[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.