[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

[JRehling]

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.

The trick is that some of the explanations combine endo- and exo-. I'm reminded of the three theories regarding the Moon's origin, which seemed to be comprehensive in addressing the possibilities, but the evidence ending up generating a fourth explanation which sort of combined two or all three of the initial set.

John Spencer's theory is that the initial emplacement of dark stuff is exogeneous, but then the darkness spreads, creeping along the surface where thermal conditions are appropriate. I'm not sure how you'd classify the second process, which is neither endo- nor exo-.

I still harbor someplace in my heart for a "big splat" origin, but Spencer's theory is growing on me.

When it comes right down to it, the quirks of Cassini Regio are such that there probably isn't a one-process explanation. We have these clues:

Generally matches the leading side.
Generally matches areas of a certain insolation/thermal constraint (reflected by latitude and local slopes).
Generally equator-facing slopes (another statement of the above).
Border is very streaky in places (eg, to the north).
Concentrates around the Snowman craters in ways implying a source from the TRAILING side.
White mountains within.
Western boundary appears jagged.
Not interrupted by many small, new, fresh craters.
The equatorial ridge runs almost right down the middle.

FWIW, other Saturnian satellites (Tethys, Dione) have dark blotchy areas, but with much less contrast.

I can't see one process matching all of those quirks.

I go with a big splat (dark impactor) that seeds an ongoing thermal creep.

[Stu]

... and the award for most "ugh, that's downright disturbing" sentence ever used in an UMSF post goes to...

( opens the envelope...)

seeds an ongoing thermal creep.

[JRehling]

Yes, the creeping crud is a disturbing notion. It's only a pity I used the phrasing to describe a neat-looking place like Iapetus instead of Hyperion, which looks like a rotting fruit.

Another armchair scientist perspective on this is to pose the question why Iapetus would have this dark stain spreading on it, but Ganymede does not. Ganymede receives about 4 times the solar radiation that Iapetus does, and despite having a much slower rotation, still gets much warmer (160K) at the equator than Iapetus does (130K). So why doesn't Ganymede boil its equatorial surface ice away, leaving dark lag deposits, to the same extent?

Not that I can't imagine an answer, but its suspicious.

Oh, how far-off September 2007 seemed when I first read the tour description...

[Gsnorgathon]

...
Another armchair scientist perspective on this is to pose the question why Iapetus would have this dark stain spreading on it, but Ganymede does not.
...

Well, Ganymede formed in a much warmer place and had higher temperatures over a long period time due to internal heat as well as solar. It might have been too warm in the first place, or so warm afterward that all the gunk burned off.

(Though I suspect the gunk on Cassini Regio is exogenous + radiation/heat processing on the surface, and not at all applicable to Ganymede. I'm just nitpicking.)