APOD: Saturn At Equinox (2009 Sep 30)

[orin stepanek]

http://apod.nasa.gov/apod/ap090930.html

Explanation: How would Saturn look if its ring plane pointed right at the Sun? Before last month, nobody knew. Every 15 years, as seen from Earth, Saturn's rings point toward the Earth and appear to disappear. The disappearing rings are no longer a mystery -- Saturn's rings are known to be so thin and the Earth is so near the Sun that when the rings point toward the Sun, they also point nearly edge-on at the Earth. Fortunately, in this third millennium, humanity is advanced enough to have a spacecraft that can see the rings during equinox from the side. Last month, that Saturn-orbiting spacecraft, Cassini, was able to snap a series of unprecedented pictures of Saturn's rings during equinox. A digital composite of 75 such images is shown above. The rings appear unusually dark, and a very thin ring shadow line can be made out on Saturn's cloud-tops. Objects sticking out of the ring plane are brightly illuminated and cast long shadows. Inspection of these images may help humanity understand the specific sizes of Saturn's ring particles and the general dynamics of orbital motion.

I'd say the rings were dark because the sun can't light them up. It does however; add a new look to our beautiful neighbor. 8)

Orin

[Craine]

There seem to be a number of moonlets visible in and around the rings. Some are clearly moonlets as they obviously disturb the nearby rings. But others could be bright background stars. I'd rather think the overall brightness might fade out any background stars though, I am not sure. Anybody wanna comment? Or perhaps somebody can even name the moonlets we can see?

[Axel]

Fortunately...humanity is advanced enough to have a spacecraft that can see the rings during equinox from the side.

So that's an earmark of how advanced humanity is: coulda fooled me.

[Indigo_Sunrise]

There seem to be a number of moonlets visible in and around the rings. Some are clearly moonlets as they obviously disturb the nearby rings. But others could be bright background stars. I'd rather think the overall brightness might fade out any background stars though, I am not sure. Anybody wanna comment? Or perhaps somebody can even name the moonlets we can see?

From here:

The moon Janus (179 kilometers, 111 miles across) is on the lower left of this image. Epimetheus (113 kilometers, 70 miles across) appears near the middle bottom. Pandora (81 kilometers, 50 miles across) orbits outside the rings on the right of the image. The small moon Atlas (30 kilometers, 19 miles across) orbits inside the thin F ring on the right of the image. The brightnesses of all the moons, relative to the planet, have been enhanced between 30 and 60 times to make them more easily visible. Other bright specks are background stars. Spokes -- ghostly radial markings on the B ring -- are visible on the right of the image.

I'd say the rings were dark because the sun can't light them up. ...

And also from here:

The novel illumination geometry that accompanies equinox lowers the sun’s angle to the ring plane, significantly darkens the rings....
At this time so close to equinox, illumination of the rings by sunlight reflected off the planet vastly dominates any meager sunlight falling on the rings. Hence, the half of the rings on the left illuminated by planetshine is, before processing, much brighter than the half of the rings on the right. On the right, it is only the vertically extended parts of the rings that catch any substantial sunlight.
With no enhancement, the rings would be essentially invisible in this mosaic. To improve their visibility, the dark (right) half of the rings has been brightened relative to the brighter (left) half by a factor of three, and then the whole ring system has been brightened by a factor of 20 relative to the planet. So the dark half of the rings is 60 times brighter, and the bright half 20 times brighter, than they would have appeared if the entire system, planet included, could have been captured in a single image.

*I visit the CICLOPS and NASA's Cassini web pages daily! Great stuff there

[wtillman1]

Has anyone found or published the manetic force (my assumption) mechanisms that hold the rings of particles in that space around the equator & not allow them to coalesce?
wtillman1

[NoelC]

My understanding (shored-up by last night's repeat of The Universe) is that orbital mechanics sufficiently explain why the particles keep from coaslescing. The particles at different distances from the planet simply orbit at different speeds.

I've always wanted to see a time lapse of the rings to make that clear. I imagine the dynamics, even from a long way off, would be spectacular to watch.

-Noel

[jerbil]

I find this magnificent composite photograph extremely difficult to interpret. I presume that the ring system and the accompanying shepherd moonlets are coplanar, so that at equinox they contribute a single dark line of shadow on the upper Saturnian cloud layer.

Underneath what I interpret as this line, there are numerous other shadow lines, at least as strong, which I can only put down from the description of the photograph as the "elongated shadows" from out-of-plane objects, though why they should be so strong escapes me, as does the fact that they are grouped together.

Furthermore, to the far right and at the immediate bottom of the image, there are bright arcs, while the insolation clearly comes from the left.

Would someone please explain this anomoly? Is it an artefact from combining many photographs into one composite?

[jerbil]

My understanding (shored-up by last night's repeat of The Universe) is that orbital mechanics sufficiently explain why the particles keep from coaslescing. The particles at different distances from the planet simply orbit at different speeds.

I've always wanted to see a time lapse of the rings to make that clear. I imagine the dynamics, even from a long way off, would be spectacular to watch.

-Noel

I disagree. The observed striations, sometimes called spokes, seen on many of the other Cassini photographs, indicate that the ring system, despite the efforts of shepherd moonlets, is inherently unstable. My earlier understanding of gravitational dynamics was that, on any astronomical timescale, a ring system such as that of Saturn would be "short-lived."

[orin stepanek]

What would happen if we suspended a spinning ball into a cloud chamber? Could we create a ring system? Probably not; but it's fun to muse!

Orin

[Chris Peterson]

I find this magnificent composite photograph extremely difficult to interpret. I presume that the ring system and the accompanying shepherd moonlets are coplanar, so that at equinox they contribute a single dark line of shadow on the upper Saturnian cloud layer.

That's correct.

Underneath what I interpret as this line, there are numerous other shadow lines, at least as strong, which I can only put down from the description of the photograph as the "elongated shadows" from out-of-plane objects, though why they should be so strong escapes me, as does the fact that they are grouped together.

All of the lines below the ring shadow are just silhouettes of the rings themselves. They are not shadows on the cloud tops.

Furthermore, to the far right and at the immediate bottom of the image, there are bright arcs, while the insolation clearly comes from the left.

The bright arcs along the bottom of the planet's disc are caused by the background light (from the illuminated cloud tops) coming through gaps and low density areas in the ring system. You can see that these gaps are dark against the background sky. The bright arcs at the right are more interesting. My guess is that we're simply seeing ring components that are a few meters above the surrounding rings, so they are in sunlight. That would happen if these bright rings are just slightly thicker.

[Chris Peterson]

I disagree. The observed striations, sometimes called spokes, seen on many of the other Cassini photographs, indicate that the ring system, despite the efforts of shepherd moonlets, is inherently unstable. My earlier understanding of gravitational dynamics was that, on any astronomical timescale, a ring system such as that of Saturn would be "short-lived."

As Noel pointed out, the rings are explainable by orbital dynamics alone. Gravity is the only relevant force that needs to be considered. The system is less stable than the Solar System (which is also unstable), but nevertheless stable over at least hundreds of millions of years. The stability is maintained by resonances with multiple moons and moonlets.

The spoke patterns have nothing to do with stability or instability. Their cause isn't absolutely established, but is probably an electrostatic effect, whereby dust is lifted a little out of the ring plane and then casts shadows. While interesting, it doesn't mean there is anything gravitationally unstable about the ring system, nor does it reduce the lifetime of the rings.

[3uu2]

I find this magnificent composite photograph extremely difficult to interpret. I presume that the ring system and the accompanying shepherd moonlets are coplanar, so that at equinox they contribute a single dark line of shadow on the upper Saturnian cloud layer.

That's correct.

Underneath what I interpret as this line, there are numerous other shadow lines, at least as strong, which I can only put down from the description of the photograph as the "elongated shadows" from out-of-plane objects, though why they should be so strong escapes me, as does the fact that they are grouped together.

All of the lines below the ring shadow are just silhouettes of the rings themselves. They are not shadows on the cloud tops.

Question: why does the moon on the middle left of the outer ring (Janus?) not cast a stripe shadow across the rings or make a spot shadow on the clouds near the ring shadow? There must be a shadow but I can't see it.

[Chris Peterson]

Question: why does the moon on the middle left of the outer ring (Janus?) not cast a stripe shadow across the rings or make a spot shadow on the clouds near the ring shadow? There must be a shadow but I can't see it.

There may be no shadow. The image is a mosaic, and presumably the moon was in a different position when the section of Saturn where you might expect the shadow was being imaged. In addition, this is a very small moon- around 180 km across. I doubt you could even see a shadow on the cloud tops. The only reason we see the moon (unresolved) is that it is reflecting a lot of light. As far as a shadow on the rings is concerned, that would only occur if the moon were right on the ring plane- unlikely since it has a small inclination.

[jerbil]

I wish to thank Chris Peterson, of the Cloudbait Observatory, for his explanatory observations, which I find most valuable.

There is, however, a further comment I would like to make on the stability in the long term of the ring system around Saturn. As has been noted in at least one previous APOD contribution, a detailed analysis of the edges of the ring systems note "frilling," namely that the inner and outer limits of a particular ring are not smooth.

There may be resonance conditions which explain this, which would require Fourier analysis to confirm (or reject.) If some sort of a wave pattern is not established for the effect, I would suggest that this observation would add further evidence for the instability of the ring system as a whole.

[Chris Peterson]

There is, however, a further comment I would like to make on the stability in the long term of the ring system around Saturn. As has been noted in at least one previous APOD contribution, a detailed analysis of the edges of the ring systems note "frilling," namely that the inner and outer limits of a particular ring are not smooth.

There may be resonance conditions which explain this, which would require Fourier analysis to confirm (or reject.) If some sort of a wave pattern is not established for the effect, I would suggest that this observation would add further evidence for the instability of the ring system as a whole.

Why?

There are certainly unstable regions within the rings- that's why the structure is so complex, with its gaps and density variations. It seems perfectly reasonable that right at boundaries between stable and unstable zones you should see odd effects, such as the frills you mention. I don't see how this reflects on the stability of the ring system as a whole, however.

[jerbil]

There are certainly unstable regions within the rings- that's why the structure is so complex, with its gaps and density variations. It seems perfectly reasonable that right at boundaries between stable and unstable zones you should see odd effects, such as the frills you mention. I don't see how this reflects on the stability of the ring system as a whole, however.

I see your point that local instability within a ring does not imply the destruction of the ring. A Shoemaker-Levi would not even dent a ring's survival, however spectacular was its impact on Jupiter. So, how did Saturn's rings develop? We know from one of the Voyager missions that Uranus has a faint ring system, but if this arises from the origin of our solar system, then why does not Jupiter possess one? Perhaps, as I have read, at an earlier stage, when there were many more rogue objects in the locality there were two catastrophic collisions in the neighborhood of each planet.

[jsizzle]

moonlets... i see one of these small white dots in the shadow of the planet is this a composite error or is this another moonlet? odd to be so bright in a dark shadow...

[bystander]

I see two white dots on either side of the outer ring just below Saturn's shadow. I believe those are indeed moonlets, probably Prometheus and Pandora.

[Chris Peterson]

I see your point that local instability within a ring does not imply the destruction of the ring. A Shoemaker-Levi would not even dent a ring's survival, however spectacular was its impact on Jupiter. So, how did Saturn's rings develop? We know from one of the Voyager missions that Uranus has a faint ring system, but if this arises from the origin of our solar system, then why does not Jupiter possess one? Perhaps, as I have read, at an earlier stage, when there were many more rogue objects in the locality there were two catastrophic collisions in the neighborhood of each planet.

All four of our gas giants, Jupiter, Saturn, Uranus, and Neptune possess ring systems.

There are different ideas about how they form, but the general idea is that you have primordial material that was too close to the planets to coalesce into moons, or you have objects that got to close and were gravitationally disrupted. I believe there are computer simulations of both mechanisms. Saturn is the only planet with a ring system that seems to be stable over a very long period, perhaps even back several billion years.

[canuck100]

A 2007 article "Saturn's Rings May be Old Timers" at http://www.nasa.gov/mission_pages/cassi ... 71212.html reports that the apparent youth of the material comprising Saturn's rings is due to its continual recycling into and out of modest sized clumps. The apparent youth of the material as observed by Voyager had led to proposals that the rings had formed as recently as 100,000 years ago. More detailed data obtained by Cassini suggests that the recycling of this material is responsible for its youthful appearance and that the rings have been in existence for at least a few million years. Furthermore, the reported analysis of the widely differing material in various rings suggests that they were not formed by a single cataclysmic event.

[NoelC]

Ice/water geysers spouting from Enceladus are known to be feeding the rings... Could it be that Saturn's system has captured by chance a particularly large (or large group) of comets? Could millions of years of ice/water geysers have simply built up the ring system as a result, perhaps, of tidal forces greadually heating/tearing up a then much larger Enceladus?

We may never really KNOW the origin of those beautiful rings, but I'm sure glad Cassini was built to go study them.

The Saturn ring system reinforces a fundamental observation about the universe: It sure evokes a sense of profound beauty when we see the macro (and micro) effects of the interaction of a huge number of particles based on a relatively few simple laws of physics. Bravo, God (or whatever may be the name of the creator of this grand "universe" computer simulation we live in)! All I can say is, I hope you're running it with an uninterruptable power supply.

-Noel

[orin stepanek]

All of Sol's giants have rings. I wonder if this is the norm? And why does Saturn have the most grandiose rings of all. Anyway Saturn has the best custom at the ball; though Earth is the prettiest.

[emc]

All of Sol's giants have rings. I wonder if this is the norm? And why does Saturn have the most grandiose rings of all. Anyway Saturn has the best custom at the ball; though Earth is the prettiest.

Hi Orin,
Of course I’m far and away from being knowledgeable on the subject… but I’m guessing Jupiter ate most of its rings, Saturn had less of an appetite and Neptune had less traffic to deal with… perhaps something to consider for those purchasing a new home??

Earth is not only the prettiest it is the easiest to get to. Which is probably why it's so crowded

[neufer]

All of Sol's giants have rings. I wonder if this is the norm?
And why does Saturn have the most grandiose rings of all.
Anyway Saturn has the best custom at the ball

Saturn's starched TUTU needed

• *NO IRONING*
  *ONION RING*

I’m guessing Jupiter ate most of its rings, Saturn had less of an appetite

<<ONION RINGs are a type of fast food commonly found in the United States, Canada, United Kingdom, Australia, and some parts of Asia. It generally consists of a small ring of onion dipped in batter or bread crumbs and then deep fried. The result varies from restaurant to restaurant. The ring-like structure of an onion lends itself well to this mode of preparation. A variant of this is made with onion paste and there are advantages and disadvantages between the two. Whole onion rings make for better presentation through a variety of sizes, while those made from a paste offer quantity through consistent size. Consumers of whole onion rings run the risk of pulling the onion out of the batter if they fail to cut it all the way through with their teeth, while onion rings made of onion paste break apart easily but oil absorbency diminishes the onion taste.

The exact origins of the onion ring is unknown, but in 1933 a recipe for deep-fried onion rings that are dipped in milk then dredged in flour appeared in a Crisco advertisement in The New York Times Magazine.>>

[orin stepanek]

*ONION RING*

Twas a chicken dinar in a neighboring city that gave you an order of onion rings that filled your plate and were heaped up about 8". Whenever we were in that fair city we loved to go there for a snack. One plate sufficed to satisfy 4 or 5 friends. They weren't as pretty as Saturn's rings but were very delicious.