[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]

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 ??

[JRehling]

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

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.

[ngunn]

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.

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.

[As old as Voyage...]

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.