[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.

[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}