Poll: Iapetus dark material source, A poll about the source of the dark material on Iapetus Options

[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.