The Return of Saturn's Spokes

CheshireCatCO writes "After more than a year in orbit around Saturn, the Cassini spacecraft has finally spotted 'spokes' in the rings. Spokes, large radial structures in the rings, where seen by the Voyager spacecraft and have remained difficult to fully explain. The reappearce of the spokes comes about two years earlier than many models predicted."

Comments

See Cassini's path

[latent_biologist]

"Unlike Voyager or Hubble, Cassini is in a unique position to study ring spoke phenomena at Saturn."

With Celestia, you can actually follow Cassini's path as it follows Saturn. It really puts the above statement in perspective (plus it makes u feel like an astronaut).

Ring Object Size

[williwilli]

While I understand your concept, another poster suggested the rings are made of smoke sized particles. Actually, the particles are sometimes quite large according to this page:

More recently, astronomers bouncing radar off the rings and analyzing the reflected signal found that ring particles must be from a few centimeters to a few meters across. When the Voyager spacecraft went behind the rings with respect to the Earth, astronomers could measure the particles sizes from how Voyager's radio signal scattered off the particles and from how sunlight scattered through the rings. The ring particles range in size from the size of a small grain of sand to the size of a large house, but on average, they are about the size of your clenched fist. Spectroscopy of the rings shows that the particles are made of frozen water. Collisions between the ring particles keeps the ring system very flat and all of the particle orbits circular.

from http://www.astronomynotes.com/solarsys/s16.htm

though there may be other information available which I am unaware of, of course. But if this particle size stands, it seems a fairly simple explaination that a meteroid could hit a large ring object and cascade debris and impact effects throughout the rings themselves...

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Re:Impact debris?

[CheshireCatCO]

No.
Some of the rings are diffuse and dusty.
The A and B rings of Saturn, however, are thick and filled with larger (figure meter-sized) particles. A typical photon doesn't make it through these rings without meeting a particle somewhere, so a meteroid would more likely than not hit something solid.

Re:Cause of the spokes?

[CheshireCatCO]

In a word, no.
The spokes must be tied to the magnetic field of Saturn. This explains why the stay at the same magnetic longitude of the planet and why they don't wind up as the inner parts of the rings orbit the planet faster. So they're not disturbances in the ring particles themselves, since the ring particles are too massive to really feel the magnetic field.

However, if you levitated dust over the rings, that would do the trick. The problem is how to levitate the dust. It turns out that you can develop a negative potential on the rings (I think it's negative... I forget, to be honest) which can repel the dust and cause it to hover. So under the right conditions, if dust were kicked up by a small collision in ther rings (say a meteroid collision), it can float over the rings and spread radially, making a spoke.

Or so we think...

Spoke mechanism

[Anonymous Coward]

The theory I heard is that the ring particles pick up a charge from being bombarded by particles trapped in the radiation belts. When a meteoroid plows through them, it breaks small, dust-like particles off. Being light and charged, these particles are picked up by the rotating magnetic field and circle the planet as a radial spoke. As the charge leaks off of the dust, the dust particles go into normal orbits with the ones farther in orbiting faster, so the spoke spreads out into a wedge and then dissipates.

Re:Spoke mechanism

[CheshireCatCO]

That doesn't really cover the seasonal nature of the spokes. (Or should I say "the apparent seasonal nature"?) It's likely that sunlight is involved in that case. It probably comes in through photo-ionization, which generates the same sort of plasma.

Re:Artifacts?

[CheshireCatCO]

There's nothing that we do that would lead to radial features. Especially across multiple images at consistent magnetic longitudes. Also, these have been seen by at least three or four cameras, now. So it's hard to imagine these being artifacts.

(In fact, the images on the space.com site might even be the raw images.)

Re:Duh!Newtons First Law

[ekimminau]

Inertia is a property of matter where objects in motion remain in motion, and objects at rest remain at rest unless acted on by some external force (Newton's First Law). The more mass an object has, the more inertia, and thus, the harder it is to change the speed and direction of it in motion. In the case of rings, we are concerned with how easy it is to make the ring spin faster (or slower) - this is angular momentum. We want to minimize angular momentum so that the rings' responsiveness will be maximized. The thing about angular momentum is that it's not just how much the ring weighs, but the distribution of weight (mass, really) that really matters. Suppose you had a ring that weighed 100 million pounds. If the rim weighed 99.99 million pounds, that'd be one tough wheel to turn. But, if instead you had that 99 million pounds in the planetary core, it wouldn't be such a chore to spin the ring. The spokes are there to just transfer the enegy from the mass at the core to the rim of the ring. Centripetal force is the force that compels a body to move in a circular path. According to the law of inertia, in the absence of forces, an object moves in a straight line at a constant speed. An outside force must act on an object to make it move in a curved path. When you whirl a stone around on a string, you must pull on the string to keep the stone from flying off in a straight line. The force the string applies to the object is the centripetal force. The word centripetal is from two Latin words meaning to seek the center. Centripetal force acts in other ways as well. For example, a speeding automobile tends to move in a straight line. Centripetal force must act on the car to make it travel around a curve. This force comes from the friction between the tires and the pavement. If the pavement is wet or icy, this frictional force is reduced. The car may then skid off the road because there is not enough centripetal force to keep it moving in a curved path. Gyroscope inertia is the tendency of a spinning body to resist change in the direction of its axis. Inertia keeps the axis of a spinning top straight up until the top slows. Use the Right Hand Rule. Form your RIGHT hand in the "hitch hiker" mode. The fingers represent the spin direction of the wheel, and the thumb represents the directional force of the gyroscope axis. Angular Momentum (A) is the force of a rotating object. A=mvr (m=mass, v=velocity, r=radius). Centrifugal force is the force arising in a rotating reference system. It points away from the center, in the direction opposite to the centripetal force. Centripetal force is the force that causes an object to change direction. For uniform circular motion, the force is directed toward the center of the circle and has a magnitude given by mv2/r.