Iapetus - Black on white or white on black? Options

[SFJCody]

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?'

[ngunn]

I think the responses you offer are inadequate. Globally I go for both on lightish grey - the zebra itself is (almost) invisible.

On local scales any of your four are also possible so individual details may be a poor guide to the big picture. For that reason I think a simplistic poll is not such a good idea.

[ugordan]

Where's the option "Black on White - black combination of endogenous and exogenous" ?

[akuo]

Is this a Michael Jackson song?

Oh Iapetus, I wouldn't have a clue. Planetary experts, save us!

[volcanopele]

I would prefer a both option: white on black in some areas, black on white in others.

[Juramike]

(Objects collide)

Object one: "Heeeey, you got white stuff on my black stuff!"

Object two: "And heeeey, your got black stuff on my white stuff!"

(together): "Two great tastes that taste great together!"



[total shameless ripoff of old Reese's Peanut Butter cup commercials.]

[[Did you know the original Reese's commercial starred John Travolta colliding with Robbie Benson?]]

-Mike

[nprev]

...of course, in the Family Guy parody of this commercial, Officer Reese shoots the other guy in order to keep the precious discovery for himself...

Interesting how uncertain we all are now. I was a solid black-on-white-exogenous bloc member until the flyby; now I've gone white-on-black-exogenous.

The craterless white highlands may hint at some internal activity still, but damned if I can think of a mechanism. Iapetus isn't well-situated orbitally for tidal heating, and its density doesn't seem high enough to make a compelling argument for enriched radioisotopes. What's left?

[JRehling]

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?

[Michael Capobian...]

I have to choose white on black on white - black endogenous, white both endo and exogenous.

Here's some of the things I see:

Globally bright water ice with a cover of endogenous black gunk of varying thicknesses. Very thick on leading hemisphere, even big craters don't break through.

Water ice brought to the surface and scattered about by impacts, some of them very large (Claude), which also contribute additional water ice. Gathers preferentially in impact-facing slopes, like Voyager Mts. White layer very thin in places, exposing dark ridges and bumps. Craters focus heat and start to remove bright material.

Water ice removed from the leading hemisphere by some exogenous process and emplaced on the trailing hemisphere and poles through sublimation.

Dark stuff spreads wherever it's exposed. Chunks of dark stuff hurled about by big impact(s), causes holes in thin bright ice layer where they fall. Crater rays can also remove white overlayer, leave dark lines. Black spreads by sublimating material around it.

It looks to me like the Claude basin and its ejecta are comparatively recent; and are the controlling factor for a large swath of the moon north and west of the basin. We need to think about what happens when a large, icy body impacts a body with a layer of gunk overlying ice.

It's interesting how a person's theories can affect what they see in these images, especially when there's the level of ambiguity we have at Iapetus. For example, I noted the little craters at the apex of certain parts of the equatorial ridge with great interest, especially considering that it looks as if portions of the ridge nearby seem to have moved and sunk. This doesn't look to me like a ridge emplaced by an infalling ring on a cold, dead world, but a ridge that was built and displaced by internal processes. But that's what I wanted it to be before I saw the images. ;-)

[lyford]

The only thing that I can say is that after reading all these posts and looking at all the pictures, I just can't get this song out of my head.

[MarcF]

It was not a good idea to go to vacation far from my computer this week. The whole week-end will not be enougth to look at all the Iapetus flyby stuff !!!

After watching some of the pictures, I have the feeling that craters in Roncevaux are very smooth, like on Enceladus. So is this region covered by a kind of snow ?
(Ejected by the Claude impact event which seems to have occured quite recently ?).
On Rhea, craters look sharper.
Moreover, as already mentioned, in some pictures white seems to cover black and in other pictures black seems to cover white. So even if I always thought that the Black was on the White, now I'm not sure about it anymore.
Iapetus might still have a certain kind of surface activity (like on Callisto?) and both black and white stuffs migth be transported from one location to the other.
Data from the other instruments (VIMS, CIRS,...) might help to solve the problem. Does someone know when they will be available ?
Marc.

[tasp]

I would pick transparent material everywhere, keyed to orbital position relative to Saturn, that only darkens upon exposure to sufficient heat/light from sun.

[David]

I would pick transparent material everywhere, keyed to orbital position relative to Saturn, that only darkens upon exposure to sufficient heat/light from sun.

I don't think this is possible; where black and white regions are adjacent, the white regions appear to be definitely higher in relief than the black (in some places high enough to cast shadows).

Now, that in itself doesn't decide white on black or black on white, because it could be produced either by icy material condensing and/or flowing over a dark background, or by a dark material heating up and causing the ice beneath it to sublimate until it ends up lower than the surrounding lighter regions.

However, it's not consistent with the same material merely undergoing a color change -- then you'd expect continuity in relief (and probably not such sharp boundarie, too).

Are we going to get information about the chemical composition of the surface materials from this pass?

[ugordan]

where black and white regions are adjacent, the white regions appear to be definitely higher in relief than the black (in some places high enough to cast shadows).

Can you give an example of this?

[David]

Can you give an example of this?

Well, you may not see the same things I do, but if you look at this one, you may see that some of the small-scale narrow bays or firths of dark material between two lighter areas are shadowed from below, as if they were running in slightly depressed channels. There are other examples, but in some of them it could be argued that the lighter material happens always to be located on higher topographic elevations -- which is probably also true, as the darker areas tend to be lower anyway, that being in itself a significant fact.