Iapetus - Black on white or white on black? |
Iapetus - Black on white or white on black? |
Sep 14 2007, 07:40 AM
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#1
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Member Group: Members Posts: 813 Joined: 8-February 04 From: Arabia Terra Member No.: 12 |
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?' |
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Sep 14 2007, 08:54 AM
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#2
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Senior Member Group: Moderator Posts: 3241 Joined: 11-February 04 From: Tucson, AZ Member No.: 23 |
I would prefer a both option: white on black in some areas, black on white in others.
-------------------- &@^^!% Jim! I'm a geologist, not a physicist!
The Gish Bar Times - A Blog all about Jupiter's Moon Io |
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Sep 14 2007, 04:15 PM
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#3
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Senior Member Group: Members Posts: 2530 Joined: 20-April 05 Member No.: 321 |
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? |
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