[X] My Assistant

[Decepticon]

Here's what extrapolating the current SPICE kernels yields:

IAPETUS 2005 NOV 12 14:34:49 185078153 415,451 km arcmin=12.1
IAPETUS 2006 JAN 25 21:13:08 191495653 873,345 km arcmin=5.7

IAPETUS 2006 APR 11 17:16:28 198047853 602,419 km arcmin=8.3
IAPETUS 2007 SEP 10 12:31:28 242699553 1,259 km arcmin=2186.8

Posted by jmknapp

This time a little further out.

Jan. 2006 - 879,000 km - 5.3 km/pixel - eastern Cassini Region and transition zone, part ofthe "moat" posted by VP


If anyone has made a animation of the encounter please post it!

[Rob Pinnegar]

A couple of points:

(1) Has anyone given much consideration to estimating the thickness of the dark layer in Cassini Regio? One would think that, if it is only a very thin layer, there should be at least one small crater somewhere that would show signs of having punched through it. (I realize frost migration could cover up such craters eventually, but it would take time.)

If the layer turns out to be a couple of kilometres thick, or even several hundred metres, this would have major ramifications. Do we have any idea?

(2) Not to kick a dead horse too much, but I'm going to throw in (one of) my old half baked hypotheses back into the ring. This one has to do with Hyperion's origin.

From what I recall, it's currently thought that Titan's atmosphere was originally something like 30 times more massive than it is now, based on isotope data. Considering the atmosphere's current height, one would therefore expect that it must have originally extended outwards quite a bit further than it does now.

This means that, back in the "old days" (i.e. the Hadean Eon) Titan's atmosphere would've made up a pretty good chunk of its total effective cross-section for collisions. From that, it's not too much of a stretch to imagine that, from time to time, large planetesimals would have come blazing through Titan's atmosphere without necessarily colliding with Titan itself.

Some of these planetesimals could have been broken up through a combination of atmospheric drag and the "disintegrative capture" process that was proposed to explain the origin of the Earth's Moon a few decades ago. Both of these effects will slow down the disrupted body, perhaps enough to allow its fragments to be captured into orbit around Saturn. I honestly don't know whether that is dynamically possible, but let's suppose for the moment that it is. (Incidentally, there are some papers on disintegrative capture in the "Origin of the Moon" conference volume from around 1986, if anyone is interested. These papers include some results of simulations, which are the source of some of the assertions I'm making here.)

Anyways, disintegrative capture tends to break the disrupted body into pieces that propagate outwards in a "spray". Since the pieces interact with each other gravitationally after disruption, they don't follow ballistic trajectories, so they wouldn't necessarily end up in Titan-crossing orbits. It wouldn't be surprising if some of the debris ended up in the 4:3 Titan resonance, where one would expect it to coalesce into a small moon. Whether or not we could get something as big as Hyperion out of this process is of course a big question.

If by some miracle this totally conjectural hypothesis were to turn out to be workable, then it would imply that the dark material on Hyperion might not be similar to what we see on Iapetus. Instead, it might be organic material from Titan's early atmosphere, that got assimilated into Hyperion, and hasn't boiled off yet because it's too deep inside the moon.

Any comments?

[JRehling]

A couple of points:

(1) Has anyone given much consideration to estimating the thickness of the dark layer in Cassini Regio? One would think that, if it is only a very thin layer, there should be at least one small crater somewhere that would show signs of having punched through it. (I realize frost migration could cover up such craters eventually, but it would take time.)

If the layer turns out to be a couple of kilometres thick, or even several hundred metres, this would have major ramifications. Do we have any idea?

(2) Not to kick a dead horse too much, but I'm going to throw in (one of) my old half baked hypotheses back into the ring. This one has to do with Hyperion's origin.

From what I recall, it's currently thought that Titan's atmosphere was originally something like 30 times more massive than it is now, based on isotope data. Considering the atmosphere's current height, one would therefore expect that it must have originally extended outwards quite a bit further than it does now.

This means that, back in the "old days" (i.e. the Hadean Eon) Titan's atmosphere would've made up a pretty good chunk of its total effective cross-section for collisions. From that, it's not too much of a stretch to imagine that, from time to time, large planetesimals would have come blazing through Titan's atmosphere without necessarily colliding with Titan itself.

Some of these planetesimals could have been broken up through a combination of atmospheric drag and the "disintegrative capture" process that was proposed to explain the origin of the Earth's Moon a few decades ago. Both of these effects will slow down the disrupted body, perhaps enough to allow its fragments to be captured into orbit around Saturn. I honestly don't know whether that is dynamically possible, but let's suppose for the moment that it is. (Incidentally, there are some papers on disintegrative capture in the "Origin of the Moon" conference volume from around 1986, if anyone is interested. These papers include some results of simulations, which are the source of some of the assertions I'm making here.)

Anyways, disintegrative capture tends to break the disrupted body into pieces that propagate outwards in a "spray". Since the pieces interact with each other gravitationally after disruption, they don't follow ballistic trajectories, so they wouldn't necessarily end up in Titan-crossing orbits. It wouldn't be surprising if some of the debris ended up in the 4:3 Titan resonance, where one would expect it to coalesce into a small moon. Whether or not we could get something as big as Hyperion out of this process is of course a big question.

If by some miracle this totally conjectural hypothesis were to turn out to be workable, then it would imply that the dark material on Hyperion might not be similar to what we see on Iapetus. Instead, it might be organic material from Titan's early atmosphere, that got assimilated into Hyperion, and hasn't boiled off yet because it's too deep inside the moon.

Any comments?

The layer is clearly either very thick (at least in some places) or fairly recent (or recently replensihed). An interesting possibility would be if the ridge is simply the thickest portion of a layer that is at least 10 to 15 km thick along that central line, with the depth tapering off to the edges. That's just a quirky possibility, but one that would exclude any of the explanations relying solely on some sort of cloud/debris that Iapetus is sopping up as it passes through it's orbit.

Your model is a two-parter: Titan's atmosphere as a source of dark stuff, and "sopping up from intersatellite space" as the means of delivery of the dark stuff onto Iapetus and Hyperion both.

I see trouble with the means-of-delivery part.

I can't see why Hyperion would have many large craters with no dark stuff (more or less inbetween ones that do have it) if "sopping up" were the means of delivery there. Or why intricate topographical features intruding into crater floors remain bright. Or even the whole of the non-crater-floor surface.

Why would Iapetus just happen to have a big ridge running down the middle of the area where fluffy dark stuff descended upon it? If Iapetus gathered many km of this stuff, how did, eg, Rhea end up with none? How are there bright mountains within western CR? How did dark stuff get to ring the western rims of the Snowman craters which are on the trailing side?

Perhaps Titan's atmosphere did end up as a source of interesting dark material, but if the means of delivery isn't there, then we wouldn't have any dark stuff thus transported to explain!

[Rob Pinnegar]

I can't see why Hyperion would have many large craters with no dark stuff (more or less inbetween ones that do have it) if "sopping up" were the means of delivery there. Or why intricate topographical features intruding into crater floors remain bright. Or even the whole of the non-crater-floor surface.

For the most part, I'd intended this disintegrative-capture/atmospheric drag hypothesis as a wild, but possibly workable, idea, that might explain where Hyperion came from, and why it is a rubble pile. The idea that it might have "sopped up" some of Titan's atmosphere was something that I added in at the last minute as I was typing it out. Admittedly, I didn't think that part through very well. The points you've raised here are certainly good ones against it being true.

By the way, I didn't say anything about Titan's atmosphere being a source for Iapetus' dark material. I agree that at first glance there doesn't seem to be any way THAT could be true.