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Iapetus - Black on white or white on black?
Ice and Gunk
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tasp
post Sep 16 2007, 04:00 AM
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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
post Sep 16 2007, 05:23 AM
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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.
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Bill Harris
post Sep 16 2007, 10:41 AM
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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


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dvandorn
post Sep 16 2007, 02:37 PM
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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


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David
post Sep 16 2007, 05:32 PM
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QUOTE (dvandorn @ Sep 16 2007, 02:37 PM) *
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?
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tasp
post Sep 16 2007, 05:38 PM
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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.

blink.gif



{I'm not at that juncture yet . . . . }
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ilbasso
post Sep 16 2007, 05:45 PM
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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"


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tasp
post Sep 17 2007, 04:10 AM
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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
post Sep 17 2007, 05:18 AM
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QUOTE (tasp @ Sep 16 2007, 10:38 AM) *
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 wink.gif

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.


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JRehling
post Sep 17 2007, 05:22 AM
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QUOTE (Bill Harris @ Sep 16 2007, 03:41 AM) *
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.
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David
post Sep 17 2007, 06:10 AM
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QUOTE (tasp @ Sep 17 2007, 04:10 AM) *
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?
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tasp
post Sep 17 2007, 01:53 PM
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Hyperion.




blink.gif
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David
post Sep 17 2007, 02:32 PM
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QUOTE (tasp @ Sep 17 2007, 01:53 PM) *
Hyperion.
blink.gif


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.
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ngunn
post Sep 17 2007, 04:00 PM
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QUOTE (David @ Sep 17 2007, 03:32 PM) *
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.
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nprev
post Sep 17 2007, 04:27 PM
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Like oDoug, I too noticed the apparent "duning" of dark material in some places; unlike him, didn't have the guts to mention it! tongue.gif

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...)


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