . . .

Posted by: ngunn Oct 17 2007, 08:40 PM

Nobody buying the idea of Radon accumulation on Iapetus? Just out of curiosity I did some back-of-envelope calculations for a Iapetan 'atmosphere' using the formula H = kT/mg. The results are interesting:

For a Radon atmosphere the scale height is 17 km. This is of the same order as the depth of large basins on Iapetus.

For an atmosphere of N2 or similar the scale height on the same simplistic model is 135 km. This is high enough for the value of g to be substantially reduced, so 135km is an underestimate for the 1/e level.

Conclusion: Radon gas could in principle 'pond ' significantly in depressions on Iapetus while gases of low molecular weight (from whatever source) could not.

Posted by: nprev Oct 17 2007, 11:55 PM

Interesting idea, but I would be very surprised if there's enough K-40 on Iapetus near enough to the surface to generate a significant--or even detectable-- amount of radon in the first place, to say nothing of the escape rate. Ditto for Titanian effluents.

As long as we're speculating, though, what about some sort of catalytic effect? If Iapetus' surface is enriched in exotic organics like another Saturnian moon we know (albeit minus a thick atmosphere), then maybe ordinary metallic meteorites might provide the seed for subsequent reactions.

Posted by: ngunn Oct 18 2007, 08:48 AM

I expect you're right (except that 222Radon comes from Uranium, not Potassium). However the Iapetan sublimation residues are probably unique in the outer solar system in concentrating heavy elements - along with organics - near a visible surface. We would be talking about a VERY thin Radon atmosphere at best, but being radioactive that could maybe still have observable effects over time.

Posted by: ngunn Oct 18 2007, 11:13 AM

OK forget the Radon atmosphere - that was one of my worst ideas. Radon is a solid at Iapetan temperatures !!!!!!!!!!! That presumably means that any that formed in the sublimation residue would remain within its own mineral grain. Anybody know which is the heaviest molecule that IS gaseous around 100K?
http://www.lenntech.com/Periodic-chart-elements/melting-point.htm

Is a radon atmosphere possible anywhere? Thinking about it, our own Moon might be the most favourable location in the solar system for radon 'ponding' in craters. Even there it would freeze out at night.

Posted by: The Messenger Oct 18 2007, 03:54 PM

Have you ever stood on the melting edge of a retreating glacier, or an iceberg? You can tell the thin areas (<2m) because looking straight down, the ice turns deep blue. If the entire underlying moon is 'reddish', but the surface ice is very thin near the leading edge craters; the 'green' could be these thinning areas of ice.

Falling off of an iceberg is just plain cold...and very hard on a camera and film - my 'blue spots' in the ice are rather washed out.

Posted by: tasp Oct 19 2007, 05:30 AM

Black crud might have variable thickness from east to west, with craters possibly having 'shades' dissimilar to surroundings in some locales.

Fiendishly complex take on things you have there.


[laugh]


That's why I come here . . .


[slinks off to bed, bemused and dead tired]

Posted by: nprev Oct 19 2007, 11:20 AM

(slaps head...) duh!!!! I knew that...for some reason I was thinking radon & typing argon...

Just to clarify...I was thinking argon & typing radon!!! I gotta drink more...noticed this iron oxide crust around my chin, clearly I'm not adhering to the necessary alcohol intake specified in my operator's manual.

Posted by: ngunn Oct 19 2007, 11:34 AM

Actually Argon would have made more sense. At least it has a chance of being gaseous in the vicinity of Iapetus! Unfortunately it's not heavy enough for the ponding idea to work. Also I don't see how it could cause a colour change on the surface.

Posted by: tasp Oct 26 2007, 04:01 AM

I am thinking 'ponding' is not a problem.

Assuming (for the moment) most of the gas transiently about Iapetus is N2 (after all, if it wafts in from Titan, that is mostly what is available), with an admixture of CH4 or perhaps C2N2 or HCN, then the lesser constituents might be subject to condensation, saturation, and/ or evaporation effects. Despite the huge difference in atmospheric pressure between earth and Mars, we see clouds, frost, etc. on both worlds.

Can we expect such effects at a further reduced atmospheric pressure ??

If N2 is stratified and/or temperature inversioned over a crater, can the gas 'trapped' in the crater 'saturate' with CH4 (or whatever)? I have been using the word 'ponding', but if there is a better word or phrase from meteorology (not my specialty, btw), let's use it instead of ponding.

If we suspect the overall makeup of the gas mixture is to progessively go 'lean' over time, and cause the resulting Cassini Regio 'crud' to smoothly change color from darkly reddish to darkly greenish from east to west (or vice versa, I forget), then we have an explanation for those seemingly 'anamolously' darkly reddish craters in the darkly greenish areas. Those craters have held a richer admixture of gas longer through the Iapetan day due to the stratification retained 'richer' gas in the crater.

Might we want to give the gaseous thermoreactive gaseous discolorant another look ??

{picture shows some low lying clouds on Mars, I am not suggesting clouds were/are visible on Iapetus, but a similar 'containment effect' might be occuring on iPateus and giving us the color differenced craters vs. adjoining plains effect}