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Iapetus - Black on white or white on black?
Ice and Gunk
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ugordan
post Sep 20 2007, 02:04 PM
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QUOTE (dvandorn @ Sep 18 2007, 03:46 PM) *
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:
QUOTE
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.


QUOTE (TheChemist)
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.


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Bill Harris
post Sep 20 2007, 04:26 PM
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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
post Sep 20 2007, 04:51 PM
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QUOTE (Bill Harris @ Sep 20 2007, 04:26 PM) *
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.
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ustrax
post Sep 24 2007, 04:35 PM
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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."


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dvandorn
post Sep 24 2007, 04:59 PM
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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


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JRehling
post Sep 24 2007, 08:03 PM
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QUOTE (dvandorn @ Sep 24 2007, 09:59 AM) *
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?)
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David
post Sep 24 2007, 09:24 PM
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QUOTE (JRehling @ Sep 24 2007, 08:03 PM) *
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.
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Bill Harris
post Sep 24 2007, 10:04 PM
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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
post Sep 25 2007, 02:48 AM
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QUOTE (Bill Harris @ Sep 24 2007, 10:04 PM) *
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.
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