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Posted by: TheChemist Oct 9 2007, 02:31 PM

Carolyn Porco comments included in this space.com article :

http://www.space.com/scienceastronomy/071008-cassini-iapetus.html

Posted by: alan Oct 9 2007, 02:54 PM

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/images/image-details.cfm?imageID=2763

VIMS Image of trailing hemisphere
http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2771

Tiny Grains on Iapetus
http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2774

Warm and Dry on Iapetus
http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2776

Posted by: Greg Hullender Oct 9 2007, 03:24 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

Posted by: ElkGroveDan Oct 9 2007, 03:29 PM

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).

Posted by: ngunn Oct 9 2007, 03:57 PM

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.

Posted by: JRehling Oct 9 2007, 03:57 PM

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.)

Posted by: AscendingNode Oct 9 2007, 04:09 PM

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.

Posted by: tasp Oct 10 2007, 05:06 AM

And, Hyperion and Iapetus are the only two major solar system moons exterior to the only significantly atmosphered moon we know of, Titan.

Posted by: ngunn Oct 10 2007, 08:36 AM

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.

Posted by: tasp Oct 11 2007, 03:51 AM

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.

Posted by: David Oct 11 2007, 07:42 AM

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.

Posted by: ngunn Oct 11 2007, 08:03 AM

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.

Posted by: Greg Hullender Oct 11 2007, 04:51 PM

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/071008-cassini-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

Posted by: ugordan Oct 11 2007, 04:56 PM

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.

Posted by: alan Oct 11 2007, 05:03 PM

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.

Posted by: nprev Oct 11 2007, 05:22 PM

Considering Titan as a potential source, I almost reluctantly ressurect http://www.unmannedspaceflight.com/index.php?s=&showtopic=4551&view=findpost&p=98309 and thread...

Posted by: Greg Hullender Oct 11 2007, 09:18 PM

Alan, ugordon: Thanks. I suppose I should have said "Keplerian orbit." Hadn't realized the light drag could have that effect, but it makes sense. I'm more skeptical about ion engines attached to the dust grains. :-)

Still raises/leaves a lot of questions, though:

Where is the dust coming from? An invisible satellite? Or was this a one-time event? In which case, when was it?

Why do we see no effect on Phoebe? (That same question would apply if one thought it was something coming from Titan and going out.) [Never mind: just realized Phoebe's rotation is just over 9 hours.]

If the dust fell in a gentle spiral, why the differentiation between leading and trailing hemispheres? Why isn't it between sub-Saturn and anti-Saturn hemispheres?

--Greg

Posted by: alan Oct 12 2007, 02:51 AM

Its the leading hemisphere because the dust is moving in a nearly circular orbit in the opposite direction as Iapetus.



Here's some fodder for the light on dark/dark on light debate. This is a piece of CICLOPS version of the Iapetus that I contrast stretched to bring out some subtle details at high latitudes.

Iapetus doesn't just have light and dark, the bright terrain near the equator are dirty in contrast to the blue-white terrain near the poles. At high latitudes you can see areas of beige on the equator facing slopes. So near the poles is it beige on white or white on beige? What was the original color of Iapetus?


I remember a paper I read before the New Year's Iapetus encounter that speculated that the dark material started hitting on Iapetus before its rotation was synchronized with its orbit. This could have made the trailing side dirty without starting the runaway process because the rotation was still to fast for the daylight temperatures to reach a critical temperature. This may have provided the seed needed to start the darking process inside the craters on the trailing side.

Posted by: stevesliva Oct 12 2007, 03:04 AM

Actually just nabbed from here:
http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=779


It was easy to miss, especially since without using the word "retrograde" it's easy to read it as "blah blah blah outer moons blah blah blah."

Posted by: Greg Hullender Oct 12 2007, 05:00 PM

steve: Good catch. Thanks!

Given, then, that Phoebe is in a retrograde orbit AND is one of the blackest objects in the solar system, I wonder why it isn't suggested as a possible source of the dark material. Could material spalled off from Phoebe account for it? Or is it more likely that Phoebe's darkness (which appears to be a relatively thin layer on ice) actually comes from the same unknown source?

--Greg

Posted by: Bill Harris Oct 13 2007, 02:46 AM

Quite possible. Doesn't spectroscopy suggest similarities in their surface materials?

Phoebe has an orbit of 160+- degrees inclination, retrograde, four times farther out than Iapetus and has several large craters. Who knows, the orbital dynamics doesn't seem to favor a transfer of spalled material, but with all the oddball things we've seen in this neighborhood, anything might be possible. The dark material on Iapetus has every appearance of being sprayed or sputtered on...

--Bill

Posted by: elakdawalla Oct 13 2007, 03:54 AM

If I recall correctly, Phoebe has long been considered a candidate for the source of the dark material, but I think that spectrally, Phoebe is, strangely enough, a better match to Iapetus' <i>bright</i> material than the dark stuff. I also seem to recall that Phoebe, though dark, is not as dark as Iapetus' dark material. I think. Somebody search around and check on these, I am not confident that I am remembering these things right.

--Emily

Posted by: ugordan Oct 13 2007, 05:06 PM

That's correct. Of all the significant moons of Saturn, Cassini Regio on Iapetus is the darkest (around 0.04 albedo, compared to about 0.06 of Phoebe).

Posted by: ngunn Oct 13 2007, 07:05 PM

Why is everyone so sure about this global dusting from another moon? If the vast majority of the Cassini Regio dark material was already in the dirty ice of Iapetus and you only need a small amount of seeding material then the problem largely goes away. The albedo and spectrum of Cassini Regio should not be expected to match the seeding material, which may be pretty hard to track down at all if most of the seeding happened a long time ago. The source could have been a population of small, exhuasted (de-iced) periodic comet nuclei with aphelia close to the orbit of Saturn. These would have long since disappeared, so the search for a source may well be futile. I think it might be more fruitful to search for a 'colour' match amongst materials that might plausibly be trapped within the ice of Iapetus, or the radiation-altered products of such.

Posted by: ugordan Oct 13 2007, 08:29 PM

The fact remains the leading and trailing hemisphere dark material has visually different look suggests whatever was deposited on the leading side wasn't only trace amounts. The leading hemisphere has a much more pronounced reddishness to it, different to the trailing side which, in stretched colors appears really greenish (those who didn't believe my calibrated stuff very much now have official proof of this). In fact, it could be postulated it's this greenish stuff that's native to Iapetus and the redder stuff was deposited (with potentially slightly impact-altered chemistry). The trailing side has a uniform subtle greenish hue to ice at equatorial latitudes as well. Interestingly enough, I recall the dark region on Dione (cliffy terrain) appears green in the same filter combination. "Green" is a relative term here, implying weak infrared and ultraviolet reflectance, not necessarily visually greenish stuff.

Invoking long-gone comets close to Saturn's orbit is IMHO stretching Occam's razor a bit too much. Why doesn't Phoebe have the same uniform coating then? Why would this only happen to occur at Saturn? If the comets were "spent", that'd mean their perihelia were much closer in so it would be seen at least in the Jovian system too. Furthermore, if the dust was coming from outside the Saturnian system perturbations by Saturn would most likely make the dusting affect a good portion of Iapetus, not just the leading hemisphere.

Posted by: tasp Oct 13 2007, 08:59 PM

Salient inferrence there, ugordan.

Curious a spalling process on Phoebe could liberate all particles at above Phoebe escape velocity, and that none would ever return to Phoebe's surface.


In one fell swoop you may have altered how we look at Phoebe in this regard.

Posted by: ElkGroveDan Oct 15 2007, 02:25 AM

Alan that is just fantastic -- bordering on my favorite Iapetus image this year.

Posted by: nprev Oct 15 2007, 02:52 AM

Fascinating ideas and discussion.

Crap; I knew there wouldn't be a simple answer, knew it knew it knew it--there never is, really.... . (Consider this paradigm in the context of future discoveries; thinking NH here).

All I have to offer is a silly idea. What about a Discovery mission equipped with a high-res imaging system designed to splat a quantity of carbon black on a bright area, then returning a few years later (assuming a highly elliptical orbit achieved around Saturn) to assess the effects? If the terrain is reactive as postulated, then either the presence or absence of observable effects would be instructive.

Posted by: ngunn Oct 15 2007, 09:07 AM

Since disconnected dark patches have formed on the 'green' trailing side too you clearly don't require the 'red' stuff to start one off. Something else must have started them. So if you invoke the 'red' stuff as the trigger on the leading side you have to postulate a completely separate triggering process for the other hemisphere. Occam doesn't like that either. There may well be a reddish dusting on one hemisphere (or a 'green' one on the other, or the red/green difference may be due to a selective removal process rather than deposition - I haven't a clue) but I don't think this is what triggered the runaway blackhouse. Since the red/green dichotomy appears to overlie frosted areas too it must be much more recent than most of the sublimation.