[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.

[RPascal]

A Model for Hyperion

For a better understanding concerning my model for Hyperion's appearance regarding the dark crater bottoms I made some diagrams.
Please note that this is not based on any official statement, so it should not been taken as an official explanation of what we see on Hyperion. This is solely my personal attempt of an explanation of the processes that may alter Hyperion's surface.
The conical shape of the craters or landslide effects are neglected in the drawings



Fig.1:
The original material of Hyperion is a porous, dark mixture of silicates (from dust to rocks), frozen water, and all the other components that are typical for a relatively primordial body of the solar system like carbon dioxide, methane, ammonia and other. Due to warming by the sun, perhaps supported my micrometeorite bombardment or by internal heating resulting from Saturn's gravitational influence, the water and the other volatile components slowly evaporate or sublimate.
This process leaves behind a brighter layer of silicate rich dust and rocks that covers the surface of Hyperion.

Fig.2:
The evaporation rate slightly decreases with growing layer thickness, but it does not stop, since the porosity of the layer still allows the volatile components to reach the surface. The timescale for this process is at least some million years.

Fig.3:
If a meteor crater is deep enough to reach down into the native material, we see this appearance that is typical for Hyperion, with the dark, water-rich material from below exposed at its bottom.
The crater floor will slowly develop a bright surface again, since the evaporation or sublimation should be increased if the layer of silicate dust is missing. On the other hand, the evaporation or sublimation rate may be reduced at the crater floor due to the originally porous material being compacted by the pressure and heat of the impact.

Fig.4:
Small craters within the dark material are also dark when they are young but...

Fig.5:
...the increased surface to volume ratio of protruding structures like the rim or the central hill speed up the loss of water. These structures become bright much faster than the surrounding terrain.

Fig.6:
After a sufficient time the complete crater is bright again. The thickness of the bright layer for a long time remains thinner than at the older surface. This may be the reason, why in this large impact basin more and smaller craters seem to have a dark floor than elsewhere.

--René

[silylene]

A Model for Hyperion

Not bad - it is rather consistent with the brightness observations. How can this model explain the cone shapes?

[RPascal]

Not bad - it is rather consistent with the brightness observations.  How can this model explain the cone shapes?

I suppose that, if this model is correct, the silicate-rich dry crust should have a consistency of fine, low density sand, while the original material should have a consistency perhaps like foamingly frozen clay.
A meteorite, especially impacting into the low density powdery crust, but also in the low density original material, would explode (explosively evaporate) very deep below the surface. This creates a steep, cone-shaped crater, as discussed for "Deep Impact".
Later landslide may occur on the crater walls, also leaving a cone-shaped crater with less steeper crater walls.
There should be a smooth transition between the "frozen clay" and the "dry sand" consistency with decreasing frozen water content, somewhere in between the material should have a low ice content, but with just enough ice between the grains to glue them together and prevent landsliding of the steep walls. With the ice content further decreasing the landslides start.

--René