Monday, May 16, 2005

Did Iapetus Consume One of Saturn's Rings?

One of the stories making the rounds lately has been this one on the formation of Iapetus' equatorial ridge. This theory, posited by Paulo C.C. Freire of Arecibo Observatory suggests that the ring formed as the result of Iapetus skimming the outer edge of Saturn's rings earlier in its history. No doubt, Iapetus is one battered and bruised body. It has no less than SEVEN impact basins larger than 400 km!! One, announced at the EGU conference in Vienna, is over 850 km wide and has a 400 km wide basin lying within it!! The shape of Iapetus is lumpy, with bulges 10-20 km above the reference ellipsoid, not to mention the ridge that is 10-20 km in places. The elllisoid itself is unusual. The shape suggest that Iapetus "froze" as a body that rotated with a period of 17 hours, compared to its current 79 day rotation.

Given all these unusual features related to this moon, what does this suggest and is this idea of the ridge being formed by the rings even possible? Somehow, I don't buy this. First, the author makes a big deal about the feature being on the equator. However, a ridge of that size need not FORM along the equator but the pole can shift so that the ridge (a rather large mass) could be on it. Secondly, if Iapetus did make short excursions into the inner Saturnian system, it would have seriously played havok on the inner moons.

Definitely, some thing seriously wrong happened to Iapetus. Maybe a satellite became disrupted and Iapetus bore the brunt of the onslaught of satellite debris?


Anonymous John Rehling said...

The difference between Iapetus and one of its many foils (say, Rhea, which is the same size, but also Dione, Tethys, etc., which are more spherical though smaller) should perhaps be thought of in terms of what happened to the others, and not Iapetus. It is easily overlooked that Iapetus orbits *much* farther from its primary than any other non-tiny satellite in the solar system. Maybe the beating Iapetus took is typical of late-phase accretion but the other satellites differ in still being warm and malleable -- somehow due to their proximity to Saturn. Perhaps the giant planets were hot enough to melt their icy satellites, as far out as Callisto and Hyperion, but not so far out as Iapetus.

The primary clue we get as to the origin of Cassini Regio will come from the directionality of its deposition, which craters near its edge provide. While we have an incomplete record of this so far, the direction of the matter seems, at most edges of CR, to indicate a ballistic deposition originating from somewhere inside CR, either central to it or in the eastern portion. The exception are the large rings to the east of CR, which show concentrations of dark matter in the opposite direction, as though there the dark matter arrived from the east. The overall account this is consistent with would be if a very large, very dark impactor was broken apart during or before impact near the eastern region of CR (as Shoemaker-Levy became many impactors before its finale), forming some rings that showed an east-to-west deposition there, with the main body(-ies) impacting into what is now eastern CR and splashing dark matter outward from there in all directions. A conventionally geological endogenic origin is going to have a tough time explaining the reverse-direction pattern of dark stuff in those rings east of CR, and a purely exogenic origin that had Iapetus sweep up dark material from Phoebe, a ring, etc., will be equally troubled. Any notion as to what darkened 1/3 of Iapetus should account for those rings east of CR in the first draft or get ready for the trash bin.

Incidentally, the CR phenomenon may be something we see to a fainter degree on Dione and Rhea, which also have large darkened areas, but with much lower contrast. CR may be the extreme version of a nonunique phenomenon.

5/17/2005 06:37:00 AM  
Anonymous Phil Stooke said...

Tethys also has a dark area of low contrast.. and in my LPSC abstract in 2002:

I pointed out that craters on its edge show a similar albedo pattern... dark walls facing the dark area. This was mentioned earlier on this blog. See especially Figure 2 in my abstract... the sun is almost overhead around the prominent craters, but we see them clearly because of the albedo distribution. So John is right, CR may only be the most extreme example of this. I never noticed the 'edge effects' on Rhea or Dione, but maybe they are waiting to be discovered. Phil

5/19/2005 08:33:00 AM  
Anonymous Phil Stooke said...

Quick update... I had a quick look, and I would now argue the same phenomenon can be seen on both Dione and Rhea as well. The trick is to look at images with sufficient resolution, which place the edges of the dark area close to the subsolar point as in my Tethys example. I find similar patterns on the east side of the dark area on Dione and the west side on Rhea (just where the better images happened to be). I'm using Voyager images but the effect ought to show in Cassini images as well, if lighting is right


5/19/2005 08:52:00 AM  
Anonymous John Rehling said...

Interesting stuff, Phil. As a strawman explanation, maybe dark impactors that create widespread fields of infall have been a common phenomenon in the saturnian system. The variation in darkness could be due to low intensity at creation time (higher ice ratio in the impactor), greater erasure over time (impact gardening or burial of the dark deposits under ice) or to a distributed nature of impacting material (a dark cloud). The rings east of CR, as I argued earlier, make the cloud explanation hard to apply to Iapetus. The fact that Dione, et al, are more rounded than Iapetus is coherent with the idea that posterior burial could have taken place -- anything that "rounds" a world would certainly bury some of the surface. We don't see much of anything like this among the Galileans (although Io and Europa would be disqualified anyway). The uranian satellites would be another place to look if this phenomenon represents a class of impactor that is/was solar-system-wide.

5/21/2005 10:51:00 PM  
Anonymous Anonymous said...

The dark areas of Tethys, Dione and Rhea are on the trailing side, the opposite of Iapetus. If there is a common cause this would imply that the source was between Iapetus and Rhea, perhaps material from Titan or Hyperion.

5/22/2005 11:09:00 AM  
Anonymous Gsnorgathon said...

Not necessarily apropos, but there's an interesting article on about shifting outer planetary orbits and the late heavy bombardment. Presumably interesting things might have happened to various moons while all this orbital shifting and flinging about of planetesimals was going on.

5/25/2005 12:20:00 PM  

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