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]

After seeing some of the contrast-enhanced images from the trailing hemisphere, I have to say that my view has changed slightly. I still firmly believe that the dark surface on the leading edge is a mantling on top of a brighter ice substrate, but there is good evidence now that where this dark material has mantled the surface along the edges of the leading hemisphere, there has been deposition of bright ices on top of it. In places.

What is so odd to me is that neither hemisphere shows much in the way of cratering that exhumes a different-albedo substrate. There are occasional bright-rimmed craters on the leading hemisphere, but almost no dark-halo craters on the trailing one. And the bright-rimmed craters are just that -- bright-rimmed. There is little to no evidence of bright ejecta around them.

I'm having a hard time believing that the impact rate is so small that there have simply been few to no impacts of any size since the materials we see were emplaced on Iapetus' surface. But at the same time, I have a similarly hard time believing that the emplacement of these materials is anything but ancient -- particularly since some of the flow patterns hint at *aeolian* deposition/deflation, which is awfully difficult to explain on a currently airless body.

So we are left with a paradox.

-the other Doug

[JRehling]

I'm having a hard time believing that the impact rate is so small that there have simply been few to no impacts of any size since the materials we see were emplaced on Iapetus' surface. But at the same time, I have a similarly hard time believing that the emplacement of these materials is anything but ancient -- particularly since some of the flow patterns hint at *aeolian* deposition/deflation, which is awfully difficult to explain on a currently airless body.

So we are left with a paradox.

-the other Doug

Again, I point to the fact that the "white" stuff isn't white. It's pretty bright, but it has an albedo of about 0.6, far from pure ice. What's happening is that when an impact surfaces some of the bright stuff from below, the thermal segregation model immediately goes to work on the excavated material, and if the situation was such that the native material turned dark in the first place, then the virgin material is going to end up the same way. Maybe it takes years, maybe centuries, but whatever the timescale, it's a blink in geological time. The only bright ray systems we see are very recent ones.

Incidentally, much the same thing is true of ray systems (usually bright on dark) on Mercury and the Moon. Those aren't the only impacts on those worlds -- they're the newer ones. For example, Tycho is estimated to be 108 MYA, Copernicus 800 MYA, and Aristarchus 450 MYA. Those all have salient ray systems. The more typical impact on the Moon, 5 to 10 times those ages, doesn't.

The dynamics of the thermal creep and the hecameter-scale shapes of the boundaries is bound to be an open and interesting question for modelers to address. There are myriad comparable issues pertaining to how winter ice and snow melts on Earth, and I doubt all of them have attracted research interest. (Not sure, even, what FIELD such research would be classified as. Geology? Meteorology? Landscaping? Poetry?)

[David]

The dynamics of the thermal creep and the hecameter-scale shapes of the boundaries is bound to be an open and interesting question for modelers to address. There are myriad comparable issues pertaining to how winter ice and snow melts on Earth, and I doubt all of them have attracted research interest. (Not sure, even, what FIELD such research would be classified as. Geology? Meteorology? Landscaping? Poetry?)

Climatology, I imagine -- we have studies of glaciation over time, and more recently studies of glacier recession, and of other results of large-scale ice-melt, like the diminution of ice shelves in the Antarctic. However, studies of how this works when there's no liquid phase for the ice to transition through are doubtless much harder to come by -- the most obvious source of information on this subject would be the study of the Martian polar regions.