[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

[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.)

[Guest_BruceMoomaw_*]

Another problem with the solar heating model of Hyperion's deep conical craters: why are they totally different from the very wide, steep-walled but flat-bottomed "pancake" craters on the nucleus of Wild 2 and Tempel 1? The latter are very likely produced by solar erosion -- but the dark lag-deposit dirt only slides down the steeper upper slopes of the crater bowl until it comes to slopes shallower than its angle of repose, at which point it builds up in a layer that prevents further sublimation of the underlying ice downward, leaving the newly cleaned upper slopes which are steeper than the dirt's angle of repose to vaporize away further and thus expand the original crater outwards horizontally without deepening it any further.

Is it that the original ice-dirt mixture in comet nuclei is darker to begin with than the original unmodified ice of Hyperion (and studies have shown that ice can be very substantially darkened by just a small admixture of dark dirt), so that the difference in albedo between the upper crater slopes and the dark crater bottom is much greater for Hyperion than for comets? Or is this genuine evidence for the alternative buried-impactor explosion theory of Hyperion's craters?

[JRehling]

[...]

[Rob Pinnegar]

It's a pity that Cassini didn't get a closer look at Himalia. It would have been interesting to compare it to Phoebe and Hyperion...

Also, I wonder to what extent Hyperion has been influenced by its proximity to Titan? Wonder if anyone's run any simulations regarding its orbital evolution. One would expect that Titan should keep it pretty much locked in place, but you never know.

[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.

[tedstryk]

It's a pity that Cassini didn't get a closer look at Himalia. It would have been interesting to compare it to Phoebe and Hyperion...

Also, I wonder to what extent Hyperion has been influenced by its proximity to Titan? Wonder if anyone's run any simulations regarding its orbital evolution. One would expect that Titan should keep it pretty much locked in place, but you never know.

It did get spectral data on Himalia...I wonder how it compares...

[David]

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.

I don't know what the line between "unusually deep" and "not so deep" is -- perhaps steep would be a better word than deep, but I can't easily estimate crater-wall angles either. But I do get the impression that the craters with dark floors are generally deeper than the craters without. Is this impression mistaken?

[Bill Harris]

The appearances of some of the Hyperion craters is unusual, whether an illusion or real. These craters appear conical (straight, steep sides), un-symmetrical, have layering and gulllies on the rim, with dark material pooled at the bottom.

The overall appearance is odd. Given that the albedo of Hyperion is 0.2, but the images appear to range from fesh snow (highlands) to pitch-black (pooled material), there has been considerable stretching done.

My 2c.

Take a look at the Hyperion Image Products thread. They have done an excellent job of stitching the hi-res images into a global view.

--Bill

[algorimancer]

What intrigues me is that there are so many apparently non-circular small craters. To me the most likely explanation is that the crater shape is largely constrained by the underlying surfaces fracture properties, similar to the reason we see so many hexagonal craters elsewhere. I would guess that the regolith is not fully established, in terms of being uniformly fractured, either due to a young surface (unlikely) or perhaps an inhomogeneity in gross material properties due to viscous creep "healing" of older regolith.

[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

[ugordan]

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.

Not to be nitpicking, but I believe the raw images are posted as they are downlinked from the spacecraft. The ISS cameras have a mode in which they actually apply a lookup table to convert 12-bit pixel intensity into 8-bit numbers. The table is not linear and is in fact pretty close to a square root function with the end result that the raw images have their dark regions brighter than they really are. Sort of like applying a gamma>1 to an image.

The actual albedo difference could then in fact be greater than your 40-50% figure.

[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