[X] My Assistant

[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.

[ugordan]

The fact remains the leading and trailing hemisphere dark material has visually different look suggests whatever was deposited on the leading side wasn't only trace amounts. The leading hemisphere has a much more pronounced reddishness to it, different to the trailing side which, in stretched colors appears really greenish (those who didn't believe my calibrated stuff very much now have official proof of this). In fact, it could be postulated it's this greenish stuff that's native to Iapetus and the redder stuff was deposited (with potentially slightly impact-altered chemistry). The trailing side has a uniform subtle greenish hue to ice at equatorial latitudes as well. Interestingly enough, I recall the dark region on Dione (cliffy terrain) appears green in the same filter combination. "Green" is a relative term here, implying weak infrared and ultraviolet reflectance, not necessarily visually greenish stuff.

Invoking long-gone comets close to Saturn's orbit is IMHO stretching Occam's razor a bit too much. Why doesn't Phoebe have the same uniform coating then? Why would this only happen to occur at Saturn? If the comets were "spent", that'd mean their perihelia were much closer in so it would be seen at least in the Jovian system too. Furthermore, if the dust was coming from outside the Saturnian system perturbations by Saturn would most likely make the dusting affect a good portion of Iapetus, not just the leading hemisphere.

[tasp]

The fact remains the leading and trailing hemisphere dark material has visually different look suggests whatever was deposited on the leading side wasn't only trace amounts. The leading hemisphere has a much more pronounced reddishness to it, different to the trailing side which, in stretched colors appears really greenish (those who didn't believe my calibrated stuff very much now have official proof of this). In fact, it could be postulated it's this greenish stuff that's native to Iapetus and the redder stuff was deposited (with potentially slightly impact-altered chemistry). The trailing side has a uniform subtle greenish hue to ice at equatorial latitudes as well. Interestingly enough, I recall the dark region on Dione (cliffy terrain) appears green in the same filter combination. "Green" is a relative term here, implying weak infrared and ultraviolet reflectance, not necessarily visually greenish stuff.

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)