"Propeller" is not a term I'm familiar with in this context. I see the phenomenon you're referring to, but what's the significance of calling it a "propeller"?

propellors

Thank you. I had missed that Cassini release in March.

The "ubergeeks" at unmannedspaceflight.com are saluted in David Grinspoon's Cosmic Relief column in July's Sky and Telescope, which arrived in my mailbox today !

Herebelow a gif animation of the 6 frames (only rotation+center crop, no tilt correction):

Will we have enough time to watch the next wave pop up before edge-on is over?

Question: Could these be considered standing waves, or maybe even solitons? They look pretty stable in that sequence.

Also, I suspect that their structure is more complicated than they look here. Been visualizing something like a polarized radar pulse with a third dimension & a helical component.

I'd like to perform some basic calculations of aproximate elevations of some moonlets and waves above the ring plane. The problem is that the position of Cassini spacecraft relative to Saturn & Sun is needed, and that requires the hour of the day the individual Cassini images were shot. Info released along with Cassini images only comprise the day, but not the hour within the day. ¿is there any way to know the hour-of-the-day info of the moment a given image was shot by Cassini?

For really intelligent answers to a lot of questions raised on this thread, see the great entry on Emily's Planetary Society Blog by guest blogger David Seal. Wow!

All that information (image time to the second) is in the PDS headers, which you can get when the validated data is released for scientific work.

The raw images do give an approximate distance to the RINGS, so that information can in principle be combined with an ephemeris to yield the time. I've had some luck doing this with raw images of the satellites. Backing out the times of ring images might be trickier as it's unclear to me which part of the rings (or Saturn) is referenced in the distance.

Two months before the equinox game of light and shadow reveals more and more details. On these pictures clearly seen that the edge of the ring extends above the plane of the ring.
http://saturn.jpl.nasa.gov/multimedia/imag...0/N00137394.jpg
http://saturn.jpl.nasa.gov/multimedia/imag...0/N00137396.jpg

Interestinger and interestinger. I'm really looking forward to the equinox - the next four or five Titan encounters pump Cassini's ringplane inclination down to below 20 degrees in August/September, and periapse goes from 570,000km currently, to half that distance in August/September - literally a ringside seat!
Can't wait!

Does anyone know at what point the rings will go dark and how long they'll stay dark? Or is that among the things we're expectingg to learn?

--Greg

They won't get completely dark because Saturn will still be illuminating them from both sides. They will get pretty dark, though, but probably only close to the actual plane crossing.

Thanks. I went and read the suggested post on Emily's blog and figured that out too, but I was too late to edit my original. :-)

It does surprise me that the reflected light from Saturn has such a strong effect, though. I'd expect it to be just a few percent of solar illumination. (I'm too lazy to work it out right this second, but it's obvious that at some amount of tilt, the two are equal.)

--Greg

Greg, I'm sure you're after more accurate information, but I did give a rough calculation in post 72 of this thread which may be of some interest (in case you missed it). I assumed a solar elevation angle of between 1 and 2 degrees there.

I did miss it, but it's quite interesting. I note that I match some of your numbers, coming at it from a different angle. I start by picking a point in the B ring (near the inner edge) where Saturn subtends exactly 60 degrees. That means it covers 6.7% of the sky. Since Saturn has a geometric albedo of 0.47, it'd be 47% as bright as the sun if it were a sheet covering half the sky. Hence our ring particle ought to get 6.3% as much light from Saturn (the whole planet) as it does from the Sun -- assuming nothing else is in the way. I think this is very close to the number you got for this part of the problem.

But the other parts of the ring do get in the way, and we can assume that the illumination from the Sun drops as the sine of the ring tilt. (Or cosine if we're talking angle of illumination, I guess.) But we should make the same assumption for the light from Saturn itself, right? Saturn's equator doesn't illuminate the rings at all, but as you go further north or south, the amount of illumination increases -- except, of course, the planet narrows too. I don't feel like setting that integral up tonight, but I'm guessing it'll drop the Saturnshine component by about a factor of 5. (And that's constant, of course; it doesn't depend on the ring tilt). At that point we can figure out the point where solar illumination drops below illumination by Saturn.

Again, I'm saying "ring tilt" to refer to the angle between the ring plane and a line connecting Saturn and the Sun.

Sound right so far?

--Greg

Okay, I think I've worked it out. I think the only naive assumption is that Saturn's disk is equally bright, even though we know there ought to be at least some limb darkening.

So using the logic above, I come up with 1.6 degrees. That is, when the rings are within 1.6 degrees of the vector to the sun, the solar illumination and the "Saturnshine" should be equal. We'd have reached that point on April 28. Of course, my naive assumption overstates how bright the Saturnshine is.

This agrees fairly well with the estimate in post 72, particularly when you consider that I've chosen a point a good bit closer to Saturn.

Summary: I think it's fair to say that by now at least the inner rings are more brightly lit by Saturn than by direct sunlight. Of course, this effect diminishes as you go around the planet, so the view from above should already show the rings being a good bit brighter in front of Saturn, getting darker as they approach the shadow in the back, and with this effect more pronounced for the inner rings. Does anyone have a pic of that?

--Greg

this one shows that effect, although I not sure it is not showing the side not directly lit by the sun

http://saturn.jpl.nasa.gov/photos/raw/rawi...?imageID=191742

edit : Ooo, triple negative, have fun parsing that

Nope, good catch alan, that was the one I was looking for

That image shows the "unlit" side of the rings, well, for now let's just call it the north side of the rings to avoid confusion..

Although on the unlit side, we definitely expect to see the effect -- especially on the B Ring.

Now let me admit to a math error that totally changes the results. The numbers from before just seemed to be too large, and, yeah, they are. I got the double integral right (I think) but made a trig error, so I integrated the wrong thing. :-( (Never trust results you got by hand while waiting in the doctor's office . . .)

At the distance of the B-ring, Saturn is 3.14% as bright as the Sun. However, because Saturn's light falls obliquely on the rings, the maximum illumination is 48 times less than that, or about 0.13% what the Sun could do, if it shone directly on the rings. Even tilting the rings, as long as there are more than two minutes of arc, the Sun should still be brighter.

And I'm still overestimating how bright Saturn really is, I think, so, based on that, I'd predict that we won't be able to see the Saturnshine effect at all on the sunlit side of the rings (that's the south side, right now) except for a few hours on the day of the equinox itself.

This hypothesis is supported by the photo. It shows that the Saturnshine effect is weak even on the backlit, north side of the rings. You'd expect direct sunlight to wash it out, given that even backlighting is brighter.

--Greg

Hello all!

I received a news release announcement from CICLOPS in my inbox yesterday and thought I would pass it along.

Saturn's Approach To Equinox Reveals Never-before-seen Vertical Structures In Planet's Rings

http://ciclops.org/view.php?id=5680

Great animation under "wave shadows in motion"

http://ciclops.org/index.php

The abstract for the article by John Weiss et al can found here.

http://www.iop.org/EJ/abstract/1538-3881/138/1/272

Looks like you can purchase the full article for $9US if anyone is interested

Enjoy

Ok, I decided I was interested and obtained a copy of the article. All and all it’s a very well written article which examines the effects of moon orbital eccentricity and inclination on ring gap edges. The resulting wave amplitudes and morphology were examined for the moon-gap cases of Pan-Encke and Daphnis-Keeler.

(Note, I was writing a much more detail article summary, but something happened and my browser page tab disappeared along with every thing I had written. I couldn’t bring myself to rewrite everything (was almost done ) so here’s a more condensed summary.

Some Daphnis-Keeler useful values:
Diameter- ~ 9 km
Keeler gap ~ 35 km
Moon-gap edge distances: 13-20 km (inner) and 14-16 (outer)

(Note: The inner edge of the Keeler Gap is in a 32:31 inner Lindblad resonance with Prometheus, causing the edge’s radial location to vary over a 15 km range and hence the larger range of Moon-gap edge distances)

Wave Amp (radial): 1.8 – 5.4 (inner) and 4.0-5.6 (outer)
Eccentricity – 3.31x10-5
Inclination – 0.0036 deg (vertical variation 8.6 km)
Orbital period – 0.594 days
Synodic orbital period – 8.5 years (5210 orbits)
(It’s interesting to note that due to angular momentum interactions with the ring, the longitude of Daphnis’ ascending node undergoes precession. This means that it will take 5210 orbits for Daphnis to reach the same relative position in space twice!)

For a circular-coplanar moon/ring orbital geometry the edge wave is primarily radial with constant wave morphology.

For an eccentric-coplanar moon/ring orbital geometry the edge wave is still primarily radial but wave amplitude now varies with time.

Orbital inclination imparts a vertical component so the wave will now display radial and vertical amplitudes. In the case of the Daphnis, the moons orbit is both eccentric and inclined relative to the ring. Because we now have 2 parameters effecting the moon ring orientation the overall wave morphology will display a large temporal variation. The orbital positions for radial and vertical wave amplitude maximum are ~ 180deg apart. Meaning when the vertical amplitude is at a maximum the radial amplitude will be at a minimum. According to the simulations performed by the authors the radial amplitudes will vary from 2.0 – 7.7 km and the vertical amplitude will vary from 0.2 – 1.7 km. Based on the shadow lengths from latest Cassini images of Daphnis (may 09) they calculated that the vertical amplitude was 1 – 1.5km and that the wave morphology was consistent with this height.

If you examine the images of Daphnis and the associated ring edge waves you will notice that the inner edge shows reduced amplitude compared to the outer edge. Apparently this is due to the eccentricity of Daphnis’ orbit which causes it to typically be closer to the outer ring edge and also in part to the fact that the inner edge particles experience a resonance effect from the moon Prometheus.

In relation to some of the discussion here regarding effect of the ring particles on Daphnis, they estimated that the mass of effected ring particles at any one time is ~ 0.0014 times the mass of Daphnis. As I noted previously, the ring particles do cause the ascending node of the orbital inclination to undergo precession. Additionally the ring particle also are causing a slight damping effect on the inclination (damping period ~ 1000 years). Apparently depending on the mass of the ring the orbital inclination can experience a damping or exciting effect and Daphnis is deemed to be right around the critical point for damping vs. excitation.

Let me know if anyone has questions or comments. Additionally I didn’t post the paper because I felt that would constitute too blatant of a violation of my single use license. However if anyone really would like to see the paper just send me a separate message and I can probably be convinced to allow you to borrow my copy

Thanks chemman for very interesting resume (is a shame not to have your initial richer report!).


This is stunning, they estimated the mass of astronomical "object" which is only 120 million tonns (comparable to the mass of a 500m wide iceberg...!). Perhaps a record (Itokawa, as example, is 3 times more massive), and for sure impressive considering also is referred to a "diffuse" assembly of tiny particles...
I guess they derived such a value from ascending node precession, do you confirm?

Hello Marco - Yes I was quite upset when I lost my initial report, half way into it I actually though to myself that I should transfer it to a different program so I would not lose it, but never did. Oh well, If I get time again soon I may make another pass at a more in depth summary.

They estimated the mass of the effected ring particles by estimating the radial and longitudinal dimensions of the interacting patch of the ring (2.5km radial by 165km longitudinal) and using a surface density of 30g/cm (based on density wave measurements from another paper). They said this then gives a combined mass for the interacting ring edges of ~ 0.0014 times the mass of Daphnis. They pointed out that this is a order of magnitude estimate only and that a 10% difference in the estimate ring mass would cause their initial calculation of orbital inclination damping to become exciting. Then end up using another more accurate method to calculate a damping period of 1000 years for the orbital inclination.

chemman--Thanks for a really great summary. I've also lost a few posts (hit go back when I meant to tab between windows), but usually copy to Word as I never catch the misspelling of typos otherwise.

Do they comment on other factors (resonance with other moons) that could keep Daphnis slightly inclined? It seems unlikely that we just happen to be viewing it a mere 1000 years before its inclination gets damped out.

They don't mention any else that might effect the inclination. I get the impression that since Daphnis-Keeler gap system seem to be right around the cutoff for orbital damping, it would not take much error in their calculations to get a different out come. Typically ring moon interactions for inclination should not be stable unless there is a counter acting force. That begin said, they did comment on the fact the Daphnis had a eccentric orbit which surprised them since they thought the eccentricity should have disappeared due to ring interactions. They didn't speculate on any reasons for the apparent stability of the eccentricity.

Shadows lengthening fast now:

http://saturn.jpl.nasa.gov/multimedia/imag...1/N00138316.jpg

http://saturn.jpl.nasa.gov/multimedia/imag...1/N00138317.jpg

A dramatic illustration of how the illumination of the dark (north) side of the rings is now dominated by reflected Saturn-light here. Note how much brighter the rings are on the day side of the planet, where Saturn-shine is stronger. Note also the contrast reversal between the B and C rings around the ring- the C ring is brighter where transmitted sunlight dominates, the B ring is brighter where Saturn-shine dominates.

John

Equinox is only 5 weeks away now - I'm wondering what light (no pun intended) will be cast on the ring-spoke phenomena when the rings darken? Will they be much more evident against the dark backdrop, or be completely invisible?

Would like to see some near-edge-on shots that might possibly capture spokes 'levitating' above or below the main ring plane; assuming that they'd scatter a bit more light at that point than the rings proper.

Agreed - this would be great!
Incidentally, further to my earlier post, doubtless it's no coincidence that we'll be right on top of the equinox. I understand that Cassini's Rev116 periapse is on the day itself (11th August), and occurs between the orbits of Janus and Mimas - the lowest for over a year. Kudos once again to those who control the orbital trajectory...

I'm hoping there might be some blog posts from the team during equinox, similar to those for the last couple of very close Enceladus flybys - I'd LOVE to see a simulation showing instrument pointing for the busiest part of periapse.

For a different perspective on the whole issue of moons casting shadows on the rings, here is a mosaic of a couple of HST pictures of Saturn (taken back in 1995, as far as I can recall):

Click to view attachment

Daphnis makes a real three-dimensional mess:

http://saturn.jpl.nasa.gov/multimedia/imag...1/N00138916.jpg

New raws up: some really strange stuff happening! My guess- sunlight coming through the Cassini division from the sunlit side and illuminating 3-D irregularities on the dark side at the outer edge of the B-ring.

Second thoughts- I don't think that works, because there are shadows cast in one direction and streaks in another direction. Maybe this is the lit side after all. Hmmm....

John

That's along the edge of the B ring yes, but this is the lit face of the rings

Let me just say whoa!

Notice how the gap to the right is wider at the top than at the bottom. This disturbance almost looks like it's waves splashing onto a beach or in this case onto the gap. Could those fuzzy white things be vertical protrusions above the plane and merely look skewed due to viewing geometry?

That's definitely my impression, Gordan. Utterly, hypnotically amazing!

Hmm, there's a tiny bit of overlap between frames 139391 and 392 and even though there's a very slight perspective change (whether primarily due to Cassini moving or rings rotating), there's no noticeable 3d effect which I'd expect if these were tall structures. In fact, if you rotate the image you can almost convince yourself these are flat features.

One thing's for sure, this disturbance looks confined to just a small segment of the ring circumference. The majority of other frames show slightly rough material, but more well-behaved.

reminds me of this image
http://pds-rings.seti.org/neptune/voyager/c1141246.html

I also notice that on the browse page the images showing the streaks are darker. Perhaps the shorter exposure plus smearing due to the orbital motion allowed the radial motion of some clumps in the rings to be captured. The streaks do appear to be in part of the ring where the distance to the edge of the ring and the narrow ring outside it is changing.

ETA: moonlet with shadow in this image?
http://saturn.jpl.nasa.gov/photos/raw/rawi...?imageID=196899

This enhanced version can be useful:
Click to view attachment

With illumination change, the images are more and more amazing! look at this Prometheus picture projecting a sharp shadow on the rings:
http://saturn.jpl.nasa.gov/photos/raw/rawi...?imageID=197449

Incredible.

In order to comprehend (with my tiny uneducated brain) what cosmic forces are at work here I downloaded the full image sequence.
It looks like the ring has been evolving up too close to the next and is refitting it's course. The white streaks might be debris from
numerous collisions as waves of moonlets are crashing into one and other. But that's just my amateur imaginative guess.

Hopefully a scientific explanation will come along. It's an absolutely amazing phenomena... I can't stop looking at it.

I don't want to jinx anything, but... HOW PRICELESS ARE THESE IMAGES?!?!

Holy schmoly, we won't duplicate them for probably 45 years. At least.

I really don't understand that image of the rings.

I can see how some of the features and some shadows correspond, but yet others don't seem to.
I'm lost.

Just a note, some of the streaking and fuzziness in the image is the result of the longer exposure times ring scientists on the imaging team have had to use as the sun get lower and lower on the ring plane.

Here's a quick-'n'-dirty mosaic of two of the raw images:

Click to view attachment

Does that account for the appearance of some of those... ring bumps? They seem like they were eroded by wind toward the top-left of the image.
(yes I know, no wind in space )

Edit: Never mind. If so, it would modify the appearance of ALL the ring bumps.

The complexity perplexes me.

Sitting here thinking how 30 years ago we were less than a month away from the Pioneer 11 Saturn encounter. With Voyager 1 to follow in 1980. These were the first spacecraft to flyby Saturn.... I do not think any of us, back then, could have imagined what was to come.

The rings are a gold mine of data for researchers ..... and just plain beautiful.

I am euphoric!

Craig

Wow, really amazing. VP, I don't think I see much streaking as there are a lot of un streaked spots (clumps/boulders). Could the diagonal ridges be due to some orbital resonance with one of the larger moons? For the F-ring, the gores point towards the moon. Is there some mechanism where the perpendicular to the ridges could be pointing towards the moon driving the ridge formation??