[X] My Assistant

[SFJCody]

Seems to be a lot of dispute on this subject... I think it's ice from the interior, but what does everyone else think?

Edit:

This world seems very complex so the question could perhaps be phrased as 'which of these options is most responsible for the Iapetan dichotomy?'

[ngunn]

I think the responses you offer are inadequate. Globally I go for both on lightish grey - the zebra itself is (almost) invisible.

On local scales any of your four are also possible so individual details may be a poor guide to the big picture. For that reason I think a simplistic poll is not such a good idea.

[ugordan]

Where's the option "Black on White - black combination of endogenous and exogenous" ?

[akuo]

Is this a Michael Jackson song?

Oh Iapetus, I wouldn't have a clue. Planetary experts, save us!

[volcanopele]

I would prefer a both option: white on black in some areas, black on white in others.

[Juramike]

(Objects collide)

Object one: "Heeeey, you got white stuff on my black stuff!"

Object two: "And heeeey, your got black stuff on my white stuff!"

(together): "Two great tastes that taste great together!"



[total shameless ripoff of old Reese's Peanut Butter cup commercials.]

[[Did you know the original Reese's commercial starred John Travolta colliding with Robbie Benson?]]

-Mike

[nprev]

...of course, in the Family Guy parody of this commercial, Officer Reese shoots the other guy in order to keep the precious discovery for himself...

Interesting how uncertain we all are now. I was a solid black-on-white-exogenous bloc member until the flyby; now I've gone white-on-black-exogenous.

The craterless white highlands may hint at some internal activity still, but damned if I can think of a mechanism. Iapetus isn't well-situated orbitally for tidal heating, and its density doesn't seem high enough to make a compelling argument for enriched radioisotopes. What's left?

[JRehling]

I would prefer a both option: white on black in some areas, black on white in others.

Yep, but 95% black on white.

White definitely gets onto black via landslides and impact events.

It *may* have the ability to creep back onto black, but aside from some possible very subtle diurnal dynamics, I don't see why there'd be anything but steady-state. If an area got black in the first place, I don't see it trapping frost to let the white cover it up.

Diurnal flux is an interesting possibility, but probably happens on a minute level on an airless world. You could have dark areas that have passed into night collecting some H2O frost boiling off the daytime side, but the opportunities for long-distance migration have to be very slight.

I think Ganymede's faint polar caps aggregated that way, but that's a steady-state thing. The high latitudes on Ganymede are always cold. If an area on Iapetus gets warm enough to turn black, it'll be that warm again within one revolution. OK, some seasonal effects could happen there too, but that'd be even more subtle since this stuff mainly happens outside the high latitudes.

To answer the main question, it's like this:

The native surface of Iapetus was white, but with some fraction of dark stuff mixed in (maybe like Rhea nowadays). Equatorial craters already were darkened by the focusing effect of what is essentially a concave mirror. The leading face was darkened a bit more by exogenous dust. That kicked off a phenomenon that caused the black to cover all but the high latitudes on the leading face and to creep around a bit to the trailing side.

The sublimation from the black stuff can migrate to produce a whiter frost on the light stuff, so it's entirely possible that the native surface was significantly darker than the whites we see now. If so, we might see some small craters that excavated darker ray systems. On the other hand, the native dark surface may itself have been a thin layer, so a crater would excavate deep fresh ice as well as a little darker surface, which would make it very hard to spot anything. Maybe such features are in the imagery from this flyby?

[Michael Capobian...]

I have to choose white on black on white - black endogenous, white both endo and exogenous.

Here's some of the things I see:

Globally bright water ice with a cover of endogenous black gunk of varying thicknesses. Very thick on leading hemisphere, even big craters don't break through.

Water ice brought to the surface and scattered about by impacts, some of them very large (Claude), which also contribute additional water ice. Gathers preferentially in impact-facing slopes, like Voyager Mts. White layer very thin in places, exposing dark ridges and bumps. Craters focus heat and start to remove bright material.

Water ice removed from the leading hemisphere by some exogenous process and emplaced on the trailing hemisphere and poles through sublimation.

Dark stuff spreads wherever it's exposed. Chunks of dark stuff hurled about by big impact(s), causes holes in thin bright ice layer where they fall. Crater rays can also remove white overlayer, leave dark lines. Black spreads by sublimating material around it.

It looks to me like the Claude basin and its ejecta are comparatively recent; and are the controlling factor for a large swath of the moon north and west of the basin. We need to think about what happens when a large, icy body impacts a body with a layer of gunk overlying ice.

It's interesting how a person's theories can affect what they see in these images, especially when there's the level of ambiguity we have at Iapetus. For example, I noted the little craters at the apex of certain parts of the equatorial ridge with great interest, especially considering that it looks as if portions of the ridge nearby seem to have moved and sunk. This doesn't look to me like a ridge emplaced by an infalling ring on a cold, dead world, but a ridge that was built and displaced by internal processes. But that's what I wanted it to be before I saw the images. ;-)

[lyford]

The only thing that I can say is that after reading all these posts and looking at all the pictures, I just can't get this song out of my head.

[MarcF]

It was not a good idea to go to vacation far from my computer this week. The whole week-end will not be enougth to look at all the Iapetus flyby stuff !!!

After watching some of the pictures, I have the feeling that craters in Roncevaux are very smooth, like on Enceladus. So is this region covered by a kind of snow ?
(Ejected by the Claude impact event which seems to have occured quite recently ?).
On Rhea, craters look sharper.
Moreover, as already mentioned, in some pictures white seems to cover black and in other pictures black seems to cover white. So even if I always thought that the Black was on the White, now I'm not sure about it anymore.
Iapetus might still have a certain kind of surface activity (like on Callisto?) and both black and white stuffs migth be transported from one location to the other.
Data from the other instruments (VIMS, CIRS,...) might help to solve the problem. Does someone know when they will be available ?
Marc.

[tasp]

I would pick transparent material everywhere, keyed to orbital position relative to Saturn, that only darkens upon exposure to sufficient heat/light from sun.

[David]

I would pick transparent material everywhere, keyed to orbital position relative to Saturn, that only darkens upon exposure to sufficient heat/light from sun.

I don't think this is possible; where black and white regions are adjacent, the white regions appear to be definitely higher in relief than the black (in some places high enough to cast shadows).

Now, that in itself doesn't decide white on black or black on white, because it could be produced either by icy material condensing and/or flowing over a dark background, or by a dark material heating up and causing the ice beneath it to sublimate until it ends up lower than the surrounding lighter regions.

However, it's not consistent with the same material merely undergoing a color change -- then you'd expect continuity in relief (and probably not such sharp boundarie, too).

Are we going to get information about the chemical composition of the surface materials from this pass?

[ugordan]

where black and white regions are adjacent, the white regions appear to be definitely higher in relief than the black (in some places high enough to cast shadows).

Can you give an example of this?

[David]

Can you give an example of this?

Well, you may not see the same things I do, but if you look at this one, you may see that some of the small-scale narrow bays or firths of dark material between two lighter areas are shadowed from below, as if they were running in slightly depressed channels. There are other examples, but in some of them it could be argued that the lighter material happens always to be located on higher topographic elevations -- which is probably also true, as the darker areas tend to be lower anyway, that being in itself a significant fact.

[tasp]

Here is another 'negative' print of Iapetus.

I am still thinking we are seeing a material not unlike silver nitrate applied to the Iapetan surface and 'developed' by exposure to sunlight.

The 'silver nitrate' analog is 'applied' as a gaseous efflux to Iapetus as it moves around the far side of Saturn, and dissipates to the void in ~40 days, with some 'ponding' in very low spots persisting longer.

The efflux may not persist as a gas in the Iapetan environment, it may absorb on to the surface materials, but it still manages to dissipate in less than 1 Iapetan day.

The material is sensitive to local heating effects, and local here can apparently be a matter of meters.

Note in the negative print the shadow like aspect of the dark areas.

The difficulty we are having in interpreting these images (regarding the black on white or white on black discussion) is perhaps due to the 'negative' visual characteristic of the actual appearance of the Cassini Regio 'crud'. Negative prints help visualize the effects of insolation on this 'silver nitrate' analog.

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[edstrick]

I get an overwhelming impression that the white and black materials are both granular (at the optically visible surface -- I can't tell about 10 cm down), mobile, and SELF-SEGREGATE, sorting each other out.

Almost nowhere do I see what look like diffuse dustings of black onto white, and the dustings of white on black, like small crater ejecta in black terrains, clearly are rapidly destroyed, presumably by sublimation of the white ice. Near the equator in light terrain, dark material lies flat in depressions, down to surface pits and irregularities of unresolved sizes. Off the equator a ways, the material forms round splotches in smallish craters that are lopsided, sitting on the sun-facing floor of the crater, but not up on the adjacent wall. In bigger, flat-floored craters, it's on the floor against the sun-facing wall. Well away from the equator, I get the overwhelming impression that the splotches have actively climbed the wall! Landslides of ice may disturb the splotches, but I get the impression that the mass of granular dark material has actively crept into a maximally sun-facing position, even against gravity.

HOW this might happen, I can only vaguely arm-wave. A few beers too many might help! Thermal expansion-and-contraction of the dark granules relative to white ones might be a method of driving a creep process, together with thermal sublimation of ice at the contact with a coherent splotch of dark. Numerical theoy and physical modelling of processes might give a clue.

[Bill Harris]

You're right, Ed, this is all very mysterious. The answer may be billions of 5cm-tall Monoliths, or something odder.

Seriously: one thing I've noticed from the closest images. We've seen a few small, recent craters with white ejecta rays in the dark regions. But I haven't seen the opposite: dark, fresh craters on the light-toned regions. This leads me to suspect that the black is a thin dusting over the white, whereas the white is a thicker layer.

I'm looking closely at the white-black-white contacts in the hi-res images for puzzle-pieces...

--Bill

[dvandorn]

I agree, Bill -- the distribution of dark materials just screams to me that it overlies the brighter, cleaner ice surfaces.

I also still believe I can see evidence of dark material piling up in craters, filling and/or deforming the "downwind" sides of some craters, and exhibiting some dune-like striations. I've been really busy and haven't had enough time to annotate images to point out what I think I'm seeing -- but I am certain that I'm seeing it.

To me, it all adds up to dark material covering light material, thickly in some places and more thinly in others.

-the other Doug

[David]

filling and/or deforming the "downwind" sides of some craters, and exhibiting some dune-like striations.

What do you literally mean by this? Obviously, there is no wind on Iapetus (hence your "quotes") and there can likewise be no dunes. So what process do you think is in fact occurring?

[tasp]

I was wondering about those streamlined (not quite actually) lumps in some of those pictures.

Rather than mention them with a question as to what everyone else thought they were, I was hoping for inspiration to strike and then I would post the question and the answer.





{I'm not at that juncture yet . . . . }

[ilbasso]

Seeing that old picture of Stevie Wonder made me think that you should add the "Michael Jackson" option to the vote - "Formerly black, now white, androgynous"

[tasp]

I can't help but think in the initial flush of excitement from the flyby we might have overlooked something quite significant in the pictures regarding the black/white white/black dichotomy:

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[volcanopele]

I was wondering about those streamlined (not quite actually) lumps in some of those pictures.

Rather than mention them with a question as to what everyone else thought they were, I was hoping for inspiration to strike and then I would post the question and the answer.

You noticed those too

Why can't they be streamlined? Keep in mind, that stream-lined forms indicate the presence of fluid flowing over a surface. The fluid could be a liquid-filled stream, an atmosphere, or dust "flowing" out from a nearby impact.

[JRehling]

Seriously: one thing I've noticed from the closest images. We've seen a few small, recent craters with white ejecta rays in the dark regions. But I haven't seen the opposite: dark, fresh craters on the light-toned regions. This leads me to suspect that the black is a thin dusting over the white, whereas the white is a thicker layer.

I think this is absolutely correct. But the truth may be a little more complex.

Most icy moons have a surface somewhat darker than ice. Impacts tend to surface whiter ice underneath. But the surface isn't dark, just "dingy".

I suppose that Iapetus's native regolith was like that -- similar to what we see on Rhea. The places where dark has spread, the top surface layer has lost its ice, leaving only the dark stuff behind. This could be, and is almost bound to be, quite thin.

If the ice that sublimated away from dark stuff has settled onto the original dingy surface in non-black areas, then we have at least three layers: fresh ice deep in the crust, a dingy regolith, and a fresh white coating of frost. So an impact in the white areas would surface a tiny bit of dingy material, but far more white ice.

[David]

I can't help but think in the initial flush of excitement from the flyby we might have overlooked something quite significant in the pictures regarding the black/white white/black dichotomy:

Where is that feature located?

[tasp]

Hyperion.

[David]

Hyperion.

Ah, that explains why it looks so unlike anything on Iapetus; no sharp boundaries between light and dark, for one thing. I shouldn't be surprised if they turned out to be quite different phenomena.

[ngunn]

no sharp boundaries between light and dark, for one thing. I shouldn't be surprised if they turned out to be quite different phenomena.

I agree, but it is possible that Hyperion shows some sublimation but little if any deposition due to it's lower gravity. Maybe it requires both competing processes to produce the sharp boundaries.

[nprev]

Like oDoug, I too noticed the apparent "duning" of dark material in some places; unlike him, didn't have the guts to mention it!

If this impression is true, then what the hell?! All I can think of is long-term outgassing that almost has to be recent & very local to the "dunes". Really want to see what the magnetometer read during this flyby; if anything's being emitted at all, that's the only shot we seemingly have of detecting it...(and, no, if any of this is really happening I have no feasible mechanism(s) to explain it...)

[dvandorn]

What do you literally mean by this? Obviously, there is no wind on Iapetus (hence your "quotes") and there can likewise be no dunes. So what process do you think is in fact occurring?

That is exactly why I put "downwind" in quotes. It's just a convenient way of indicating that I see apparent flow vectors in some of the dark surficial material.

Such a flow could be caused by any number of factors, and I'd be surprised if there is just a single factor controlling it. It could be anything from material creep forced along by interactions with Saturn's magnetotail to atmospheric movements caused by transient atmospheres generated by (relatively) immense amounts of sublimation or outgassing from the leading edge (as has been speculated). Heck, it could just be some odd form of impact ejecta interaction. I will point out that you see some "duning" landforms on other airless bodies, such as the Moon, which are almost definitely formed by ejecta processes. Those are just at much larger scales than you see on Iapetus.

I still think that those who dismiss the dark material as simply "the natural surface of Iapetus below the bright ice" are missing the point. This dark material is unlike any other surficial material on any other Saturnian moon -- Hyperion included. These bodies are made up primarily of ices. You'd have to postulate some mechanism that would first emplace a dark surface on Iapetus before you can start to speculate that the dark material is simply that surface with an overlay of clean ice removed from the leading edge. Having heard absolutely no such theory about the emplacement of a surface that is significantly darker (and of a significantly different composition) than we see on *any* other moon in the system, seeing this dark material having been moved along the leading edge in apparent flow patterns, and seeing a "true" bright ice surface exposed by impact from under the dark surface in many areas, I simply have to regard the possibility of the dark surface being Iapetus' "natural" surface as the least likely of all of the theories.

So, all of my observations and an application of Occam's razor leads me to the conclusion that the dark material is emplaced on top of a bright, icy surface that is far more representative of what we see on the rest of the icy moons. Hence my conclusion that we're seeing black on white and not vice-versa.

-the other Doug

[dvandorn]

Like oDoug, I too noticed the apparent "duning" of dark material in some places; unlike him, didn't have the guts to mention it!

Yep, that's me -- all guts, no glory...

-the other Doug

[tasp]

Here is some more 'ponding' to contemplate.

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[tasp]

And the comparison.



{I suppose it would be nice if I knew how to 'twin' pics in single post}

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[tasp]

Crikey, there is something hinky going on tweenst Hyperion and Iapetus.

Most of the difference is from the drastically different spin periods of the 2 objects affecting the sustained insolation induced darkening of the same exogenous efflux.

[nprev]

I simply have to regard the possibility of the dark surface being Iapetus' "natural" surface as the least likely of all of the theories.
-the other Doug

Mmm...Disagree based on one hypothesis: that Iapetus is a captured object, possibly a large, errant KBO. If it was a "native" Saturnian satellite, I'd think that its orbital inclination would have been a lot less with respect to Saturn's equatorial plane.

Would give a lot for a close-up view of Chiron right now; willing to bet that it just might have some gross similarities to Iapetus.

[tasp]

We note earth's moon to have an orbit about earth rather suspiciously 'relaxed' into alignment with the ecliptic.

We note something rather similar in regards to Iapetus.

Not sure we can infer an exogenous origin for Iapetus from such an orbital characteristic, Saturn having such a nice big Hill sphere, and Iapetus orbiting, albeit inclinedly, circularly and comfortably inside.

[nprev]

True, but Earth's Moon is thought to be an artifact of a massive collision (therefore technically of exogenic origin?)...not quite the same situation as would be expected for a satellite of a gas giant.

[alan]

There was a paper a couple of years ago linking Titan's eccentricity to an interaction with Jupiter.
http://adsabs.harvard.edu/abs/2005AGUFM.P33C0263B

Perhaps the inclination of Iapetus has a similar cause.

[edstrick]

What can I say.. just look at this d__n stuff... At this latitude, ALMOST none of it is on a horizontal surface, it's almost ALL on slopes, facing noonday sun. Not quite all, but the stuff in the lower left basin rim setment is the totally classic example. I can't avoid the conclusion, as I said above, that some process has caused it to creep against gravity, to accumulate on surfaces perpendicular to the sun's rays at mid-day.

Igor... it's alive..... IT'S ALIVE!!!!!!!!

<master.... you didn't take your medicine this morning!>

(data taken and sharpend from UGordon's big b&w mosaic)

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[dvandorn]

OK -- I understand your conclusion, here. But I still want to hear your theory for how Iapetus got a dark, reddish natural surface, and from where the bright ices that are covering this dark surface came. If insolation can "burn off" the snow from sun-facing slopes, over billions of years, shouldn't it have burned off ALL of the snow?

Iapetus has no atmosphere, so we're not talking about a hydrocycle, here. If what everyone has been saying is true and it's stone-cold dead, it hasn't vented icy plumes for billions of years. So why hasn't all of the snow been burned off, the vapor sputtered into space?

And, to reiterate -- how did this entire moon get surfaced with a dark reddish material that's different in composition from any of the other icy moons, and yet seems similar in composition to Titan's atmosphere?

Before I can accept the concept that the dark material is the natural surface and the icy snow lies on top of it, I need to hear a mechanism postulated that accounts for this dark surfacing. And just saying "Well, it's probably a KBO, that explains it" doesn't explain a thing for me... show me where KBOs follow a pattern of having dark, reddish surfaces and explain how they *all* got surfaced that way, and maybe I'll start to consider it. But until then, Occam's Razor tells me that any airless icy body that was born anywhere near Saturn ought to have a bright icy surface, and that any significant darkening must be an overlay on top of that icy surface.

-the other Doug

[The Messenger]

And, to reiterate -- how did this entire moon get surfaced with a dark reddish material that's different in composition from any of the other icy moons, and yet seems similar in composition to Titan's atmosphere?

It is a lot easier to explain the shallow craters if the surface is similar to terrestrial planet regoth.
Rocks and sand, just like Titan.

[TheChemist]

What can I say.. just look at this d__n stuff... At this latitude, ALMOST none of it is on a horizontal surface, it's almost ALL on slopes, facing noonday sun. Not quite all, but the stuff in the lower left basin rim setment is the totally classic example. I can't avoid the conclusion, as I said above, that some process has caused it to creep against gravity, to accumulate on surfaces perpendicular to the sun's rays at mid-day.

(data taken and sharpend from UGordon's big b&w mosaic)

Ed,
I think that the image you posted is from an area where we see white ice on top of black soil.
So it is just the walls of the basin that are black.
The original released image can be found here.
I attached a rotated version that matches the orientation in your image.

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[ugordan]

I think that the image you posted is from an area where we see white ice on top of black soil.

White ice on top of black soil - how do you figure that?

[TheChemist]

Ugordan,
I think that I see wide openings (holes) on the ice in this particular image, that seem to have thick white walls and are partially black at the bottom.
This gives me the impression of melted ice on top of black terrain.

If it was just white holes filled with black material melting the ice, one has to explain the black basin walls, which I find hard to do

[ugordan]

And, to reiterate -- how did this entire moon get surfaced with a dark reddish material that's different in composition from any of the other icy moons, and yet seems similar in composition to Titan's atmosphere?

I haven't seen this one mentioned before. Is there any reference to this? All I've seen is comparisons to Phoebe's spectra and Hyperion.
Some related links:
http://saturn.jpl.nasa.gov/multimedia/prod...ARM_verH_FC.pdf
http://saturn.jpl.nasa.gov/multimedia/prod...HARM_050125.pdf

The first CHARM presentation has an interesting bit:

The IR spectra show that Phoebe dark material is similar to Iapetus dark material, but the visual spectra show that Hyperion and Iapetus are more similar.

If it was just white holes filled with black material melting the ice, one has to explain the black basin walls, which I find hard to do

I'm seeing lots of very small craters that have dark bottoms, but I guess everyone sees things differently. I wouldn't say the dark material is doing any melting here, far too cold for that, but the ice can slowly sublimate away from the regolith, at least from the depth through which sun can penetrate into.

[Bill Harris]

RE: edstrick and the chemist, posts 40 & 42--

One of the things that has amazed and puzzled me from the first day are the flow lobes visible on the downhill side of these scarps/crater walls. Tha attached is an adaption of "iapetus_snow_1" showing multiple flow lobes that look all the world like snowy avalanches. Not unusual in the Alps, but this is a small airless world with temperatures in the -100's. And you can find these flow lobes frequently. Dunno what it means, capt'n, but it's gotta be significant...

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[David]

RE: edstrick and the chemist, posts 40 & 42--

One of the things that has amazed and puzzled me from the first day are the flow lobes visible on the downhill side of these scarps/crater walls. Tha attached is an adaption of "iapetus_snow_1" showing multiple flow lobes that look all the world like snowy avalanches. Not unusual in the Alps, but this is a small airless world with temperatures in the -100's. And you can find these flow lobes frequently. Dunno what it means, capt'n, but it's gotta be significant...

There's another one, just as striking, above the one you outlined; it seems to curve down and cut off two other flows on the slopes that come down from the left, apparently having flowed over them.

[ustrax]

Tilmann Denk's "vote" at spacEurope:

"Denk thinks so...he goes even further, he believes that “we are close to solving the 335 years old riddle of the brightness dichotomy.”
He told spacEurope is reasons for this positive perspective, “in earlier years, I usually remarked in presentations that the only commonality of all explanation attempts is that none of these is widely accepted. Now, I think different.”Interesting will be to see “how the data of the ridge will be interpreted”, also the “good coverage of most parts of the trailing side give us now an insight into the "last unknown" hemisphere of Iapetus.”

Black and white, white and black, yin and yang, yang and yin...lots of speculation about the processes envolved, about the true nature of Iapetus...does Tilmann Denk have a definitive opinion about this? He does, ”black is on top of white. I'm pretty certain on this, now even more thanbefore the flyby”, where he was already quite sure about his look over this particular enigma."

[dvandorn]

After seeing some of the contrast-enhanced images from the trailing hemisphere, I have to say that my view has changed slightly. I still firmly believe that the dark surface on the leading edge is a mantling on top of a brighter ice substrate, but there is good evidence now that where this dark material has mantled the surface along the edges of the leading hemisphere, there has been deposition of bright ices on top of it. In places.

What is so odd to me is that neither hemisphere shows much in the way of cratering that exhumes a different-albedo substrate. There are occasional bright-rimmed craters on the leading hemisphere, but almost no dark-halo craters on the trailing one. And the bright-rimmed craters are just that -- bright-rimmed. There is little to no evidence of bright ejecta around them.

I'm having a hard time believing that the impact rate is so small that there have simply been few to no impacts of any size since the materials we see were emplaced on Iapetus' surface. But at the same time, I have a similarly hard time believing that the emplacement of these materials is anything but ancient -- particularly since some of the flow patterns hint at *aeolian* deposition/deflation, which is awfully difficult to explain on a currently airless body.

So we are left with a paradox.

-the other Doug

[JRehling]

I'm having a hard time believing that the impact rate is so small that there have simply been few to no impacts of any size since the materials we see were emplaced on Iapetus' surface. But at the same time, I have a similarly hard time believing that the emplacement of these materials is anything but ancient -- particularly since some of the flow patterns hint at *aeolian* deposition/deflation, which is awfully difficult to explain on a currently airless body.

So we are left with a paradox.

-the other Doug

Again, I point to the fact that the "white" stuff isn't white. It's pretty bright, but it has an albedo of about 0.6, far from pure ice. What's happening is that when an impact surfaces some of the bright stuff from below, the thermal segregation model immediately goes to work on the excavated material, and if the situation was such that the native material turned dark in the first place, then the virgin material is going to end up the same way. Maybe it takes years, maybe centuries, but whatever the timescale, it's a blink in geological time. The only bright ray systems we see are very recent ones.

Incidentally, much the same thing is true of ray systems (usually bright on dark) on Mercury and the Moon. Those aren't the only impacts on those worlds -- they're the newer ones. For example, Tycho is estimated to be 108 MYA, Copernicus 800 MYA, and Aristarchus 450 MYA. Those all have salient ray systems. The more typical impact on the Moon, 5 to 10 times those ages, doesn't.

The dynamics of the thermal creep and the hecameter-scale shapes of the boundaries is bound to be an open and interesting question for modelers to address. There are myriad comparable issues pertaining to how winter ice and snow melts on Earth, and I doubt all of them have attracted research interest. (Not sure, even, what FIELD such research would be classified as. Geology? Meteorology? Landscaping? Poetry?)

[David]

The dynamics of the thermal creep and the hecameter-scale shapes of the boundaries is bound to be an open and interesting question for modelers to address. There are myriad comparable issues pertaining to how winter ice and snow melts on Earth, and I doubt all of them have attracted research interest. (Not sure, even, what FIELD such research would be classified as. Geology? Meteorology? Landscaping? Poetry?)

Climatology, I imagine -- we have studies of glaciation over time, and more recently studies of glacier recession, and of other results of large-scale ice-melt, like the diminution of ice shelves in the Antarctic. However, studies of how this works when there's no liquid phase for the ice to transition through are doubtless much harder to come by -- the most obvious source of information on this subject would be the study of the Martian polar regions.

[Bill Harris]

My first, and continuing, impression of the dichotomy on Iapetus is that the object got splatted with a cosmic tomato. It looks like something was spattered, sputtered or applied to the leading side.

I've found my first example of an impact on the white snowy areas leaving rays from the dark reqolith. N00092009, located in the Voyager Mountains. Looks like an impact dead-centered on a peak.

I did an "RGB" composite image from IR1-GRN-UV3 frames taken at the time. Registration was poor, colr balance was pitiful and I won't inflict it upon you, but the color value of the rays was the same as the weathered regolith surface. An observation presented FWIW.

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[David]

I've found my first example of an impact on the white snowy areas leaving rays from the dark reqolith. N00092009, located in the Voyager Mountains. Looks like an impact dead-centered on a peak.

I don't have a problem believing that in that case we have dark layered on top of white which is in turn (over on the opposite side from the impact site) layered on top of dark again.