Saturn's A-ring Soaks Up Debris Ejected from Nearby Moon

ScienceDaily is running a story about the recently discovered interaction between Saturn's A-ring and one of Saturn's small moons, Enceladus. Thanks to data from Cassini, scientists have discovered that ejected matter from Enceladus' ice geysers is absorbed into the A-ring, where it is then trapped. We discussed the geysers themselves a few years ago, and researchers have been working since then to determine where the material was going. Quoting: "This is the latest surprising phenomenon associated with the ice geysers of Enceladus to be discovered or confirmed by Cassini scientists. Earlier, the geysers were found to be responsible for the content of the E-ring. Next, the whole magnetic environment of Saturn was found to be weighed down by the material spewing from Enceladus, which becomes plasma -- a gas of electrically charged particles. Now, Cassini scientists confirm that the plasma, which creates a donut-shaped cloud around Saturn, is being snatched by Saturn's A-ring, which acts like a giant sponge where the plasma is absorbed."

Re:Moons creating moons?

[CheshireCatCO]

To put it more exactly, tidal forces will tear apart anything within, roughly, the A ring. (Cavaets: this applies to bodies with no internal cohesion and cases where there's a large size differential move the limit inward, a la Porco et al. 2006.)

Re:Moons creating moons?

[CheshireCatCO]

Tides are a big problem for forming a moon within, roughly, the A ring. A bigger problem is that Enceladus produces "dust" (very small particles, the size of particles in cigarette smoke, approximately), albeit made mostly of ice. It takes a long time to form this stuff into macroscopic bodies when the conditions are friendly (which they aren't). Worse still, Enceladus isn't really putting out that much mass per time. It'd take a very long time to put out enough to make another decent-sized moon, even at 100% efficiency.

Re:Moons creating moons?

[TripMaster Monkey]

The debris being generated cannot form into a new moon, as the A-ring is within the Roche radius [wikipedia.org] of Saturn.

Is This New?

[CheshireCatCO]

I'm not sure that this is really news. I can vaguely recall people talking about exactly this sort of thing happening in papers from years ago. I'm not 100% certain that the topic was E ring particles, but I rather thing that it is. Sure, before we found the plume on Enceladus, that moon's connection wasn't apparent, but the issue of contamination of the A ring has come up before. I even remember discussion as to how far into the A ring you'd have to go before the contamination stopped. (Which probably played back into older photometric and spectroscopic measurements of the outer A ring, which has a a rather distinct character.)

It's a bit difficult to tell from the article what the point of the new research is, but I will say that even confirming this with new, perhaps more telling, measurements is still useful result, even if I'm remembering correctly and this isn't a new idea.

Re:Moons creating moons?

[techno-vampire]

No. With the exception of the E ring, all of Saturn's rings are within its Roche Limit. [wikipedia.org] Tidal forces would prevent a new moon from forming that close just as they'd break up an existing one.

ice geysers?

[NotZed]

Do they really look like geysers to anybody? Wouldn't they be more columnar, or conical?

If anything it looks like the solar corona, or a comet perhaps?

ahh, here we go ... 2 years ago, same story - with a cometary conclusion:

http://www.astrobio.net/news/modules.php?file=article&name=News&op=modload&sid=1797 [astrobio.net]

Re:Moons creating moons?

[CheshireCatCO]

You don't need collisions for such viscous spreading (and most stuff wants to move inward anyway). Also, the A ring is somewhat held in check by the larger moons, so spreading is very slow.

Re:Moons creating moons?

[CheshireCatCO]

The Roche limit, as usually defined, is for a body with *no* internal strength. This is a very reasonable limit to use as anything trying to accrete from small particles would not have internal strength (between these particles) immediately. It's difficult to invoke any kind of strong enough force to overcome the tidal stretching: electrostatic charges, for example, are too small to work on macroscopic bodies --- large charges don't persist for long as they attract opposite charges quickly and cancel out. (Besides, as soon as two opposite-charged ring particles meet, I suspect that they'd exchange charges faster than you can sinter the particles together. Once the charges are equalized, there's no more force to hold them together.) The only way of triggering growth in the main rings that I know of is to seed the growth with larger cores which are dense enough to attract the smaller ring particles and hold on to them for long enough for the ices to bond together. Of course, this process is self-limiting. (http://science.slashdot.org/article.pl?sid=07/12/07/1326240)

On the other hand, bodies *with* internal strength have a very different tidal limit. If the strength is similar to what we are used to on Earth, then there's no real reason to worry about Saturn right down to the planet's "surface". (Consider Earth: we're well inside the Roche limit, but satellites (and astronauts!) don't fall apart.)