Iapetus - Black on white or white on black? Options

[dvandorn]

What do you literally mean by this? Obviously, there is no wind on Iapetus (hence your "quotes") and there can likewise be no dunes. So what process do you think is in fact occurring?

That is exactly why I put "downwind" in quotes. It's just a convenient way of indicating that I see apparent flow vectors in some of the dark surficial material.

Such a flow could be caused by any number of factors, and I'd be surprised if there is just a single factor controlling it. It could be anything from material creep forced along by interactions with Saturn's magnetotail to atmospheric movements caused by transient atmospheres generated by (relatively) immense amounts of sublimation or outgassing from the leading edge (as has been speculated). Heck, it could just be some odd form of impact ejecta interaction. I will point out that you see some "duning" landforms on other airless bodies, such as the Moon, which are almost definitely formed by ejecta processes. Those are just at much larger scales than you see on Iapetus.

I still think that those who dismiss the dark material as simply "the natural surface of Iapetus below the bright ice" are missing the point. This dark material is unlike any other surficial material on any other Saturnian moon -- Hyperion included. These bodies are made up primarily of ices. You'd have to postulate some mechanism that would first emplace a dark surface on Iapetus before you can start to speculate that the dark material is simply that surface with an overlay of clean ice removed from the leading edge. Having heard absolutely no such theory about the emplacement of a surface that is significantly darker (and of a significantly different composition) than we see on *any* other moon in the system, seeing this dark material having been moved along the leading edge in apparent flow patterns, and seeing a "true" bright ice surface exposed by impact from under the dark surface in many areas, I simply have to regard the possibility of the dark surface being Iapetus' "natural" surface as the least likely of all of the theories.

So, all of my observations and an application of Occam's razor leads me to the conclusion that the dark material is emplaced on top of a bright, icy surface that is far more representative of what we see on the rest of the icy moons. Hence my conclusion that we're seeing black on white and not vice-versa.

-the other Doug

[dvandorn]

Like oDoug, I too noticed the apparent "duning" of dark material in some places; unlike him, didn't have the guts to mention it!

Yep, that's me -- all guts, no glory...

-the other Doug

[tasp]

Here is some more 'ponding' to contemplate.

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[tasp]

And the comparison.



{I suppose it would be nice if I knew how to 'twin' pics in single post}

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[tasp]

Crikey, there is something hinky going on tweenst Hyperion and Iapetus.

Most of the difference is from the drastically different spin periods of the 2 objects affecting the sustained insolation induced darkening of the same exogenous efflux.

[nprev]

I simply have to regard the possibility of the dark surface being Iapetus' "natural" surface as the least likely of all of the theories.
-the other Doug

Mmm...Disagree based on one hypothesis: that Iapetus is a captured object, possibly a large, errant KBO. If it was a "native" Saturnian satellite, I'd think that its orbital inclination would have been a lot less with respect to Saturn's equatorial plane.

Would give a lot for a close-up view of Chiron right now; willing to bet that it just might have some gross similarities to Iapetus.

[tasp]

We note earth's moon to have an orbit about earth rather suspiciously 'relaxed' into alignment with the ecliptic.

We note something rather similar in regards to Iapetus.

Not sure we can infer an exogenous origin for Iapetus from such an orbital characteristic, Saturn having such a nice big Hill sphere, and Iapetus orbiting, albeit inclinedly, circularly and comfortably inside.

[nprev]

True, but Earth's Moon is thought to be an artifact of a massive collision (therefore technically of exogenic origin?)...not quite the same situation as would be expected for a satellite of a gas giant.

[alan]

There was a paper a couple of years ago linking Titan's eccentricity to an interaction with Jupiter.
http://adsabs.harvard.edu/abs/2005AGUFM.P33C0263B

Perhaps the inclination of Iapetus has a similar cause.

[edstrick]

What can I say.. just look at this d__n stuff... At this latitude, ALMOST none of it is on a horizontal surface, it's almost ALL on slopes, facing noonday sun. Not quite all, but the stuff in the lower left basin rim setment is the totally classic example. I can't avoid the conclusion, as I said above, that some process has caused it to creep against gravity, to accumulate on surfaces perpendicular to the sun's rays at mid-day.

Igor... it's alive..... IT'S ALIVE!!!!!!!!

<master.... you didn't take your medicine this morning!>

(data taken and sharpend from UGordon's big b&w mosaic)

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[dvandorn]

OK -- I understand your conclusion, here. But I still want to hear your theory for how Iapetus got a dark, reddish natural surface, and from where the bright ices that are covering this dark surface came. If insolation can "burn off" the snow from sun-facing slopes, over billions of years, shouldn't it have burned off ALL of the snow?

Iapetus has no atmosphere, so we're not talking about a hydrocycle, here. If what everyone has been saying is true and it's stone-cold dead, it hasn't vented icy plumes for billions of years. So why hasn't all of the snow been burned off, the vapor sputtered into space?

And, to reiterate -- how did this entire moon get surfaced with a dark reddish material that's different in composition from any of the other icy moons, and yet seems similar in composition to Titan's atmosphere?

Before I can accept the concept that the dark material is the natural surface and the icy snow lies on top of it, I need to hear a mechanism postulated that accounts for this dark surfacing. And just saying "Well, it's probably a KBO, that explains it" doesn't explain a thing for me... show me where KBOs follow a pattern of having dark, reddish surfaces and explain how they *all* got surfaced that way, and maybe I'll start to consider it. But until then, Occam's Razor tells me that any airless icy body that was born anywhere near Saturn ought to have a bright icy surface, and that any significant darkening must be an overlay on top of that icy surface.

-the other Doug

[The Messenger]

And, to reiterate -- how did this entire moon get surfaced with a dark reddish material that's different in composition from any of the other icy moons, and yet seems similar in composition to Titan's atmosphere?

It is a lot easier to explain the shallow craters if the surface is similar to terrestrial planet regoth.
Rocks and sand, just like Titan.

[TheChemist]

What can I say.. just look at this d__n stuff... At this latitude, ALMOST none of it is on a horizontal surface, it's almost ALL on slopes, facing noonday sun. Not quite all, but the stuff in the lower left basin rim setment is the totally classic example. I can't avoid the conclusion, as I said above, that some process has caused it to creep against gravity, to accumulate on surfaces perpendicular to the sun's rays at mid-day.

(data taken and sharpend from UGordon's big b&w mosaic)

Ed,
I think that the image you posted is from an area where we see white ice on top of black soil.
So it is just the walls of the basin that are black.
The original released image can be found here.
I attached a rotated version that matches the orientation in your image.

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[ugordan]

I think that the image you posted is from an area where we see white ice on top of black soil.

White ice on top of black soil - how do you figure that?

[TheChemist]

Ugordan,
I think that I see wide openings (holes) on the ice in this particular image, that seem to have thick white walls and are partially black at the bottom.
This gives me the impression of melted ice on top of black terrain.

If it was just white holes filled with black material melting the ice, one has to explain the black basin walls, which I find hard to do