[X] My Assistant

[Michael Capobian...]

As we approach Iapetus for this close fly-by, I thought it might be fun to try and predict if, after we see the close up images of the transition region and Voyager Mountains, the source of the dark material will be obviously endogenic or exogenic, or there will still be no obvious explanation.

I'll go for the longshot, endogenic.

Michael

[nprev]

Mmm...might not be obvious from this encounter, but gotta go with exogenic, probably from Phoebe, and probably also the same stuff that's at the bottom of Hyperion's 'craters'.

The deposition pattern on Iapetus just doesn't look like it could have happened from an internal source. (Of course, it also doesn't look much like one would expect from infalling material...in fact, the whole thing looks like a baseball that had 1/2 of its outer covering dyed, right along the stitch lines, which is weird...wondering if Iapetus was 'nodding' along its orbit at the time, or if its orbital inclination was a factor...but I yield to Occam's Razor).

[David]

The black stuff is all on the leading edge of Iapetus' revolution, isn't it? As if Iapetus had gone plowing through a cloud of ash (that was restricted to its own orbital plane) sometime in its history. That seems unlikely to be coincidental, and an endogenic origin would seem unlikely to have such a neat distribution.

[tasp]

Gunk on Iapetus and Hyperion dark crater bottoms all came from Titan atmosphere blow off.

Drastically reduced level of outgassing of similar materials (CH4, N2) fom Ariel has darkened up Umbriel, too.

[dvandorn]

I also have to go with exogenic. From the closest images of Iapetus thus far, I could swear I've seen images of dark material piled up like drifts along the "back" rims of small craters ("back" in relation to Iapetus' orbital motion vector). If that pattern persists to finer scales, then we'll have our answer...

-the other Doug

[ngunn]

I thought it was supposed to be just the residue left by net sublimation of dirty ice, whereas the bright areas are places where there is net deposition of (clean) frost. Only a little exogenic dark stuff on the leading hemisphere, possibly in the distant past, is necessary to create the thermal asymmetry to start the process off. That's easily the neatest explanation I've read and I'll be very surprised if it's wrong.

[ugordan]

I agree with ngunn here, the sublimation model appears to be the most elegant one. As for the equatorial ridge, I don't see why the two phenomenons would have to be related.

[ngunn]

I don't see why the two phenomenons would have to be related.

Well if you go for sublimation for the 'paint job' and fossil ring for the ridge there is actually a nice link. Both are consequences of Iapetus' exceptional remoteness from its primary. (Very long month/day for the first and low tidal disruption for the second.)

[ugordan]

Yes, they might both be a consequence of a third factor, but I was actually getting to the point the ridge not being a direct or indirect cause for the dark stuff.

[ngunn]

Agreed. The ridge is ancient, the repainting must be ongoing.

[tasp]

Ridge and splotch are unrelated other than high end of ridge (and Voyager Mountains) is aligned directly away from Saturn.

Formation of ridge predates splotch. Tidal effects of Saturn may have forced Iapetus into tide lock in this alignment. Splotch is result of thermo-reactive gaseous efflux from Titan being temporarily (<80 days)retained in Iapetan realm from it's repeated applications as Iapetus encounters it during Saturnian magnetotail traverses.

[Pavel]

Do we have any proof that it's dark material on top of white material and not vice versa? Can it be that Iapetus was black originally and got sprayed with water that condensated as bright ice on the white side?

[Juramike]

I agree.

For all it's amazing curiosity and exotic beauty, Titan is a stinkball in Saturn orbit.

-Mike

[nprev]

...well, to be fair, a hypothetical Titanian Juramike would probably say something similar about Earth, even as he was fascinated by our geochemistry...

Starting to like the thought of Titanian contamination as the root cause of the splotch, though. Again, I'm struck by the fact that Iapetus really does look a lot like a bi-colored baseball. Assuming that Titan's effluents are distributed in a torus that's aligned with Saturn's equatorial plane (and densest there, naturally), would Iapetus' orbital inclination account for this seemingly odd surface distribution...?

Also, does anyone know if Iapetus' rotation axis is perpendicular to its own orbital plane, or to Saturn's equatorial plane?

[ngunn]

Do we have any proof that it's dark material on top of white material and not vice versa?

No. If its sublimation that's doing it then both the light depositional frost and the dark sublimation residue would overlie a bulk material that is the source of both and so presumably of intermediate hue.

Also, does anyone know if Iapetus' rotation axis is perpendicular to its own orbital plane, or to Saturn's equatorial plane?

The former.

[Bjorn Jonsson]

Do we have any proof that it's dark material on top of white material and not vice versa? Can it be that Iapetus was black originally and got sprayed with water that condensated as bright ice on the white side?

Looking at the highest resolution (so far!) images gives a strong impression of dark stuff 'sprayed' over white stuff and not vice versa, especially when looking at the dark/bright boundary. At least that is the impression I get when looking at the images. There is also some exposed ice in the dark terrain, especially in crater walls and central peaks.

[nprev]

I agree, Bjorn, and thanks ngunn for answering my question re rotation axis alignment; that's what I thought.

What's interesting here is that if there is a Titan Schmutz Torus (TST), Iapetus intersects its densest section twice every orbit. Given the 19 deg orbital inclination, the sub-TST point during the descending plane passage is offset to the north by this amount, and to the south during the ascending plane passage. (The same hemisphere takes the brunt of this each time, of course).

Therefore, if this model is valid, the TST residue area should be offset slightly towards both of the poles and roughly centered at the point on Iapetus that corresponds with its direction along its orbit. If Iapetus' orbit wasn't inclined, the distribution would be uniform over a full hemisphere rather than exhibiting this baseball/yin-yang appearance.

Does this agree at all with the observed distribution of the splotch (or, alternatively, with the light area if the TST's effects are erosive rather than depositional, since I'm just assuming that the dark side is also the orbital leading side?)

[djellison]

I don't 'do' Saturnian moons (havn't a clue what's going on beyond Mars frankly) - My vote is for 85% Cocoa Chocolate. It's what it looks like.

Doug

[nprev]

Er...you can try the first cup, Big Guy...I'll wait!

[antoniseb]

My take is pretty close to ngunn's. It looks to me like there was some very large local heating event that cause part of the surface to boil, and that the boiled area turned black, and that some sputtering sprayed the neighboring white areas, giving some false hint of exogenesis.

I don't think that this one close encounter will reveal conclusive evidence one way or the other. I suspect that the probe doesn't have the tools that could tell us. It seems more designed to examine Titan. I am grateful for this one pass, and hope the images are spectacular.

[nprev]

I don't think that this one close encounter will reveal conclusive evidence one way or the other.

Depressing, but probably true given the history of UMSF in general.

I think that the splotch's global distribution will provide the most important clues, though, and Cassini's been doing a great job of obtaining global albedo data from a distance. Maybe we'll get lucky during C/A and get a much better idea of the physical consistency of this material...

[Juramike]

VIMS might be able to provide some further data:

If the spectra look very similar to Titan gunk, there might be a common origin. ('Course, Iapetus may have done it's own subsurface chemistry that mirrors Titan as well).

If it is extremely different, then you can at least put some constraints on the system. (Titan stuff but modified on Iapetus, Iapetus's own home-brewed novel recipe stuff, or stuff from another source [Hyperion? comet? other?]).

I'm really hopeful that this pass will at least give us a hint.

(Actually, I'm totally psyched that this pass will give us a hint.)

-Mike

[ugordan]

You can't really directly compare Titan's gunk with Iapetus because we only see Titan's surface through a few narrow windows so we don't get spectra, only snapshots.

[JRehling]

As we approach Iapetus for this close fly-by, I thought it might be fun to try and predict if, after we see the close up images of the transition region and Voyager Mountains, the source of the dark material will be obviously endogenic or exogenic, or there will still be no obvious explanation.

The trick is that some of the explanations combine endo- and exo-. I'm reminded of the three theories regarding the Moon's origin, which seemed to be comprehensive in addressing the possibilities, but the evidence ending up generating a fourth explanation which sort of combined two or all three of the initial set.

John Spencer's theory is that the initial emplacement of dark stuff is exogeneous, but then the darkness spreads, creeping along the surface where thermal conditions are appropriate. I'm not sure how you'd classify the second process, which is neither endo- nor exo-.

I still harbor someplace in my heart for a "big splat" origin, but Spencer's theory is growing on me.

When it comes right down to it, the quirks of Cassini Regio are such that there probably isn't a one-process explanation. We have these clues:

Generally matches the leading side.
Generally matches areas of a certain insolation/thermal constraint (reflected by latitude and local slopes).
Generally equator-facing slopes (another statement of the above).
Border is very streaky in places (eg, to the north).
Concentrates around the Snowman craters in ways implying a source from the TRAILING side.
White mountains within.
Western boundary appears jagged.
Not interrupted by many small, new, fresh craters.
The equatorial ridge runs almost right down the middle.

FWIW, other Saturnian satellites (Tethys, Dione) have dark blotchy areas, but with much less contrast.

I can't see one process matching all of those quirks.

I go with a big splat (dark impactor) that seeds an ongoing thermal creep.

[Juramike]

You can't really directly compare Titan's gunk with Iapetus because we only see Titan's surface through a few narrow windows so we don't get spectra, only snapshots.

Not easily, no.

But you just might (cross fingers, touch wood, drink stumpwater, rub lucky rabbits foot) be able to compare the ratios of absorbances between the Titan methane windows with the corresponding ratios for spectra from Iapetus.

-Mike

[volcanopele]

My money is exogenic. Iapetus is dead. It is deader than a doornail. Good grief, it is deader than Rhea for goodness sake. I very much doubt anything has been boiled, geysered, expelled, or outgassed there in a VERY long time, if ever. So given the lack of an obvious endogenic source, one needs to look outside Iapetus for the source.

We know a few things about this material. We know it contains a lot of carbon-based materials, both organic (like PAHs) and cyanides. We know that it is recently emplaced or is being emplaced as we speak. John Spencer's theory seems to explain the specifics of its distribution, that the lower albedo material has led to increased sublimation of H2O from the dark area, leading to brighter poles and an extended range of dark material along the equator. It seems to be concentrated either in depressions (suggesting that this material "flows" downslope) or along equator-facing slopes.

Again, my money is on a planetocentric origin, likely do to accumulation of dust, rather than of ejecta from a large impact. We should, however, be careful about immediately attributing to Phoebe. There are plenty of small, little moons out in the outer Saturnian system that could be the donor. I'm guessing Hati...

[Stu]

... and the award for most "ugh, that's downright disturbing" sentence ever used in an UMSF post goes to...

( opens the envelope...)

seeds an ongoing thermal creep.

[nprev]

...

Valid as that really interesting idea is, I'm afraid that the first thing I thought of when reading about it was the Creeping Terror...

Fortunately, future explorers would have no trouble at all outrunning it, even in full suits!

[JRehling]

Yes, the creeping crud is a disturbing notion. It's only a pity I used the phrasing to describe a neat-looking place like Iapetus instead of Hyperion, which looks like a rotting fruit.

Another armchair scientist perspective on this is to pose the question why Iapetus would have this dark stain spreading on it, but Ganymede does not. Ganymede receives about 4 times the solar radiation that Iapetus does, and despite having a much slower rotation, still gets much warmer (160K) at the equator than Iapetus does (130K). So why doesn't Ganymede boil its equatorial surface ice away, leaving dark lag deposits, to the same extent?

Not that I can't imagine an answer, but its suspicious.

Oh, how far-off September 2007 seemed when I first read the tour description...

[The Messenger]

endogenic - natural composition of the crust of Iapetus.

The cratering tells us the surface is ancient. The shallow depth of the craters suggest a resilient surface. I get to guess the light side is also dark, but spattered with ices: Either Iapetus took a backside graffiti hit from Enceladus (unlikely) or there was a recent thermal event on the front face that boiled-off most of the ice deposited in the last hundreds of millennia, exposing the moon's native surface.

I also get to speculate that the composition includes clays, including pyroxene, sulfates, silicates and other terrestial compounds not on anyone's list; and low levels of organics, if they are found at all.

[nprev]

Hmm. Anybody got some quatloos they want to risk?

I'm down for 100 Qs, exogenic, dark overlay, Titanian source.

[Gsnorgathon]

...
Another armchair scientist perspective on this is to pose the question why Iapetus would have this dark stain spreading on it, but Ganymede does not.
...

Well, Ganymede formed in a much warmer place and had higher temperatures over a long period time due to internal heat as well as solar. It might have been too warm in the first place, or so warm afterward that all the gunk burned off.

(Though I suspect the gunk on Cassini Regio is exogenous + radiation/heat processing on the surface, and not at all applicable to Ganymede. I'm just nitpicking.)

[dvandorn]

To the argument that we can't tell if the dark material is Titanian in origin, since we don't have spectra of Titan's surface:

If the material is being accreted from a Titanian smutz torus (I like that term, Nick!), then that material comes not fromTitan's surface, but from the outer layers of Titan's atmosphere.

Correct me if I'm wrong, but we have a *lot* of spectroscopic information about the outer layers of Titan's atmosphere, don't we? You don't have to look down through Titan's thick atmosphere in order to see its outer layers, do you? You basically just have to look at Titan from a distance and take spectroscopic readings of what the outer layers are made of... right?

According to the best theories I've been reading, if there is a torus of Titanian origin that gets swept back through Saturn's magnetotail, it would be composed of molecules sputtered off the top of the atmosphere by solar wind interactions and by interactions with Saturn's magnetic field. You don't have to postulate some huge impact that lofted megatons of Titan's surface material out beyond its Hill sphere to construct such a torus -- and to be honest, I haven't seen anything in the surface imaging of Titan to hint at such an enormous impact. Unlike several other Saturnian moons, Titan doesn't sport any obvious basins that are a sixth or more the size of the moon itself.

I think that if the dark material on Iapetus is of Titanian origin, it's got to be remnants of Titan's depleted upper atmosphere. Which we have a fair amount of compositional data for.

-the other Doug

[ngunn]

Well, Ganymede formed in a much warmer place and had higher temperatures over a long period time due to internal heat as well as solar. It might have been too warm in the first place, or so warm afterward that all the gunk burned off.

Exactly so. Ganymede is much more differentiated; in fact its ice could have melted all the way to the surface at some points in the past when tidal heating went through a peak. There would have been a net loss of volatiles like ammonia that lower the melting point of the icy mixture. In addition the differentiation would have made the surface ice much cleaner than on Iapetus. Any sublimation that does occur therefore fails to leave behind much residue of dark materials, an essential factor in the spreading of Cassini Regio beyond the originally seeded area on Spencer's Iapetus model.

[ngunn]

I also get to speculate that the composition includes clays, including pyroxene, sulfates, silicates and other terrestial compounds not on anyone's list

You may recall that I am among the large majority who dismiss your hypothesis that Titan's darker surface materials, and especially the dunes, consist largely of silicate sand. However I have to admit that on the poorly differentiated Iapetus it is much more likely that the dirty surface ices do contain some silcates as well as organics.

[nprev]

I don't know if Alex has posted this yet, but the Iapetus mission description is up. There's an excellent summary discussion of the splotch that has a lot of relevance to this thread.

Also, one little offhand tidbit in this document was that there might be a water/ammonia slurry "several decameters" below the surface. Correct me if I'm wrong, but if true wouldn't this make Iapetus the body with the most accessible liquid water we know of? I was very surprised by that statement.

Hell, maybe it is the Creeping Terror!

[Gsnorgathon]

... You don't have to postulate some huge impact that lofted megatons of Titan's surface material out beyond its Hill sphere to construct such a torus
...

Here's a question: has anyone taken a shot at calculating the mass of the dark gunk that comprises Cassini Regio and compared it to the mass of smutz lofted from Titan's atmosphere?

[nprev]

Great question, GS, though I don't think that we know how thick it is (much less its composition and therefore molar mass, assuming it's homogeneous) to figure that out yet. The other big issues associated with this idea are:

1. Why aren't all the icy moons coated in smutz, if Titan is the source? See no obvious reason why this stuff should preferentially migrate outward from Saturn, unless Hyperion's irregular orbit helps to pump it out somehow (and assuming that Hyperion has enough mass to make a difference).

2. What the hell is smutz if it exists, anyhow? Complex, heavy organic molecules presumably do not escape from Titan at any great rate; most effluents must be very light (mostly H2, in fact). Shooting from the hip, here, I'd guess that propane (C3H8) might be close to the upper limit of molecular weight for escaping compounds.

3. Given #2, there must be a significant amount of decomposition of compounds due to solar radiation while they're drifting around in the TST awaiting Iapetus impact, decreasing yield.

4. Once they're on the surface, then additional reactions must be postulated in order to generate the dark material, unless it's just elemental carbon.

Playing devil's advocate here for a theory I personally support...please feel free to kick holes in all this!

[Gsnorgathon]

Well, I figured the error bars might be pretty big...

But I figured it'd be better than nothing. The thickness is somewhat constrained (from radar data), the density... well, it's not solid lead, right? So that's somewhat constrained as well.

As far as why the rest of the moons aren't so smutzy, I thought (if you can call it thinking) that the smutz was ending up on Iapetus due to interaction with Saturn's magnetotail. But I have no idea if that's plausible or not.

[nprev]

As far as why the rest of the moons aren't so smutzy, I thought... that the smutz was ending up on Iapetus due to interaction with Saturn's magnetotail. But I have no idea if that's plausible or not.

Quite plausible, I'd say, and good thinking! Possibly a much more efficient transport mechanism than TST intersections. Two more variables in that scenario, IMHO:

1. Smutz charging potential (whatever it is, if it is)

2. Frequency in which the ascending & descending nodes of Iapetus' orbit intersect with Saturn's magnetotail (thinking once every 29 Earth years for each, but not sure; if that's the driver, then there might be considerable deposition during each passage). This would also put some pretty hard upwards constraints on smutz production, since it can only hit Iapetus seasonally.

3. Possible seasonal variations in smutz production by Titan due to probable sudden changes in hemispherical climate; is there any correlation with #2 for Iapetus? Be interesting if there was...

[Del Palmer]

1. Why aren't all the icy moons coated in smutz, if Titan is the source?

Perhaps they are, but Enceladus is continuously re-coating them. After all, Enceladus spewings touch at least 11 other moons...

[Big_Gazza]

This armchair boffin-wannabee (using the term in its loosest possible context!!) has a endogenic-exogenic theory:

With the Cassini mission description hinting that radar studies show ammonia-water slurries just beneath the surface, is it not possible that Iapetus' interior contains significant quantities of organic volatiles locked deep in the interior? Given that this moon has an obvious history of massive impacts, could a particulary penetrating impactor (eg an iron-nickel object from the asteroid belt) not have ejected large quantities of these deep lying organics to form a orbital debris belt along Iapetus' path ie endogenic source? Once swept up by Iapetus, the organic debris would subsequently darken due to UV-induced breakdown (or would darken while in orbit), ie exogenic deposition.

Are there any relatively young deep impact features that also show signs of dark ejecta? Smowman/Moat is poorly imaged, but could this not be the source?

I guess this theory is nothing new, but is it likely? Reading through the postings, it doesn't seem to be considered a contender.

[JRehling]

Iapetus is dead. It is deader than a doornail. Good grief, it is deader than Rhea for goodness sake.

Pre-flyby quote of the week!

It may just be deader than Deimos.

[David]

Which of Saturn's outer satellites comes closest to Iapetus' orbit? Bestla? Could one of them be an exogenic source for the dark material?

[tty]

What the hell is smutz if it exists, anyhow? Complex, heavy organic molecules presumably do not escape from Titan at any great rate; most effluents must be very light (mostly H2, in fact). Shooting from the hip, here, I'd guess that propane (C3H8) might be close to the upper limit of molecular weight for escaping compounds.

How about elemental carbon? It is very dark and fairly light, and might be be produced from e. g. CH4 or CN. Also it is extremely stable, so once created it will probably stay around for billions of years. Even in Earths highly unstable/oxidizing environment elemental carbon several hundred million years old is very common.

[The Messenger]

How about elemental carbon? It is very dark and fairly light, and might be be produced from e. g. CH4 or CN. Also it is extremely stable, so once created it will probably stay around for billions of years. Even in Earths highly unstable/oxidizing environment elemental carbon several hundred million years old is very common.

The problem is the utter lack of relatively new craters exposing any white stuff underneath; as pointed out by Emily in her PS post on Iapetus. Ethenal loaded with even a few ppm elemental carbon traces of benzene could create an extremely slow moving tar than carmelizes every thing...but that would also quickly fill the craters.

[JRehling]

This armchair boffin-wannabee (using the term in its loosest possible context!!) has a endogenic-exogenic theory:

With the Cassini mission description hinting that radar studies show ammonia-water slurries just beneath the surface, is it not possible that Iapetus' interior contains significant quantities of organic volatiles locked deep in the interior? Given that this moon has an obvious history of massive impacts, could a particulary penetrating impactor (eg an iron-nickel object from the asteroid belt) not have ejected large quantities of these deep lying organics to form a orbital debris belt along Iapetus' path ie endogenic source? Once swept up by Iapetus, the organic debris would subsequently darken due to UV-induced breakdown (or would darken while in orbit), ie exogenic deposition.

Are there any relatively young deep impact features that also show signs of dark ejecta? Smowman/Moat is poorly imaged, but could this not be the source?

I guess this theory is nothing new, but is it likely? Reading through the postings, it doesn't seem to be considered a contender.

This has similar elements to my Big Splat theory, which is that an impactor (or series of impactors) moving from east to west hit around where is now Eastern Cassini Regio, and one of them contributed a huge amount of dark stuff, laying out an ejecta blanket the size of CR. The length of CR is about what you get when you take ray systems on the Moon and imagine the same events in Iapetus's lighter gravity.

The dark stuff on the inner WESTERN slopes of the Snowman craters definitely shake up the usual exogenous theories that the leading face swept up stuff from above.

Clearly, though, there's been some "upkeep" on anything that may have happened in the ancient past. There is certainly a mechanism maintaining the dark stuff, and there must be SOME other mechanism that led to its creation in the first place.

[monitorlizard]

Those of you who have read Arthur C. Clarke's original novel "2001: A Space Odyssey" will remember that the Discovery went to Saturn, not Jupiter as in the movie, and the Stargate was the dark side of Iapetus. While today's findings will probably fall short of being that awe-inspiring, I do believe we will find at least one or two unexpected things. That's what keeps us all in a continual state of excitement.

[antoniseb]

the Stargate was the dark side of Iapetus.

My recollection was that the monolith was in the center of a large white circle, which ACC referred to as the Eye of Iapetus.

[monitorlizard]

Please, you're harshing my buzz.

Actually, I read the book about 35 years ago, so I could be remembering it wrong, but I'd have sworn it was somewhere on the dark half of Iapetus. If not, it would have made a cool plot point. I'll leave it to Cassini to tell us where the actual Stargate is.