[X] My Assistant

[RPascal]

The images of Hyperion are fascinating, and its appearance with the many dark crater bottoms and the very steep crater walls seems to be unique, never seen on any other Solar System body before (?). For this reason I wanted to start a special topic solely about Hyperion and the origin of its appearance here.
(By the way: phantastic mosaics and color images, Jason!)

In most discussions here I have read the idea, that the dark crater bottoms is material that slides down the steep crater walls after the volatile icy component has evaporated or sublimated away. But looking at the many images I could not find a single (small or large) crater in the dark material that would expose bright material beneath. Shouldn't we expect this? If the dark component would indeed be a more or less thin layer deposited on the crater bottoms one should find many small craters were this thin layer was blasted away by the impact.
Because of this, my impression is that the dark stuff is the material from inside Hyperion, exposed by impacts, and the bright material is the crust.

[dvandorn]

...looking at the many images I could not find a single (small or large) crater in the dark material that would expose bright material beneath. Shouldn't we expect this? If the dark component would indeed be a more or less thin layer deposited on the crater bottoms one should find many small craters were this thin layer was blasted away by the impact...

The dark floors of these craters seem quite "level," especially in relation to the very steep crater walls. And, in the highest resolution images, you can see that most of these dark units display cratering, in some cases down to the limits of resolution. The dark areas *seem* to be less heavily cratered at these smaller size ranges than the brighter inter-crater areas, but they do retain cratering morphologies. There are also some odd, almost viscous-looking morphologies in a few of the dark units.

If the dark units were just dark dust that's settled to the bottoms of craters in a relatively light-colored surface, you wouldn't expect them to be solid enough to retain a cratering record at such small scales.

This stuff just feels to me like stuff that seeped up into the craters from below, rather than stuff that filtered in from above. That's a gut feeling, and it will quite probably prove to be wrong, but that's what it looks like...

-the other Doug

[RPascal]

...
This stuff just feels to me like stuff that seeped up into the craters from below, rather than stuff that filtered in from above.  That's a gut feeling, and it will quite probably prove to be wrong, but that's what it looks like... 

-the other Doug

I can only agree, this was also my impression, while I don't think it is exactly like this.
What is also striking, and not so easy to explain is, that most of the craters in the dark material have a more or less bright rim that lets them appear like rings, and you can only find very few craters that are completely dark.
But perhaps I have an idea for a model. What about the possibility, that the dark material has a high amount of volatile components (frozen water, hydrocarbons), and after evaporation or sublimation the bright stuff remains. This seems to be the other way round than we expect it (we expect ice to be bright), but ice mixed with other fine grained stuff in most cases is darker than the same stuff alone.
Originally Hyperion was dark, but due to evaporation or sublimation of water and the other volatile components the bright loose crust formed. This evaporation process continues even through the low density crust, that very slowly grows in thickness on the cost of the dark core. If a meteorite strikes the surface, the original dark material is exposed again, the evaporation is increased at that place, and after some million years the crater floor is bright again.
But why should the rims of the small craters be bright? This model could explain it very easily: Any structure protruding above the original plain has a higher surface- to volume-ratio and would "dry out" faster than the surrounding flat floor. The fact that the rim is made of shattered material should support this process. The small craters that are completely dark would then be the youngest craters of all craters existing in the dark material.
If this is true, Hyperion would indeed be very close to a comet's nucleus.

--René