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Mystery of Saturn's Two-Faced Moon Solved
TheChemist
post Oct 9 2007, 02:31 PM
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Carolyn Porco comments included in this space.com article :

http://www.space.com/scienceastronomy/0710...ni-iapetus.html
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alan
post Oct 9 2007, 02:54 PM
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Some press releases from the Cassini site, the links to them on the Cassini main page are broken for some reason.

Mosaic of trailing hemisphere
http://saturn.jpl.nasa.gov/multimedia/imag...fm?imageID=2763

VIMS Image of trailing hemisphere
http://saturn.jpl.nasa.gov/multimedia/imag...fm?imageID=2771

Tiny Grains on Iapetus
http://saturn.jpl.nasa.gov/multimedia/imag...fm?imageID=2774

Warm and Dry on Iapetus
http://saturn.jpl.nasa.gov/multimedia/imag...fm?imageID=2776
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Greg Hullender
post Oct 9 2007, 03:24 PM
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In that case, why only Iapetus? Why don't we see the same effect on other moons of Saturn and the other gas giants?
I'm sure there must be an answer, but I don't see it on that site.

--Greg
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ElkGroveDan
post Oct 9 2007, 03:29 PM
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QUOTE (Greg Hullender @ Oct 9 2007, 07:24 AM) *
In that case, why only Iapetus?

My guess would be the nature of the orbital dust. Something about the mechanics of the Saturn system is creating a dark-particle mass co-orbital with Iapetus. It could even be as simple as the debris from a single impact elsewhere in the not too-distant past that created this debris cloud - although the uniform nature of the particle size and composition makes this unlikely. Nonetheless, perhaps Iapetus' yin-yang condition is a temporary condition (on the appropriate time scale).


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If Occam had heard my theory, things would be very different now.
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ngunn
post Oct 9 2007, 03:57 PM
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QUOTE (Greg Hullender @ Oct 9 2007, 04:24 PM) *
In that case, why only Iapetus? Why don't we see the same effect on other moons of Saturn and the other gas giants?
I'm sure there must be an answer, but I don't see it on that site.

--Greg


I'd list 3 factors:
1/ Exceptionally long days
2/ Poorly differentiated, so plenty of dirt in the surface ices
3/ Strong enough gravity to retain some sublimed ice molecules

No other moon meets all three criteria.
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JRehling
post Oct 9 2007, 03:57 PM
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QUOTE (Greg Hullender @ Oct 9 2007, 08:24 AM) *
In that case, why only Iapetus? Why don't we see the same effect on other moons of Saturn and the other gas giants?
I'm sure there must be an answer, but I don't see it on that site.

--Greg


Basically, there's a certain temperature required to make this happen. And a certain level of impurity of surface. The quirks of Iapetus are that it is nondifferentiated (the dark stuff never sank into the core) and it rotates VERY slowly and it is just the right temperature to have some of its surface meet the threshold for the effect, but not all of its surface.

We do see this effect on Callisto, except that Callisto is warm enough that it is basically ALL dark stuff.

We do see this effect on Ganymede, whose most ancient crust is also basically all dark stuff. It's younger, groovier terrain probably doesn't have enough dark stuff to allow the effect to happen.

We do see this effect on mature comet nuclei, which are black with lag deposits. They lose their ice in short episodes around perihelion.

We do see this effect on Hyperion, whose craters act as concave mirrors to darken their floors.

The other saturnian satellites rotate too rapidly. Noon comes and goes quickly.

The trailing side (and high latitudes) of Iapetus aren't dark enough to kickstart the process.

The uranian satellites are presumably too cold for the same process to work there, but it may work on some level? (I'm not too familiar with their absolute albedos.)
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AscendingNode
post Oct 9 2007, 04:09 PM
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QUOTE (Greg Hullender @ Oct 9 2007, 08:24 AM) *
In that case, why only Iapetus? Why don't we see the same effect on other moons of Saturn and the other gas giants?
I'm sure there must be an answer, but I don't see it on that site.


I think Hyperion's weird pitting may be a similar mechanism where dark material heats the water ice and causes it to sublime. What make Iapetus unique is the dark material being isolated to a hemisphere. But I bet the part of dark material getting darker by subliming ice probably is something we'd see at Uranian moons too.
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tasp
post Oct 10 2007, 05:06 AM
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And, Hyperion and Iapetus are the only two major solar system moons exterior to the only significantly atmosphered moon we know of, Titan.
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ngunn
post Oct 10 2007, 08:36 AM
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I've no problem with the feedback process, or with the idea that the despinning to synchronous rotation plus increase in solar luminosity is what got Iapetus into the supercritical state. I still have doubts about the 'seeding' though. I don't think a light dusting of dark material could have done the trick. I don't think you can trigger the runaway by just slightly darkening a large area. I think you need discrete black spots to create the necessary temperature contrast. How big do they have to be? I don't know but maybe quite small. I'm thinking of radii just a few times the distance heat conducts horizontally though the ground in one Iapetan day.
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tasp
post Oct 11 2007, 03:51 AM
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The 'seeding' material for the darkening process is not 'dust' or a granular solid.

It is gaseous.

We see 'ponding' effects in craters. A 'dusting' cannot do that. Ponding, ala a gaseous insolation activated agent, can.
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David
post Oct 11 2007, 07:42 AM
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Iapetus doesn't need a huge differentiation in brightness to start with; it just needs any amount of differentiation that will preferentially and predictably distribute ice from one location to another, rather than randomly; so that sublimation events remove ice from a slightly darkened area (making it darker and warmer) and deposit it in a slightly brighter area (making it brighter and colder). The (ice-)rich regions get richer, and the (ice-)poor get poorer.

The "natural" state of Iapetus, unaffected by external forces, would be, I think, an overall icy globe, all-white at the poles, but with darker sun-facing ridges and crater floors in the lower latitudes, just as we see in the middle of Roncevaux Terra. Cassini Regio is then a deviation from this basic state, motivated by an external force. That force would have to be solid particles, I think; surely a gas (of what origin?) suspended in circumSaturnian space would quickly become far too diffuse to have any real effect? But a very thin dusting, not needing to do more than to turn the ice cover from white to off-white, if it covered enough area would be enough to start the ball rolling. My question is whether it was a one-time event (something that disintegrated near Iapetus, for instance, leaving the dusting) or a series of repeated events, possibly still going on. I imagine the latter is more probable.
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ngunn
post Oct 11 2007, 08:03 AM
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QUOTE (David @ Oct 11 2007, 08:42 AM) *
But a very thin dusting, not needing to do more than to turn the ice cover from white to off-white, if it covered enough area would be enough to start the ball rolling.


Do you know this, or is it just an opinion? I'm not convinced, but willing to be convinced. I think discrete black spots would be much more effective triggers. My question is - How does a new dark patch start in a white area? This has undoubtedly happened on innumerable occasions, even in 'undusted' areas.
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Greg Hullender
post Oct 11 2007, 04:51 PM
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Thinking about this some more, this comment

"Dusty material spiraling in from outer moons hits Iapetus head-on and causes the forward-facing side of Iapetus to look different than the rest of the moon," said Tilmann Denk, Cassini imaging scientist at the Free University in Germany.

From the original article

http://www.space.com/scienceastronomy/0710...ni-iapetus.html

strikes me as really odd. First, a spiral isn't a possible orbit. Second, material in elliptical orbits with periapsis near Iapetus' orbit should be moving faster than it so they should impact the trailing edge, not the leading one. I'd expect material impacting the leading edge to have been in orbits with apapsis near Iapetus.

At the expense of appearing to agree with Tasp on something :-) perhaps Titan is a reasonable source for this material. Perhaps we should be looking for signs of tar stuck to Cassini's camera. :-)

--Greg
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ugordan
post Oct 11 2007, 04:56 PM
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QUOTE (Greg Hullender @ Oct 11 2007, 06:51 PM) *
First, a spiral isn't a possible orbit.

Ion engines give you spiralling 'orbits', it's perfectly plausible as long as the orbital energy change is very gradual. Dust indeed can spiral in due to solar light pressure, probably electrostatic effects come into play as well.


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alan
post Oct 11 2007, 05:03 PM
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The dust is theorized to have originated from a outer moon in a retrograde orbit either Phoebe or one of the small irregular moons. Small particles experience Poynting-Robertson orbital decay caused by the drag created by light striking slightly more form the direction of motion the the opposite direction. This causes their orbits to get smaller, or spiral in. Since the dust is in a retrograde orbit it will strike the leading side of a moon in a prograde orbit such as Iapetus.

edit: I see ugordon types faster than I do.
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