[X] My Assistant

[TheChemist]

Carolyn Porco comments included in this space.com article :

http://www.space.com/scienceastronomy/0710...ni-iapetus.html

[ngunn]

Why is everyone so sure about this global dusting from another moon? If the vast majority of the Cassini Regio dark material was already in the dirty ice of Iapetus and you only need a small amount of seeding material then the problem largely goes away. The albedo and spectrum of Cassini Regio should not be expected to match the seeding material, which may be pretty hard to track down at all if most of the seeding happened a long time ago. The source could have been a population of small, exhuasted (de-iced) periodic comet nuclei with aphelia close to the orbit of Saturn. These would have long since disappeared, so the search for a source may well be futile. I think it might be more fruitful to search for a 'colour' match amongst materials that might plausibly be trapped within the ice of Iapetus, or the radiation-altered products of such.

[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.