Saturn's bared mini-moons Options

[Guest_AlexBlackwell_*]

There is a new Cassini-related paper by Tiscareno et al. (and accompanying News and Views piece by Spahn and Schmidt) in the March 30, 2006, issue of Nature. See also the Editor's Summary: Propelled into the limelight.

[Guest_BruceMoomaw_*]

What we are seeing here is simply the latest in an impressive line of evidence from Cassini virtually proving Esposito, Colwell and Canup's "recycling" theory of the nature of Saturn's rings.

The lifetime of Saturn's rings has long been a puzzle. Quoting Jeffrey Cuzzi in 2002: "On geological timescales, it has long been believed that dynamical evolution of the rings under moonlet torques removes both the rings and the inner ringmoons from their current locations in times much shorter than the age of the solar system (Goldreich and Tremaine 1982, Lissauer et al. 1985, Esposito 1986). As also discussed above, it has more recently been realized that meteoroid bombardment will alter the ring structure and composition on comparably short timescales. Ip (1984), Lissauer (1984) and Cuzzi and Durisen (1990) show that angular momentum dilution by infalling material can be important in causing orbits of ring material to decay. Northrop and Connerney (1987) suggest that the entire C ring might erode into the planet via small, charged grains ejected by impacts. While both the dynamical and the meteoroid effects have their uncertainties, they independently point in the same direction and to comparable ring ages of ∼ 1/10 the age of the solar system." And, quoting Esposito: "The processes of collisions, diffusion and transport should have homogenized the rings over the age of the solar system. Instead, these differences persist. The mass density in the Cassini division inferred from density waves is so low, that the material there would be ground to dust in 30,000 years. The observed moons that cause such interesting structure in the rings have short lifetimes against disruption by cometary bombardment and against the angular momentum transfers that push them away from the rings."

Esposito et al propose a wonderful why-the-hell-didn't-I-think-of-that solution to all this that doesn't require the relatively unlikely coincidence of Saturn's rings being created only a short time ago. As they pointed out in "Icarus" in 1995, the Roche limit applies only to two objects of relatively equal size: when they bump into each other, a majority of their volumes -- and thus of their masses -- is outside their "Hill spheres" (the zones within which their weak gravitational attraction toward each other overcomes Saturn's tidal forces pulling them apart), so that they don't stick together gravitationally. But when one ring particle bumps into another much larger than it is, most or all of the smaller particle is within the Hill sphere of the larger one -- so the smaller particle DOES stick to the bigger one gravitationally, even within the Roche limit. (The range of relative sizes for which this operates depends, of course, on just how close the two particles are to Saturn, and thus how powerful the tidal forces pulling them apart are.) But what this can lead to is a situation in which Saturn's rings consist of "a bimodal population, with one component consisting of small, meter-sized objects and the other of a few tens of bodies of hundreds of meters to kilometers in size. These large bodies continue to sweep up small bodies and dust, but are precluded from accreting with each other by the tidal potential." Thus the larger "ring moonlets" keep sweeping up finely pulverized ring material before more than a small amount of it has a chance to be ground up into dust so fine that sunlight pressure or Saturn's own charged-particle radiation can vaporize it, sweep it completely away from the planet, or sweep it INTO the planet. Then, occasionally, these growing moonlets are hit by each other or by an incoming meteoroid, and the material they contain is re-splattered through the rings. "This recycling could extend the ring lifetime almost indefinitely." That is, Saturn's rings are the scene of an eternal natural snowball fight.

Almost as soon as it arrived, Cassini started finding solid evidence on several different fronts that this theory is correct. First, its UVIS saw compositional "variations over scales of 1000 - 3000 km", which are hard to ecxplain in any way but by events "in which a small moon residing within the rings is shattered by an external impactor. The interior of such a body has been shielded from external meteoritic bombardment, and thus contains purer ice. Since the amount of meteoroid pollution provides a rough clock to estimate the age of the rings, these random events reset that clock locally, making the material at that radial location younger and purer. As these purer ring particles collide with others, they exchange regolith, and the range of purer water ice spectrum spreads radially. The radial variation we interpret as due to differential pollution in our data set is consistent with the disruption of several small bodies in the A ring in the last 10 million to 100 million years. When the small moon Pan (R ~ 10 km, now residing in the nearby Encke Gap) is eventually shattered by an external impact, the gap will close up, and for some 10 to 100 million years thereafter a brighter radial swath of purer water ice at its former location will gradually spread and darken." ( http://www.agu.org/cgi-bin/SFgate/SFgate?&...t;P51C-02" )

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.)

Third, Cassini has actually photographed Prometheus in the process of gravitationally attracting a streamer of the fine debris from the F Ring back to itself, thus slowly growing in size -- and showing that it hasn't QUITE hit its maximum possible size yet.

Now we have the fourth piece of evidence -- confirmation of the existence of a large number of smaller moonlets, just a few hundred meters across and thus too small to be seen directly by Cassini, within the rings along with the larger Pan and Daphnis. The one major respect in which Esposito's theory seems wrong is that it appears that there may be not "tens" of such moonlets embedded in the rings, but millions of them. The rings may still have been formed originally by a collision that shattered one of Saturn's early moons - but that collision could have occurred in the Solar System's earliest days when huge amounts of material were still zooming around throughout the Solar Sysyem and such a collision was thus tremendously more likely. (Preliminary indications are that the "natural lifetimes of the rings of Uranus and Neptune, and the densities of their own innermost moons, are also so low that they too must be the product of an eternal recycling of material between little particles and large rubble-pile moonlets ever since their initial creation.) At any rate, this -- like Iapetus' dark/light dichotomy -- seems to be one very long-lived mystery about Saturn that Cassini has already conclusively cracked.

[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.)