Saturn's bared mini-moons Options

[Guest_BruceMoomaw_*]

Second, it proved (http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2289.pdf ) that all of Saturn's inmost moonlets -- Pan, Atlas, Prometheus and Pandora -- had very low densities of around 0.4 g/cc, proving that they must be loose rubble piles of ground-up ice. And, moreover, the inner three had maximum diameters almost exactly equal to the maximum size to which an accumulating snowball moonlet could grow at that distance from Saturn before it completely filled up its own Hill sphere in that orbit, after which no more material could stick gravitationally to its outermost surface. (Pandora is a bit smaller than its Hill sphere, indicating that at that distance it ran out of loose material to accumulate before it could grow to its own maximum possible size. Logical conclusion: they are indeed such accumulating snowballs, with temporary rather than permanent lives. Some calculations had already indicated that Prometheus and Pandora, within at most a few hunded thousand years, would collide with each other. Atlas presumably congealed out of the original material at the outer edge of Saturn's ring, grew to its maximum possible size, and since then has been tidally shepherding the next-inmost ring material inwards while slowly spiralling outwards itself -- but at some point before it gets too far away, it will be hit by a meteoroid and turn into a new thin ringlet outside the current edge of the A Ring. (Janus and Epimetheus, by contrast, have somewhat higher densities of about 0.65 g/cc, suggesting that they may be more permanent rubble piles accreted out of bigger chunks of icy debris -- the remains of a single original moon that was shattered long ago, with its two surviving remnants forever trying unsuccessfully to rejoin into a single moon again.)

I need to clarify a point here. An accreting and growing moonlet does NOT come closer and closer to completely filling its Hill sphere just by growing in SIZE -- if its density stayed the same as it grew, its Hill sphere would continue to grow at the same rate and thus remain larger than the moonlet itself. What makes an object come closer and closer to filling up its total Hill sphere -- so that it finally reaches a point at which its surface gravity is no longer strong enough to overcome the tidal forces of Saturn and pull more small bits of material down to cling to its surface -- is a decrease in its average DENSITY.

So, what has actually happened is that the innermost moonlets of Saturn (its "ring moonlets", including Prometheus and Pandora) started out as one large solid chunk of water ice, which then attracted a thicker and thicker overlying loose layer of small ice fragments. But this larger and larger proportion of LOOSE ice fragments meant that the total density of the growing moonlet also dropped steadily from the initial density of its big central fragment of solid water ice, until the moonlet's average density finally dropped to the level that it completely filled up the volume of its Hill sphere and could no longer make more bits of material cling to its outer surface. (More precisely, the sizes and densities of Pan, Atlas and Prometheus show that this is what happened to them -- they've have completely filled up their hill spheres. Pandora, being a bit farther from Saturn, apparently ran out of available small debris to accumulate before it could finish filling up its own Hill sphere.)

[scalbers]

Looking at this older thread, I'm wondering how we would summarize the size distribution of moonlets in Saturn's rings? This would help in visualizations of ring fly throughs. Roughly speaking it seems that Daphnis and Pan are the only moonlets larger than 1km (in the main part of the rings). Yet the number of moonlets 100m in size could be on the order of a million. Has anyone been thinking of refining this size distribution? Here is an older reference on this: http://arxiv.org/pdf/0710.4547v2.pdf

This rendering looks pretty realistic in fact (possibly from Bjorn?): https://www.youtube.com/watch?v=KPwNrop6KN8