[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.

[volcanopele]

Just a thought for the day: Are all of you sure that Hyperion's craters are unusally deep? I think the strange topography and the dark material on the floors of many craters give the optical illusion that many craters are quite deep, when in fact, many are quite shallow.

[RPascal]

Just a thought for the day: Are all of you sure that Hyperion's craters are unusally deep?  I think the strange topography and the dark material on the floors of many craters give the optical illusion that many craters are quite deep, when in fact, many are quite shallow.

My impression is, that the craters are not unusually deep (or steep), compared with Phoebe for instance, but the craters are more often funnel shaped (as on Phoebe too) than the typical bowl shape when compared with a larger, (more solid?) moon. One can find also a lot of shallow craters on Hyperion, and for me it does not look as if a dark crater floor is correlated with the slope of the walls.

For a good impression of the true brighness contrast between the dark and the bright material this image may be useful:
http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=50265

It shows the dark stuff in sunlight, as well as the total darkness of the shadows. With the help of this image I estimated the albedo of the dark material being about 40-50% of the albedo of the bright material.
One crater above the center has the shadow on its bright, as well as on its dark region. Concerning the shape of the shadow, there seems to be no significant difference in slope between the two zones for this particular crater.

-- René

[JRehling]

[...]

[RPascal]

Compare this Callisto closeup with Hyperion. It is tempting to consider that something similar is going on, but that Callisto is drenched with the dark stuff while Hyperion has much less -- perhaps as though Callisto has a higher "water table" of the dark stuff, with it only "ponding" away from the highest few percentiles of the local topography, while on Hyperion, the dark stuff is ponded only in the lowest percentiles of the lowest topography. That analogy being presented, I don't think in either case, we're truly seeing some subsurface dark layer being uncovered. Rather, I think there's a component of nonice dark stuff in both places, and Callisto just has a higher share of it, so it is more nearly covered in lag deposit.

http://nssdc.gsfc.nasa.gov/image/planetary...l_cl3_48125.jpg

http://photojournal.jpl.nasa.gov/jpegMod/PIA07741_modest.jpg

At first glance there seem to be similarities, since Callisto also has these bright crater rims. But at Callisto you can see that all the terrain without a steep slope is covered with the dark stuff, while on Hyperion most of the flat terrain shows no trace of it, if it is not a crater bottom. I can't imagine how it should get there without leaving a trace anywhere else, especially a trace of sliding down the crater walls.

--René