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?'

[Bill Harris]

You're right, Ed, this is all very mysterious. The answer may be billions of 5cm-tall Monoliths, or something odder.

Seriously: one thing I've noticed from the closest images. We've seen a few small, recent craters with white ejecta rays in the dark regions. But I haven't seen the opposite: dark, fresh craters on the light-toned regions. This leads me to suspect that the black is a thin dusting over the white, whereas the white is a thicker layer.

I'm looking closely at the white-black-white contacts in the hi-res images for puzzle-pieces...

--Bill

[JRehling]

Seriously: one thing I've noticed from the closest images. We've seen a few small, recent craters with white ejecta rays in the dark regions. But I haven't seen the opposite: dark, fresh craters on the light-toned regions. This leads me to suspect that the black is a thin dusting over the white, whereas the white is a thicker layer.

I think this is absolutely correct. But the truth may be a little more complex.

Most icy moons have a surface somewhat darker than ice. Impacts tend to surface whiter ice underneath. But the surface isn't dark, just "dingy".

I suppose that Iapetus's native regolith was like that -- similar to what we see on Rhea. The places where dark has spread, the top surface layer has lost its ice, leaving only the dark stuff behind. This could be, and is almost bound to be, quite thin.

If the ice that sublimated away from dark stuff has settled onto the original dingy surface in non-black areas, then we have at least three layers: fresh ice deep in the crust, a dingy regolith, and a fresh white coating of frost. So an impact in the white areas would surface a tiny bit of dingy material, but far more white ice.