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Posted by: RPascal Oct 2 2005, 12:01 AM

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.

Posted by: dvandorn Oct 2 2005, 03:39 AM

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

Posted by: RPascal Oct 2 2005, 11:08 PM

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

Posted by: BruceMoomaw Oct 3 2005, 12:46 AM

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-hyperion1oct01,1,1919164.story?coll=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.)

Posted by: BruceMoomaw Oct 3 2005, 01:02 AM

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?

Posted by: JRehling Oct 3 2005, 01:19 PM

[...]

Posted by: Rob Pinnegar Oct 3 2005, 02:42 PM

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.

Posted by: volcanopele Oct 3 2005, 05:34 PM

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.

Posted by: tedstryk Oct 3 2005, 09:50 PM

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

Posted by: David Oct 4 2005, 03:46 AM

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?

Posted by: Bill Harris Oct 4 2005, 09:27 AM

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 http://www.unmannedspaceflight.com/index.php?showtopic=1497&hl= thread. They have done an excellent job of stitching the hi-res images into a global view.

--Bill

Posted by: algorimancer Oct 4 2005, 01:00 PM

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.

Posted by: RPascal Oct 4 2005, 03:40 PM

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/images/raw/raw-images-details.cfm?feiImageID=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

Posted by: ugordan Oct 4 2005, 04:00 PM

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.

Posted by: RPascal Oct 4 2005, 04:27 PM

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

Posted by: JRehling Oct 4 2005, 04:32 PM

[...]

Posted by: alan Oct 5 2005, 11:30 AM

I suspect the perceived cone shape of the craters is due to the landslides visible in many of them. The landslides have buried the dark material in the outer part of the crater so it is only visible near the center or more often just on one side of the crater. The contrast makes the dark areas appear deeper than it actually is. Look at some of the high resolution images, the dark areas don't appear much lower than the surrounding bright areas in them.

Posted by: RPascal Oct 9 2005, 02:12 PM

Your impression may be correct, in many craters there seem to be landslides visible that have buried the dark floor partially or completely. But I cannot see features that piont to any sliding of the dark material. Looking at the landslides it seems to my that the bright material should have a consistency of dry sand.

--René

Posted by: BruceMoomaw Oct 9 2005, 09:56 PM

No. Take a look at the many obliquely-viewed Hyperion craters. They really ARE extremely deep relative to their width; they really are cone-shaped, and they really do usually have pools of dark material in their bottoms that are clearly distinguishable from the shadows there.

Posted by: RPascal Oct 10 2005, 03:51 PM

Sorry, my last answer was perhaps verbalized a little bit sloppy and therefore misleading:
I meant the impression concerning the landslides.
Concerning the crater wall steepness, as I already wrote as an answer to Jason's "thought for the day", my impression is as follows:
Compared to Phoebe for instance, the craters are not much steeper, but compared to other much larger (more solid?) moons many craters are really very steep, and the walls very often have this typical conical shape that can be found on Phoebe too.
--René

Posted by: RPascal Oct 10 2005, 04:12 PM

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

Posted by: volcanopele Oct 10 2005, 04:50 PM

My thought of the day, and my thought for today as well, was that most of the craters on Hyperion, particularly within Bond-Lassell crater, are neither unusally deep or have steep walls, those with dark floors anyway. Most of the craters appear to be quite shallow. True, there are deep and steep walled craters. Bahloo is one example. But most appear to be shallow.

Posted by: RPascal Oct 11 2005, 08:45 PM

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

Posted by: silylene Oct 17 2005, 02:37 AM

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

Posted by: RPascal Oct 18 2005, 04:06 PM

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

Posted by: RNeuhaus Oct 18 2005, 04:28 PM

It is strange that there is landslides on the world where the temperature never rises above than 0 centigrades...I don't think that Hyperion might have landslides since the ice is very hard and firm on their surfaces.

Rodolfo

Posted by: JRehling Oct 18 2005, 05:29 PM

[...]

Posted by: tasp Nov 4 2005, 02:54 AM

I will go out on limb for this;

The dark stuff on Iapetus, is exactly the same dark stuff in the crater bottoms of Hyperion.

?

Yep. Same stuff.

How......

Major impacts on Titan that exceed Edward Tellers criteria for blasting atmospheric materials into space (from the old H-bomb days) send methane and nitrogen to Hyperion and Iapetus via Saturn's magnetotail. As we have seen on Enceladus, even low grav objects can have some atmospheric retention capabilities. The gases persist in the deep holes of Hyperion, and are polymerized by solar radiation. In the case of Hyperion, the steep crater walls make a hot spot at the crater bottom whenever the sun is over head. Due to the chaotic rotation, the sun is eventually overhead everywhere on Hyperion.

On Iapetus, the long orbit period around Saturn implies a 'day' 40 days long. After Iapetus transits through the magnetotail of Saturn, the leading hemisphere is turning sunward and the photo- or thermo- chemical reaction processes the methane and nitrogen into dark brown goo. Same stuff in Titans atmosphere, aerosolized is orange, puddled, its dark brown. The spot is symetrical north and south due to the symetrical insolation of the surface. The trailing hemisphere doesn't develop the dark coating because the methane and nitrogen are lost to the void or used up in 40 days and when that side of Iapetus is exposed to the sun, no gas, no darkening.

Posted by: nprev Dec 8 2005, 07:00 PM

Interesting and plausible hypothesis, Tasp. Here is an alternative, but not so nearly well thought out as yours. Two assumptions:

1. Hyperion is a recently (>1 By) captured outer system object, possibly an errant KBO, that probably formed at an even greater distance from the Sun than Phoebe. This would explain its unusual orbital parameters, rotation period, and apparent lack of internal chemical differentiation.

2. KBOs are known to be rich in dark surface organics, which probably also permeate the interiors as well.

During the capture event, Hyperion experienced substantial tidal heating as its orbit was gradually regularized by interactions with both Saturn and, later, Titan. This caused localized phase changes of material within the otherwise undifferentiated body that produced the "foamy" appearance of the moon. Note that many of the "craters" viewed in cross-section on the walls of the large impact basin have an almost tubular appearance; they might thus be explained as chimneys through which fluidized or gaseous materials rose to the surface, producing localized, explosive eruptions.

Some of the dark material was ejected onto Iapetus during the process of orbital stabilization, where it remains visible today; most of it probably rained on Titan and disappeared into the murk.

Posted by: Phil Stooke Dec 8 2005, 08:03 PM

"Note that many of the "craters" viewed in cross-section on the walls of the large impact basin have an almost tubular appearance; they might thus be explained as chimneys through which fluidized or gaseous materials rose to the surface, producing localized, explosive eruptions. "

I think this is just slope failure above a crater forming at the foot of a scarp.

Also, I suspect most of the sponge-like effect is an optical illusion caused by the dark floors of so many craters. The dark floors are the really interesting thing here.

Phil

Posted by: nprev Dec 9 2005, 04:29 AM

Could be...but even the ragged, non-spherical shape of the moon (anomalous for its size) and the extreme apparent crater density (a grazing encounter with the rings?) argue for a capture scenario, odd orbital and rotational characteristics aside...I still have to wonder how much tidal heating a capture event might generate.

Posted by: ljk4-1 Feb 4 2006, 02:55 PM

Rough and Tumble Hyperion

Summary - (Fri, 03 Feb 2006) Cassini captured this image of Saturn's moon Hyperion during a distant encounter in December, 2005. Hyperion is only 280 km (174 miles) across, and is covered with closely packed pits, giving it a spongy appearance. This photo was taken when Cassini was 228,000 kilometers (142,000 miles) from Hyperion.

http://www.universetoday.com/am/publish/tumble_hyperion.html?322006

Posted by: Gsnorgathon Feb 4 2006, 10:07 PM

And if you liked the book, http://saturn.jpl.nasa.gov/multimedia/videos/video-details.cfm?videoID=109!

Posted by: nprev Feb 4 2006, 11:59 PM

Nice!!!

The thing that strikes me the most about Hyperion is that it's so battered, more so than any other known body, IMHO. Phoebe or any of the main-Belt asteroids don't even come close to this crater density, and if I'm not mistaken Phoebe is the only other object in Hyperion's size range that we've examined closely.

So...I reiterate my hypothesis that Hyperion was captured after one or more passes through the rings followed by a close encounter with Titan that placed it in its current orbit. Alternatively, it may have smashed through a ring that used to be beyond Titan that was eventually swept up by Hyperion after being braked thereby into its present position.

Posted by: tasp Feb 5 2006, 06:54 AM

Maybe Hyperion was a satellite of Titan, formed concurrently, and 'spun off' not unlike what our own moon currently is experiencing tidally from earth.

Granted the spin off thing only works (if it works at all) prior to Titan achieving tide lock with Saturn.

Hyperion, while close to a still accreting Titan will take quite a beating.

Posted by: nprev Feb 5 2006, 09:22 AM

It would be interesting to see if there is a well-defined cluster of similar crater sizes on Hyperion; at first glance, this appears possible. If there is, this might indicate that there was a localized bombardment of objects of similar masses at similar velocities, which might support the ring-collision/capture hypothesis.

Correct me if I'm wrong here, but I am assuming that we are fairly confident that each of the major ring regions are composed of objects of similar sizes and compositions...

Tasp, your former Titanian moon hypothesis is also interesting. Do you have an estimate of how much time would be required for such an orbital evolution, or are there too many variables to consider? Ballpark guess here, but I'd say that Titan probably achieved tidal lock with Saturn within a billion years.

Posted by: JRehling Feb 5 2006, 01:51 PM

[...]

Posted by: tasp Feb 5 2006, 02:59 PM

Tide lock for Titan would have been achieved prior to Iapetus doing it. But the time scale (I think) depends on the 'stiffness' of Titan.

The dissipation of energy in the body of Titan (in my view) would be tough to guess.

Degree of molteness, rigidity of solid portion etc.

My 'big thing' in that scenario was having Hyperion moving slowly in the vicinity of Titan to have become captured in the 4:3 resonance. 'Spinning off' a satellite (if possible) would give you a slowly moving (relative to Titan) object in the vicinity of Titan for something interesting to happen.

Maybe something similar explains the little guys, too (Polydueces, Telesto, etc.)

Mass ratio of Hyperion to Titan, and Calypso to Dione (or was it Tethys? tooo many rocks to keep track of!) is similar.

Co-inky-dink? Heck if I know . . . . . . . .

Posted by: JRehling Feb 9 2006, 08:43 PM

[...]

Posted by: Phil Stooke Feb 24 2006, 07:17 PM

These pics of Hyperion just came down. I've enlarged them by a factor of two, and each is a composite of three frames.

Phil

Posted by: ljk4-1 Feb 24 2006, 09:40 PM

This recent paper claims that Hyperion looks the way it does now
due to a literal run-in with Titan as it was being captured by Saturn:

http://arxiv.org/abs/astro-ph/0602512

And that other Saturnian moons which "met" Titan were not so lucky.

Posted by: Phil Stooke Feb 25 2006, 12:07 AM

Based solely on the abstract, it doesn't really claim it, it just suggests it as a rather vague possibility. There couldn't be any evidence, I would imagine. Maybe the paper goes beyond that.

Phil

Posted by: tasp Feb 28 2006, 04:10 AM

Was poking around at the Cassini website looking at pictures of Hyperion.

Noticed something about picture N00040336.jpg (there are some similar frames of this picture available too)

The photo shows a typical Hyperion crater, except that it is on the edge of a depression and a good chunk of the crater wall is 'open' to the depression.

While the crater has a small dark shadow at its bottom due to angle of illumination, it does not appear to have the black stuff virtually all the other craters do.

Why?

The crater, open on one side, can not trap a pool of gas to be polymerized (or whatever, maybe thermo-catalyzed) at the bottom like all the other intact craters do. The gas runs (wafts? whatever word you wish to describe it exiting the crater) out and 'pools' in other craters downslope. The darkening effect does not occur here because even though there is enough crater left to focus the sun at the bottom, there is no material for the UV or heat to process.

I am doing a great job of convincing me I know what is going on on Hyperion and Iapetus, ( ), but I don't know about the rest of you . . . .