Iapetus - Black on white or white on black? Options

[SFJCody]

Seems to be a lot of dispute on this subject... I think it's ice from the interior, but what does everyone else think?

Edit:

This world seems very complex so the question could perhaps be phrased as 'which of these options is most responsible for the Iapetan dichotomy?'

[edstrick]

I get an overwhelming impression that the white and black materials are both granular (at the optically visible surface -- I can't tell about 10 cm down), mobile, and SELF-SEGREGATE, sorting each other out.

Almost nowhere do I see what look like diffuse dustings of black onto white, and the dustings of white on black, like small crater ejecta in black terrains, clearly are rapidly destroyed, presumably by sublimation of the white ice. Near the equator in light terrain, dark material lies flat in depressions, down to surface pits and irregularities of unresolved sizes. Off the equator a ways, the material forms round splotches in smallish craters that are lopsided, sitting on the sun-facing floor of the crater, but not up on the adjacent wall. In bigger, flat-floored craters, it's on the floor against the sun-facing wall. Well away from the equator, I get the overwhelming impression that the splotches have actively climbed the wall! Landslides of ice may disturb the splotches, but I get the impression that the mass of granular dark material has actively crept into a maximally sun-facing position, even against gravity.

HOW this might happen, I can only vaguely arm-wave. A few beers too many might help! Thermal expansion-and-contraction of the dark granules relative to white ones might be a method of driving a creep process, together with thermal sublimation of ice at the contact with a coherent splotch of dark. Numerical theoy and physical modelling of processes might give a clue.