[X] My Assistant

[TheChemist]

Carolyn Porco comments included in this space.com article :

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

[David]

Iapetus doesn't need a huge differentiation in brightness to start with; it just needs any amount of differentiation that will preferentially and predictably distribute ice from one location to another, rather than randomly; so that sublimation events remove ice from a slightly darkened area (making it darker and warmer) and deposit it in a slightly brighter area (making it brighter and colder). The (ice-)rich regions get richer, and the (ice-)poor get poorer.

The "natural" state of Iapetus, unaffected by external forces, would be, I think, an overall icy globe, all-white at the poles, but with darker sun-facing ridges and crater floors in the lower latitudes, just as we see in the middle of Roncevaux Terra. Cassini Regio is then a deviation from this basic state, motivated by an external force. That force would have to be solid particles, I think; surely a gas (of what origin?) suspended in circumSaturnian space would quickly become far too diffuse to have any real effect? But a very thin dusting, not needing to do more than to turn the ice cover from white to off-white, if it covered enough area would be enough to start the ball rolling. My question is whether it was a one-time event (something that disintegrated near Iapetus, for instance, leaving the dusting) or a series of repeated events, possibly still going on. I imagine the latter is more probable.