Spokes on Saturns Rings (2009 June 2)

[Mentad]

Why not a purely optical phenomenon? Slight variations in the flatness of the rings or certain alaignements in ice chunks/crystals ought to be able to produce effects similar to this one when the rings are almost on edge in reference to the sun.

[neufer]

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http://apod.nasa.gov/apod/ap090602.html
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<<The B Ring is the largest, brightest, and most massive of the rings. Its thickness is estimated as 5 to 15 metres, its mass at 28 × 10^18 kg (almost as much as Mimas ~ 37.5 × 10^18 kg), and its optical depth varies from 0.4 to 2.5, meaning that well over 99% of the light passing through some parts of the B Ring is blocked. The B Ring contains a great deal of variation in its density and brightness, nearly all of it unexplained. These are concentric, appearing in the form of narrow ringlets.

Up until 1980, the structure of the rings of Saturn was explained as being caused exclusively by the action of gravitational forces. Then images from the Voyager spacecraft showed radial features in the B ring, known as spokes, which could not be explained in this manner, as their persistence and rotation around the rings was not consistent with orbital mechanics. The spokes appear dark in backscattered light, and bright in forward-scattered light. The leading theory regarding the spokes' composition is that they consist of microscopic dust particles suspended away from the main ring by electrostatic repulsion, as they rotate almost synchronously with the magnetosphere of Saturn. The precise mechanism generating the spokes is still unknown, although it has been suggested that the electrical disturbances might be caused by either lightning bolts in Saturn's atmosphere or micrometeoroid impacts on the rings.

The spokes were not observed again until some twenty-five years later, this time by the Cassini space probe. The spokes were not visible when Cassini arrived at Saturn in early 2004. Some scientists speculated that the spokes would not be visible again until 2007, based on models attempting to describe their formation. Nevertheless, the Cassini imaging team kept looking for spokes in images of the rings, and they were next seen in images taken on September 5, 2005. The spokes appear to be a seasonal phenomenon, disappearing in the Saturnian midwinter/midsummer and reappearing as Saturn comes closer to equinox.>>

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• `the best thing to get us dry would be a Caucus-race.'
  
  `What is a Caucus-race?' said Alice; not that she wanted much to know, but the Dodo had paused as if it thought that somebody ought to speak, and no one else seemed inclined to say anything.
  
  `Why,' said the Dodo, `the best way to explain it is to do it.' (And, as you might like to try the thing yourself, some winter day, I will tell you how the Dodo managed it.)
  
  First it marked out a race-course, in a sort of circle, (`the exact shape doesn't matter,' it said,) and then all the party were placed along the course, here and there. There was no `One, two, three, and away,' but they began running when they liked, and left off when they liked, so that it was not easy to know when the race was over. However, when they had been running half an hour or so, and were quite dry again, the Dodo suddenly called out `The race is over!' and they all crowded round it, panting, and asking, `But who has won?'
  
  This question the Dodo could not answer without a great deal of thought, and it sat for a long time with one finger pressed upon its forehead (the position in which you usually see Shakespeare, in the pictures of him), while the rest waited in silence. At last the Dodo said, `everybody has won, and all must have prizes.'

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<<Dark spokes dance around Saturn's B ring in this series of movies comprised of images taken with Cassini's wide-angle camera.



This animation is a concatenation of spoke movies acquired by NASA's Cassini spacecraft in the second half of 2008 on Aug. 21, Sept. 19, Sept. 26, Oct. 11, and Nov. 25.

As Saturn nears equinox in August 2009 and the sun angle on the ring plane decreases, spokes become common sights in Cassini images, just as they were in Voyager images. The planet's orbital period is 29.5 years, so Saturn has nearly made one complete trip around the sun since the flybys of the two Voyager spacecraft in 1980 and 1981, allowing Cassini to closely match Voyager's viewing geometry.

Each of these five movies shows the sunlit side of the rings at low solar phase, or spacecraft-rings-sun, angles. The spokes appear dark against Saturn's B ring at low phase angles because the particles within them scatter light more efficiently in the forward direction (away from Cassini) than the surrounding larger ring particles. In the opposite viewing geometry, at high phase angles, spokes appear bright relative to surrounding ring particles.>>
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[neufer]

Why not a purely optical phenomenon? Slight variations in the flatness of the rings or certain alignments in ice chunks/crystals ought to be able to produce effects similar to this one when the rings are almost on edge in reference to the sun.

The spokes rotate almost synchronously with the magnetosphere of Saturn.

Only the finest of dust particles could respond so rapidly to such weak electromagnetic effects.

[Chris Peterson]

Why not a purely optical phenomenon? Slight variations in the flatness of the rings or certain alaignements in ice chunks/crystals ought to be able to produce effects similar to this one when the rings are almost on edge in reference to the sun.

There are no mechanisms known that could explain ripples or variations in the flatness of the rings. Such a thing would violate simple orbital dynamics.

[renglish]

Quoting the article: "Visible in the above image as light ghostlike impressions, spokes were first discovered by the Voyager spacecraft that buzzed by Saturn in the early 1980s. Their existence was unexpected. Oddly, the spokes more commonly observed when Saturn's rings are more nearly edge on to the Sun, and so were conspicuously absent from initial images sent back by the robot Cassini spacecraft now orbiting Saturn."

All well and good, except for the fact (acknowledged by NASA websites) that then amateur astronomer Steven O'Meara observed the "dark radial features" in Saturn's rings and sketched them in 1977. His observations were widely discounted until the Voyager spacecraft produced photographic evidence. Which leads me to the conclusion that the most compelling evidence must by necessity, be the most expensive.

[neufer]

All well and good, except for the fact (acknowledged by NASA websites) that then amateur astronomer Steven O'Meara observed the "dark radial features" in Saturn's rings and sketched them in 1977. His observations were widely discounted until the Voyager spacecraft produced photographic evidence. Which leads me to the conclusion that the most compelling evidence must by necessity, be the most expensive.

O'Meara should never have referred to them as "canals."

[Chris Peterson]

All well and good, except for the fact (acknowledged by NASA websites) that then amateur astronomer Steven O'Meara observed the "dark radial features" in Saturn's rings and sketched them in 1977. His observations were widely discounted until the Voyager spacecraft produced photographic evidence. Which leads me to the conclusion that the most compelling evidence must by necessity, be the most expensive. :-)

Well, you have to admit that photographic evidence anyone can see is a darn site more convincing than the report of a single eagle-eyed observer, no matter how credible he is (and there was never any suggestion O'Meara isn't credible). In the absence of either a plausible mechanism, or other observations, a high degree of skepticism is called for in such cases.

[aristarchusinexile]

All well and good, except for the fact (acknowledged by NASA websites) that then amateur astronomer Steven O'Meara observed the "dark radial features" in Saturn's rings and sketched them in 1977. His observations were widely discounted until the Voyager spacecraft produced photographic evidence. Which leads me to the conclusion that the most compelling evidence must by necessity, be the most expensive.

Consensus is a hard nut to crack.

I suppose shadow has been ruled out?

[Chris Peterson]

I suppose shadow has been ruled out?

Yes, because they are only dark when viewed from the same direction as the Sun. When viewed from the other side, with light coming through them, they are brighter than the rings. So their appearance depends on how light scatters from the particles.

[Mentad]

Chris P Wrote: "There are no mechanisms known that could explain ripples or variations in the flatness of the rings. Such a thing would violate simple orbital dynamics."

Shouldn't an object passing near or through the rings be able to, at least temporarily, displace areas of ordinary "ring stuff" by gravitation, to degrees varying according to the stuffs proximity to the object, the objects mass and the objects velocity?

[Chris Peterson]

Chris P Wrote: "There are no mechanisms known that could explain ripples or variations in the flatness of the rings. Such a thing would violate simple orbital dynamics."

Shouldn't an object passing near or through the rings be able to, at least temporarily, displace areas of ordinary "ring stuff" by gravitation, to degrees varying according to the stuffs proximity to the object, the objects mass and the objects velocity?

Sure, but such objects should be observable, and they aren't observed. Also, you wouldn't expect a "ripple" out of the ring plane. A change in inclination requires a lot of energy, and there is nothing that would restore the inclination back to the ring plane. If material were dragged out of the ring plane, it would continue in the direction it was dragged. I can't think of a mechanism that would create a stable or metastable "ripple" in the rings that wasn't radial.

You do see disturbances in the rings from moons. But they are primarily radial- shifts in material eccentricity, for example.