The BBC is reporting a 'new' theory about the bulge on Iapetus.
http://news.bbc.co.uk/1/hi/sci/tech/7965332.stm

If this theory is true, wouldn't the bulge be across the entire equator? Not just the darker Cassini Regio?

Yeah, the despinning idea is not that "new;" it's been proposed as a possible mechanism before, but this modeling work is new, it shows what things happen during despinning that could plausibly cause something to happen at Iapetus' equator. It's true that the discontinuity of the ridge is a problem, but the discontinuity is actually a problem for nearly any geophysical explanation for the ridge. The ridge is a very old feature though -- lots of stuff could have happened in 4+ billion years to obliterate part of it. I think it's safe to say that no one who's given any thought to the problem of the ridge is fully satisfied with any of the proposed explanations yet.

--Emily

I agree with Emily. This looks like a classic case of 'predicting' what is already observed. The problem is that if you have enough parameters you can always tweak them to do this but that doesn't mean you're onto the correct explanation.

I confess to a problem here. I liked, and still like, Wing Ip's fossil ring hypothesis. (I think that rings or debris dics are very common phenomena - perhaps almost every world had one at some point - but because they are short-lived we only see a few at our particular moment in time.) There is supposed to be a paper out there that refutes Ip but try as I may I cannot access it on the internet. Can anybody help?

Since the last time we discussed this here we have had interesting discoveries at Rhea.

Here's one for the 'wild speculation' file. I would welcome comments.

I’ve now had a chance to read and digest the transcript of the January 2009 CHARM presentation on Iapetus. One major problem for the models seems to be reconciling the rigidity needed to support the anomalously oblate ellipsoid with the internal flexibility required for Saturn’s tidal influence to de-spin it from a presumed rotation period of 17 hours to its present value of 80 days. Extreme solutions are considered, such as Iapetus once having been much closer to Saturn. The other ‘elephant in the room’ is the still unexplained equatorial ridge (recent papers notwithstanding). I’ve always been a fan of the fossil ring hypothesis for the ridge, and I’ve suggested before that the ring could have originated from a former sub-satellite of Iapetus in a decaying orbit. What occurred to me on reading the transcript is a development of that idea. Maybe if the sub-satellite was in a retrograde orbit it could also have been responsible for de-spinning Iapetus.

Here’s the scenario. Iapetus formed as a double moon. (We don’t see any of those now but they may have been common when the solar system was young. There is still a high population of binaries in the asteroid and Kuiper belts where they are relatively free from disruptive influences.) The companion is in a retrograde orbit which decays quickly. Eventually it disintegrates and its material ends up on Iapetus. This, rather than the tidal influence of Saturn, accomplishes most of the despinning at a very early date, reducing the need for long lived internal flexibility for which there is no other evidence. Saturn just has to fine tune the job of locking the rotation. Possibly some of the debris piling up on a rigid Iapetus contributes directly to its overall oblateness, making the original spin period of 17 hours an underestimate and reducing the amount of despinning required. Finally the last of the debris emplaces the geometrically perfect equatorial ridge.

An interesting article about a new theory which explains the equatorial ridge and the flattening of Iapetus: the compaction of a porous interior under a thick shell of at least 120 km:
http://topex.ucsd.edu/sandwell/publications/130.pdf

This is a very interesting alternate model to the despinning theory and which does not require a very high initial spin rate of the moon.
Marc.