[X] My Assistant

[nprev]

Well, this is probably not a unique observation, but has anyone else been struck (ta da, da!) by the fact that most of the larger icy moons have one or more very large impact craters in relation to their diameters? Makes me think that the rings are ancient, and formerly much more extensive and dense...either that, or there was a much more intense LHB in Saturn's vicinity, which seems much less probable.

Just throwin' this out here for thought...fire away!

[dvandorn]

I've been giving this matter quite a bit of thought, actually. I don't necessarily see the apparent very-high paleoimpact rate at Saturn as being indicative of a heavy bombardment that was greater than what was seen in the inner system -- I see it as a set of markers for an intense gravitational disruption that occurred in the outer system early in its history.

If you look at the moons of all of the three outer giant planets, you see objects that have literally been ripped apart and formed into rings, objects that came close to being ripped apart and still show the aftermath of being tremendously stressed, and objects that were ripped apart and then re-formed into intact (if savaged) moons.

Yes, I think it's possible to postulate that all of these effects were caused simply by impacts, and if your model doesn't include the possibility of intense gravitational gradients, impacts fit Occam's Razor quite well. But recall that, according to "best" theories right now, Jupiter formed farther out than Saturn and migrated inwards to its current location. Jupiter is one huge mother, with the second-largest gravitational field in the system. I think, if the theory of its migration is sound, that we're seeing effects from gravitational shear that occurred as Jupiter migrated.

One of the things that tends to support this perspective, for me, is that Jupiter's own moons show the *least* of these observed effects. Since they were very close to Jupiter and their orbits were so dominated by the immense Jovian gravitational field, you would expect them to suffer the fewest effects. Et voila, that's what we observe.

You can also postulate that Jupiter's migration wasn't entirely responsible for all of the effects we see -- our system formed within the debris cloud of a supernova explosion, after all, it's unreasnable to assume that there were no other large masses (from stars-in-formation to objects the size of several Jupiters) being accreted nearby out of the same cloud. Any number of close encounters with such masses could also account for the disruptions we see.

Truly, I think we're looking not at an impact cataclysm, but at a gravitational one.

-the other Doug