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Posted by: nprev Jul 7 2007, 02:38 AM

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!

Posted by: dvandorn Jul 8 2007, 04:37 PM

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

Posted by: Rob Pinnegar Jul 12 2007, 05:26 AM

I've wondered on occasion how much of the mass of Saturn's ring system is "original", and how much comes from small comets that have hit the rings and been incorporated into them.

Anything sizeable would of course just plow straight through the rings, but a "Tunguska-sized" object passing through the B Ring could conceivably get slowed down enough to get captured -- after which it would, of course, pass through the rings again at the end of its first orbit around Saturn, as would any ring material scattered by the impact. It wouldn't take long for the rings to act like a gigantic broom and sweep up all the evidence of the collision.

Considering the enormous cross-sectional size of the rings, impacts like this must happen at least once a year to once a decade. You'd think Cassini would pick up them up from time to time. If it can spot those tiny emissions coming from Enceladus, it ought to be able to pick up the out-of-the-ring-plane snowstorm caused by a 10- or 20-metre impactor whacking into the main rings.

Posted by: nprev Jul 12 2007, 06:04 AM

Hmm. Interesting thought, Ron. Of course, one thing we really don't know very well is the modern frequency of meteoroid impacts in the outer system, though I suspect that the overall probability density is much lower than that of the inner system. A disproportionate amount of our impacts come from Asteroid Belt residual debris, and probably not nearly as much of that makes it beyond the orbit of Jupiter aside from the occasional skimming & ejection by Big J.

Dvandorn, your thoughts strike a chord. What's weird is how each of the other three gas giant systems seem to exhibit different patterns of disruption within the framework of your hypothesis. Most of Saturn's moons seem to have been whacked by one or more proportionally substantial objects just below the threshold needed to blow them into smithereens; Uranus is not only tipped, but also has rings in its bizarre equatorial plane along with some diminutive moons that have only a few smaller hits (notwithstanding whatever the hell happened to Miranda, of course). Neptune shows clear evidence of a whole lot of housecleaning in its immediate vicinity (thin, patchy rings, ragged, small inner moons), all of which may or may not have been primarily due to the presumed capture of Triton.

What I'm getting at here is whether a single smoking gun--the inward migration of Jupiter--is in fact a likely cause for all these effects. It may well be, but not sure if we understand all we've seen so far well enough to make that call. The really odd things about Saturn like Hyperion, Iapetus' orbit, the lack of Sun-Saturn Trojans, and the 'big hits' on the icy moons I originally mentioned...those might be all due to a single event, but I'm skeptical about any relationship between that hypothetical catastrophe and the histories of Uranus & Neptune.

Posted by: tty Jul 12 2007, 06:42 AM

Any migration of a large body (read Jupiter, Saturn, Uranus or Neptune) in the solar system would cause a storm of impacts through "resonance-sweeping".

A small body with an orbit in resonance with a large planet is quickly ejected from it by repeated perturbations, and as a large planet migrates the resonance zones migrate too causing mass ejection of KBO's and/or asteroids.

It has been hypothesized that the Late Heavy Bombardment c. 3.8-3.9 10^9 years ago was caused by Neptune migrating inwards and "sweeping" the Kuiper belt. The signs of very violent things happening out around Saturn does suggest that the Kuiper belt may have been more involved than the asteroids.