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?'

[dvandorn]

I agree, Bill -- the distribution of dark materials just screams to me that it overlies the brighter, cleaner ice surfaces.

I also still believe I can see evidence of dark material piling up in craters, filling and/or deforming the "downwind" sides of some craters, and exhibiting some dune-like striations. I've been really busy and haven't had enough time to annotate images to point out what I think I'm seeing -- but I am certain that I'm seeing it.

To me, it all adds up to dark material covering light material, thickly in some places and more thinly in others.

-the other Doug

[David]

filling and/or deforming the "downwind" sides of some craters, and exhibiting some dune-like striations.

What do you literally mean by this? Obviously, there is no wind on Iapetus (hence your "quotes") and there can likewise be no dunes. So what process do you think is in fact occurring?

[dvandorn]

What do you literally mean by this? Obviously, there is no wind on Iapetus (hence your "quotes") and there can likewise be no dunes. So what process do you think is in fact occurring?

That is exactly why I put "downwind" in quotes. It's just a convenient way of indicating that I see apparent flow vectors in some of the dark surficial material.

Such a flow could be caused by any number of factors, and I'd be surprised if there is just a single factor controlling it. It could be anything from material creep forced along by interactions with Saturn's magnetotail to atmospheric movements caused by transient atmospheres generated by (relatively) immense amounts of sublimation or outgassing from the leading edge (as has been speculated). Heck, it could just be some odd form of impact ejecta interaction. I will point out that you see some "duning" landforms on other airless bodies, such as the Moon, which are almost definitely formed by ejecta processes. Those are just at much larger scales than you see on Iapetus.

I still think that those who dismiss the dark material as simply "the natural surface of Iapetus below the bright ice" are missing the point. This dark material is unlike any other surficial material on any other Saturnian moon -- Hyperion included. These bodies are made up primarily of ices. You'd have to postulate some mechanism that would first emplace a dark surface on Iapetus before you can start to speculate that the dark material is simply that surface with an overlay of clean ice removed from the leading edge. Having heard absolutely no such theory about the emplacement of a surface that is significantly darker (and of a significantly different composition) than we see on *any* other moon in the system, seeing this dark material having been moved along the leading edge in apparent flow patterns, and seeing a "true" bright ice surface exposed by impact from under the dark surface in many areas, I simply have to regard the possibility of the dark surface being Iapetus' "natural" surface as the least likely of all of the theories.

So, all of my observations and an application of Occam's razor leads me to the conclusion that the dark material is emplaced on top of a bright, icy surface that is far more representative of what we see on the rest of the icy moons. Hence my conclusion that we're seeing black on white and not vice-versa.

-the other Doug