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_AlexBlackwell_*]

Saturn’s Rings Created by Collision
By Ker Than
Staff Writer, Space.com
posted: 29 March 2006
12:37 pm ET

[volcanopele]

Here's the link to the image releases for this news item:

http://ciclops.org/view_event.php?id=50

[Guest_Richard Trigaux_*]

Before jumping to conclusions, I would like to see a statistics of the ring particule sizes.

Since there was working models found to expain the Moon by a collision, everybody see collisions everywhere and present collisions as the standard and only theory. There are still several other theories to explain Saturn rings, including a formation in the same time as Saturn, or a larger satellite comming too close to Saturn and breaking appart with tidal forces.

We should note too that the moonlets travelling with the F ring show many craters, evidence that they are very old. But they also show a kind of snow softening the crater shapes: very probably matter gathered from the F ring. But this snow can be only a tiny fraction of the moonlets, showing that they were not formed in the same time than this F ring, or that they interacted only little with it. This frankly rules out the theory as what the moonlets would be in equilibrium with the F ring, gathering matter from it, and then breaking with tidal forces and reforming the ring and again.

At last, the killing argument: it seems that the craters in the moonlets are not all softened in the same way. If this is true, that means that the moonlets are gathering snow from the F ring since billions years, and thus that the rings are very old too, formed together with Saturn or from an early satellite breaking.

[ljk4-1]

Cassini spacecraft finds evidence of football-field sized moonlets in Saturn's A ring

http://www.news.cornell.edu/stories/March0....Saturn.lg.html

March 29, 2006

By Lauren Gold
lg34@cornell.edu


New observations of propeller-shaped disturbances in Saturn's A ring indicate the presence of four small, embedded moons -- and most likely millions more, Cornell University astronomers report.

This is the first evidence of the existence of moonlets bridging the gap in size between the larger ring moons Pan and Daphnis (several miles each in diameter) and the much smaller ice particles that comprise the bulk of the rings. The discovery could lead to a better understanding of the origin and formation of Saturn's rings and the solar system as a whole.

Matthew Tiscareno, a Cornell research associate, is lead author of a paper describing the discovery in the March 30 issue of the journal Nature.

The four disturbances, which appear as pairs of slightly offset bright horizontal streaks in an otherwise bland region of the ring, were captured in two images taken in 2004 by NASA's Cassini spacecraft. Astronomers say the streaks are indicators of orbiting moons about 100 meters (328 feet) in diameter: about the length of a football field, but still too small for even Cassini's highly sensitive Imaging Science Subsystem (ISS) to see directly, but large enough to exert an observable gravitational pull on the particles around them.

"The discovery of these intermediate-sized particles tells us that Pan and Daphnis are probably just the largest members of the ring population, rather than interlopers from somewhere else," said Tiscareno.

A continuum of particle sizes lends strong support to the theory that Saturn's rings were formed when another object fragmented close to the planet, breaking into pieces which were then captured by Saturn's gravitational pull.

"There has always been the question about whether the rings were primordial material that was unable to grow into a moon or debris left over from a breakup event," said Joseph Burns, Cornell professor of astronomy and of theoretical and applied mechanics and paper co-author, along with Cornell research associate Matthew Hedman and researchers at other institutions. The discovery doesn't rule out the accretion model, but "it's a step in that direction," said Tiscareno. "It's hard for direct accretion to produce particles this large. It's much easier if you start with a solid icy core, like a shard from a breakup."

The discovery also helps explain fully cleared openings such as the Encke and Keeler gaps within the rings. The gravitational influence of a larger moon like Pan or Daphnis wraps around the circumference of the rings, creating a gap. The smaller moonlets begin to create this effect, the researchers say, but their influence is not strong enough to prevent particles from falling into the rings ahead of and behind them.

Like a motorboat's wake on a smooth lake, the four observed disturbances are particularly visible since the area they inhabit is otherwise smooth. But the fact that four were found in just two images covering only a tiny fraction of the ring makes it likely that millions more exist. By studying them further, researchers hope to gain a better understanding of how Saturn's rings formed -- and even about how solar systems form around stars.

"The structures we observe with Cassini are strikingly similar to those seen in many numerical models of the early stages of planetary formation, even though the scales are vastly different," said Carl Murray, a co-author and astronomer at Queen Mary College, University of London. "In this way, Cassini is giving us unique insight into the origin of planets."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology, manages the mission for NASA.

During its Saturn tour, as currently planned, Cassini will complete 74 orbits of the ringed planet, 44 close flybys of the moon Titan and numerous flybys of Saturn's other icy moons.

##

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[Guest_AlexBlackwell_*]

Cassini Finds 'Missing Link' Moonlet Evidence in Saturn's Rings
NASA/JPL/Space Science Institute
March 29, 2006

[ElkGroveDan]

This kind of discovery is one of the reasons why I'm in favor of closer and closer ring dives as Cassini nears the end of it's mission, sort of the way they got bolder and bolder with Galileo near Io as its days were numbered.

(...assuming of course the orbital mechanics would allow it. I put away my slide rule long ago, so I'll leave it up to you guys who crunch numbers for a living.)

[Guest_Richard Trigaux_*]

This kind of discovery is one of the reasons why I'm in favor of closer and closer ring dives as Cassini nears the end of it's mission, sort of the way they got bolder and bolder with Galileo near Io as its days were numbered.

(...assuming of course the orbital mechanics would allow it. I put away my slide rule long ago, so I'll leave it up to you guys who crunch numbers for a living.)

With my opinion, the best "last" orbit for Cassini would be
-periastron just grazing Saturn atmosphere
-apoastron lower than Enceladus (to avoid biological contamination)
-crossing the ring plane at periastron, just above saturn atmosphere, the place where there are the less particules (not stable in this place).
-with a very lonw inclination, so that it will graze the rings throughout their width.

Pros:
-lower radioactivity
-close view to the rings, perhaps possibility to separate larger particules
-possibility to radar probe the rings and even Saturn itself
-safe end of Cassini when out of fuel (such an orbit will soon lead to a crash on Saturn or into the rings, but avoids Enceladus).

Cons:
-Would need lot of fuel or moon assistance. Don't know if it is possible
-if it is possible, the fuel consumed would be at the cost of an extended mission.

[Holder of the Tw...]

With my opinion, the best "last" orbit for Cassini would be
-periastron just grazing Saturn atmosphere
-apoastron lower than Enceladus (to avoid biological contamination)
........
Cons:
-Would need lot of fuel or moon assistance. Don't know if it is possible
-if it is possible, the fuel consumed would be at the cost of an extended mission.

Once you get your apoastron down below Titan's orbit, it will be very, very difficult to get it lower. You can no longer use Titan for gravity assist. To get it down to below Enceladus, as suggested here, means using, at best, Mimas for orbit reduction. In a word: impossible. More practical but still out of reach would be to use Saturn aerobraking, but fuel and time become factors here, along with the fact that the high gain antenna would have to be put to a job it was not designed for. There is not near enough fuel on board to accomplish anything close to achieving a tight orbit directly.

Some day there will likely be a spacecraft in the orbit you describe. But unfortunately it won't be Cassini.

In the meantime Cassini can do what Cassini does best, investigate Titan.

[ljk4-1]

I am surprised that no one has addressed the real issues here:

Are these mini-moons actual moons?

How small or large does an object have to be to differentiate
between being a ring particle and a moon?

What are we going to name them all?

Will any of us still be alive by the time the IAU makes a decision
on these matters?

[JRehling]

With my opinion, the best "last" orbit for Cassini would be
-periastron just grazing Saturn atmosphere
-apoastron lower than Enceladus (to avoid biological contamination)
[...]
Cons:
-Would need lot of fuel or moon assistance. Don't know if it is possible
-if it is possible, the fuel consumed would be at the cost of an extended mission.

It's totally impossible. Once you get apoapsis inside Titan's orbit, there are no more gravity assists to be had. And circularizing the orbit that deep in the gravity well would require a LOT of delta-v. Unless aerobraking at Saturn were possible, this cannot be done, and aerobraking at Saturn is a much more difficult than at Mars.

[Guest_BruceMoomaw_*]

Yes -- and don't forget that Alex has already pointed out a reason why aerobraking Cassini is impossible in any case: its magnetometer boom (and plasma-wave antennas) would throw it out of alignment.

[Guest_Richard Trigaux_*]

It's totally impossible. Once you get apoapsis inside Titan's orbit, there are no more gravity assists to be had. And circularizing the orbit that deep in the gravity well would require a LOT of delta-v. Unless aerobraking at Saturn were possible, this cannot be done, and aerobraking at Saturn is a much more difficult than at Mars.

The matter is not to circularise the orbit: it is impossible because the orbit would have to cross the rings. I rather speak of a very elliptic ellipse, which periastron is grazing Saturn atmosphere, or at least under the ring (the rings have no defined lower limit, but seeminly the particule density decreases with altitude). After, apoastron is where we can set in, probably near Titan, as it is, as you note, the only strong help for gravity assist. Of course it would be fine to set the apoastron below Enceladus.

Such orbit is, for instance, grazing the ring on the north side on the descending branch; then it crosses the ring plane very close to Saturn (where it can do a radar probe of the atmosphere); after it climbs back to the apoastron, on the southern side of the rings. At both times it may have a much closer view of them than anything previously done.

The only question is if Titan can deflect Cassini trajectory at nearby 90° to bring it on a trajectory toward Saturn. I noted that, on the programs, the planned orbits of Cassini have a lower and lower apoastron, so that its energy is decreasing. Maybe what is not possible today will be possible in two or three years. If the fuel is not already spent, of course.

[Holder of the Tw...]

I am surprised that no one has addressed the real issues here:

Are these mini-moons actual moons?

How small or large does an object have to be to differentiate
between being a ring particle and a moon?

What are we going to name them all?

Will any of us still be alive by the time the IAU makes a decision
on these matters?

How about this idea? You have to be able to carve out an entire ring gap before you can be considered a moon. If you're creating "propellers", then you're a moonlet.

Edit- the orbit I described earlier in this posting is impossible, and I've taken it out. It's been a long week.

[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.