[X] My Assistant

[Michael Capobian...]

As we approach Iapetus for this close fly-by, I thought it might be fun to try and predict if, after we see the close up images of the transition region and Voyager Mountains, the source of the dark material will be obviously endogenic or exogenic, or there will still be no obvious explanation.

I'll go for the longshot, endogenic.

Michael

[volcanopele]

My money is exogenic. Iapetus is dead. It is deader than a doornail. Good grief, it is deader than Rhea for goodness sake. I very much doubt anything has been boiled, geysered, expelled, or outgassed there in a VERY long time, if ever. So given the lack of an obvious endogenic source, one needs to look outside Iapetus for the source.

We know a few things about this material. We know it contains a lot of carbon-based materials, both organic (like PAHs) and cyanides. We know that it is recently emplaced or is being emplaced as we speak. John Spencer's theory seems to explain the specifics of its distribution, that the lower albedo material has led to increased sublimation of H2O from the dark area, leading to brighter poles and an extended range of dark material along the equator. It seems to be concentrated either in depressions (suggesting that this material "flows" downslope) or along equator-facing slopes.

Again, my money is on a planetocentric origin, likely do to accumulation of dust, rather than of ejecta from a large impact. We should, however, be careful about immediately attributing to Phoebe. There are plenty of small, little moons out in the outer Saturnian system that could be the donor. I'm guessing Hati...