[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.

[Guest_BruceMoomaw_*]

We're starting to hear from the science team themselves, and they do seem to be oriented toward the view that the lighter-colored ice is sublimating away and leaving a lag deposit of dark stuff that then slides to the bottom of the craters.

http://www.nature.com/news/2005/050926/full/050926-15.html : "The Cassini pictures also show that many of these craters have been partly filled with dark red stuff. 'The general idea is that this is some kind of organic material,' says Denk.

"Some astronomers think that the material comes from Saturn's outer satellites, which are peppered by the stream of energetic particles in the solar wind. This ejects dirt, which is then swept up by Hyperion and one of Saturn's other moons, Iapetus.

" 'But there are big problems with this idea,' says [Tillman] Denk. He points out that the tiny outer moons could not release enough material to produce more than a light dusting on Hyperion, yet images of some relatively recent craters show that the dark stuff may be metres thick. This suggests that much of the dingy dust comes from an unknown source, says Denk.

"The crater's sharp rims may provide a clue, says Carolyn Porco, who leads Cassini's imaging team. These well-defined edges indicate that ice around the rim is slowly subliming, releasing trapped dirt that tumbles down into the crater pits. 'The question is why would this happen on Hyperion to this extent,' she asks. 'No other body in the saturnian system looks like this.' "

http://www.latimes.com/news/science/la-sci...la-news-science : "Researchers are particularly eager to learn the identity of the dark material that fills many craters on this moon. Features suggest it may be only tens of yards thick, with a brighter material underneath."

Sounds very much like Jeffrey Bell's model in which a relatively lightweight but fast micrometeoroid bombardment is vaporizing away large amounts of ice, leaving behind a relatively large lag deposit of native dark material that then slides down to the crater bottoms. The difference is that with Iapetus that sparse but fast meteoroid bombardment comes from Phoebe, whereas in the case of Hyperion it may be a remnant of in-spiralling Phoebe material, or may be ejecta blasted off Hyperion itself by large impacts that then gets trapped in the 4:3 Titan resonance zone until it finally comes back and crashes into Hyperion again (also producing that overdose of small craters). It will be very interesting to get a closeup look at small craters on Iapetus for similar phenomena.

However, there's one additional possible twist. From a photo caption at http://sciencenow.sciencemag.org/ : "The dark material visible at the bottom of the densely packed impact craters seems to have absorbed extra solar heat that ate into the underlying ice, deepening normally saucer-shaped craters into honeycombs." So such solar erosion may be a contributing factor to -- or the sole cause of -- the craters' strange depth, rather than my model of Hyperion's surface being so fluffy that impactors plow down through it a short distance before exploding and thus excavate deep conical craters. (The Cassini data on the thermal inertia and radar reflectivity -- and thus the looseness -- of Hyperion's surface will be important here.)

[RPascal]

We're starting to hear from the science team themselves, and they do seem to be oriented toward the view that the lighter-colored ice is sublimating away and leaving a lag deposit of dark stuff that then slides to the bottom of the craters. 
...

Concerning all the models that assume accumulation of the dark stuff from above I see many discrepancies with the Cassini high res images.
At first, a layer of material should exhibit many spots were meteorite impacts blasted away the layer, leaving a bright spot behind. Beyound the hundreds of craters visible I could not find a single crater within the dark stuff that looks this way. An inspection of the general process of crater formation shows that at least the center of a crater exposes material from beneath. The center of the small craters would have to be bright if the dark stuff was just a thin layer.
On the other hand one could assume that nearly all craters are older than the layer, but then there would be new discrepancies: if the material falls in from space, one has to explain why there are so many craters without any trace of the black stuff.
Assuming it is material that slides down the walls of the large craters after its icy component has disappeared, we have to explain why we do not see the small craters or other structures within the large craters to hinder the material sliding down. Below these small craters a fan shaped area should be visible where less material is present because it has to flow around the protruding structures (see the drawing for discussion, the red circle symbolizes a crater with the expected fan shaped area below drawn into the image).

On Phoebe I am quite shure that we can see dark material sliding down the steep slopes and exposing a bright layer (ice?) below, it looks exactly as one would expect it to look, but comparing this with Hyperion I think that there must be a different origin for the dark crater floors on Hyperion.

--René