APOD: Daphnis the Wavemaker (2017 Jan 21)

[APOD Robot]

Daphnis the Wavemaker

Explanation: Plunging close to the outer edges of Saturn's rings, on January 16 the Cassini spacecraft captured this closest yet view of Daphnis. About 8 kilometers across and orbiting within the bright ring system's Keeler gap, the small moon is making waves. The 42-kilometer wide outer gap is foreshortened in the image by Cassini's viewing angle. Raised by the influenced of the small moon's weak gravity as it crosses the frame from left to right, the waves are formed in the ring material at the edge of the gap. A faint wave-like trace of ring material is just visible trailing close behind Daphnis. Remarkable details on Daphnis can also be seen, including a narrow ridge around its equator, likely an accumulation of particles from the ring.

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[Ann]

I saw this picture on Astronomy Now yesterday, and thought it looked fantastic. I have a question about the image, though. Right next to Daphnis, the wave displays a pale, pencil-thin outline. Is that for real? If so, what causes it?

Click to view full size image

Vertical structures created by Saturn’s small moon Daphnis cast long shadows across
the rings in this dramatic image taken by Cassini on June 8, 2009, at a distance of
414,000 miles (666,000 kilometers). Credit: NASA/JPL/Space Science Institute

Astronomy Now reminded me of another stunning Daphnis-shadows-and-waves image, namely the one at left.

Ann

[heehaw]

I remain annoyed that the NASA mission at Saturn is not going to end by having it give us a closeup of the rings or rather a tiny part of a ring. I can't remember how thick the rings are supposed to be, but it is meters not miles. Just imagine being 100 yards above a ring! Below you, hundreds of giant snowballs, apparently completely stationary, yet moving around Saturn at high speed. Never colliding with one another. In effect, billions of moons of Saturn. I don't remember reading about anyone simply monitoring the variable brightness of stars passing behind rings to give some idea of the size distribution of the lumps that make up the rings. The rings of Saturn are perhaps the best adornment of our solar system. I want a closeup of a ring!

[rstevenson]

... I can't remember how thick the rings are supposed to be, but it is meters not miles. Just imagine being 100 yards above a ring! Below you, hundreds of giant snowballs, apparently completely stationary, yet moving around Saturn at high speed. Never colliding with one another. ...

Our current understanding is that the rings range from about 10 m thick to perhaps as much as 1 km thick in some areas. Particle size ranges from about 1 cm to about 10 m. They are thought to at least nudge one another as they gradually clump up and unclump in reaction to gravitational tugs from the many moons and moonlets.

I agree a closeup pic would be very interesting to see.

Rob

[Homer]

So are the "waves" just on one side, and if so why, or is that just the result of the viewing angle?

[neufer]

So are the "waves" just on one side, and if so why, or is that just the result of the viewing angle?

• As Ann's picture clearly shows: both sides.

[Lasse H]

It looks a bit like a deserted spaceship...

[neufer]

Click to play embedded YouTube video.

I remain annoyed that the NASA mission at Saturn is not going to end by having it give us a closeup of the rings or rather a tiny part of a ring.

Which is why the Planetary Society sent geckzilla on a mission to Saturn in order to give us a closeup of the rings.

[tomatoherd]

Daphnis looks like a pecan. A really big nut.

[rstevenson]

I remain annoyed that the NASA mission at Saturn is not going to end
by having it give us a closeup of the rings or rather a tiny part of a ring.

Which is why the Planetary Society sent geckzilla on a mission to Saturn in order to give us a closeup of the rings.

There's also this NASA image. It's a lot less realistic looking than geck's, but shows the expected clumping. However I'm not entirely convinced that this 2007 image, based on a 1984 image which was itself based on earlier research, is reflective of current thinking about the rings.

Rob

[Ann]

Thanks, Geck. Superb illustration. It looks very "real", I must say.

Ann

[Case]

It seems to me that, in this image, Daphnis is distorting the ring ‘below’ much more than the ring ‘above’, although it the small moon does seem to be located somewhat in the middle of the gap. Another image of Daphnis shows the making waves at different sides (left/right), which is much less apparent in todays APOD. Are there any thoughts on why the rings do show this appearance of point reflection around the small moon?
What forces are at play here? Is the distortion in the ring caused be the gravity of the small moon alone?

[ta152h0]

let's have more Cassini's ( and New Horizon's )

[Steve Dutch]

Pictures like this are puzzling because they appear static. The reality is that everything is orbiting Saturn from left to right. Particles at the top of the picture are moving a little faster, those near the bottom are moving a little slower. Particles above the gap are overtaking Daphnis, and particles near the bottom are being overtaken. But the relative velocities are very small, meters per second or less. So the waves aren't bouncing up and down quickly. Instead, particles at the wave crest have been disturbed into an orbit with its peri-saturn a few km closer, out of a radius of 136,500 km.

The page https://spacemath.gsfc.nasa.gov/weekly/10Page28.pdf gives the velocity of a ring particle as 29.4/R^1/2 where R is in Saturn Radii. Unfortunately, some of the other results on the page don't check out. But if the velocity formula is correct, Daphnis is doing 19.5 km/sec and a particle 1 km further out is going 7 cm/sec slower. It will take Daphnis many years to lap the particles on the edges of the gap. But it only takes Daphnis and the ring particles a couple of days to orbit Saturn, so the disturbed ring particles have plenty of time to trace out their new orbits and form the waves.

Short story: Everything in the picture is moving around Saturn at about 20 km/sec, taking just a couple of says to orbit. But, the relative velocities of everything within the picture are extremely small.

I'd love to see a close-up of the rings, too, but matching the ring planes is out of the question and an inclined flyby would have to have pictures timed to the millisecond. The end of the mission calls for some dives between Saturn and the innermost ring, so that will be our best hope.

[neufer]

Are there any thoughts on why the rings do show this appearance of point reflection around the small moon?

Pictures like this are puzzling because they appear static. The reality is that everything is orbiting Saturn from left to right. Particles at the top of the picture are moving a little faster, those near the bottom are moving a little slower. Particles above the gap are overtaking Daphnis, and particles near the bottom are being overtaken. But the relative velocities are very small, meters per second or less. So the waves aren't bouncing up and down quickly. Instead, particles at the wave crest have been disturbed into an orbit with its peri-saturn a few km closer, out of a radius of 136,500 km.

The relative velocities are very small (±5 m/s) so the waves aren't bouncing up & down quickly (once per ~14 hour Daphnis orbital period so as to produce ~265 km long wavelengths). Particles at the wave crest have been disturbed vertically primarily because the orbit of Daphnis is slightly inclined to the ring plane so as to bob up & down by ±1.5 km.

<<The Keeler Gap is a 42-kilometre-wide gap in the A Ring, approximately 250 kilometres from the ring's outer edge. The small moon Daphnis, discovered 1 May 2005, orbits within it, keeping it clear. The moon induces waves in the edges of the gap. Because the orbit of Daphnis is slightly inclined to the ring plane, the waves have a component that is perpendicular to the ring plane, reaching a distance of 1.5 km "above" the plane.>>

[NoelC]

Pictures like this are puzzling because they appear static. The reality is that everything is orbiting Saturn from left to right. Particles at the top of the picture are moving a little faster, those near the bottom are moving a little slower. Particles above the gap are overtaking Daphnis, and particles near the bottom are being overtaken.

That completely cleared up a quick question that occurred to me "why is Daphnis orbiting at a different speed than the ring material". Had I thought it out throroughly I'd have come to that conclusion, especially after having another few looks at the image and seeing the slight wave just to the right of Daphnis.

Thank you, Steve, for putting the image in context.

Now the next questions come to mind, for the astrophysicists...

If the moonlet causes disruption of the ring material, and it eventually rubs against itself and sooner or later comes back to being smooth and even, I imagine that causes some heat as the ring particles disturb one another and ultimately settle back down, so...

• From where does the energy for that heat come? Is there a lessening of the angular velocity of the material?
• If so, does this mean that Daphnis and the nearby ring material are losing a small amount of velocity, and the Keeler gap is slowly moving inward toward Saturn? If not, is something replenishing the energy?
• And lastly... Is the moonlet likely to disappear eons from now, with the ring filling in - or grow bigger, making a bigger gap?

-Noel

[Boomer12k]

It would be interesting to have a video of this...because the previous wave is pulled out, but not the interval between the waves... though Daphnis should be pulling the whole way....

:---[===] *

[BillBixby]

It would be interesting to have a video of this...because the previous wave is pulled out, but not the interval between the waves... though Daphnis should be pulling the whole way....

:---[===] *

Take a look at the "making waves" link. It seems like an animation like you are asking about. Or I misunderstood what you wanted. But it is still a good time lapse show.

[Catalina]

So is the moonlet the cause and source and maintainer of the gap in the rings?

[Ann]

So is the moonlet the cause and source and maintainer of the gap in the rings?

I believe so, yes.

This article, where you can only read the abstract, talks about gaps in protoplanetary disks as signs of planets.

So if planets cause gaps in protoplanetary disks, then maybe it is not so strange if moonlets cause gaps in Saturn's rings.

Ann

[neufer]

The relative velocities are very small (±5 m/s) so the waves aren't bouncing up & down quickly (once per ~14 hour Daphnis orbital period so as to produce ~265 km long wavelengths). Particles at the wave crest have been disturbed vertically primarily because the orbit of Daphnis is slightly inclined to the ring plane so as to bob up & down by ±1.5 km.

<<The Keeler Gap is a 42-kilometre-wide gap in the A Ring, approximately 250 kilometres from the ring's outer edge. The small moon Daphnis, discovered 1 May 2005, orbits within it, keeping it clear. The moon induces waves in the edges of the gap. Because the orbit of Daphnis is slightly inclined to the ring plane, the waves have a component that is perpendicular to the ring plane, reaching a distance of 1.5 km "above" the plane.>>

I wish to correct my above statement.

The bobbing of Daphnis above & below the ring plane by ±1.5 km is a secondary factor in producing the ring waves.

The primary factor is Daphnis's orbital eccentricity which causes it to move radially ±4.5 km within the very narrow (42km) Keeler Gap. This produces tremendous modulating tidal forces which are out of sync for the inner and outer rings. Today's APOD clearly shows the tidal pulling on the outer ring due to the apokrone of Daphnis. A short orbital rotation period of ~14 hours Coriolis forces then causes such horizontal extensions to snap back into vertical extensions (as seen on the left in response to a previous apokrone of Daphnis).

Note that the ring particles near the edges of the Keeler Gap must include slightly elliptical as well as circular orbits making for a spread of orbital periods. Hence the 'downstream ripples' (from previous apokrones of Daphnis) include a spread of different wavelengths which generate the fuzziness of APOD's left side vertical wave and which eventually interfere with each other so as to damp out 'downstream' oscillations.

[neufer]

• A faint wave-like trace of ring material is just visible trailing close behind Daphnis.

To my eye that wave-like trace of ring material appear to be coming
from a separate ring layer slightly above the ring itself.

I wonder of this constitutes a separate (electrostatically charged) dust layer
that is hypothesized to exist hovering slightly above optically thick rings:

Click to play embedded YouTube video.

Dark spokes on the B ring's sunlit side in low phase angle Cassini images.

<<The B Ring is the largest, brightest, and most massive of the rings. Its thickness is estimated as 5 to 15 metres and its optical depth varies from 0.4 to greater than 5, meaning that >99% of the light passing through some parts of the B Ring is blocked. 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 gravitational orbital mechanics. The spokes appear dark in backscattered light, and bright in forward-scattered light; the transition occurs at a phase angle near 60°. 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 appear to be a seasonal phenomenon, disappearing in the Saturnian midwinter and midsummer and reappearing as Saturn comes closer to equinox. Suggestions that the spokes may be a seasonal effect, varying with Saturn's 29.7-year orbit, were supported by their gradual reappearance in the later years of the Cassini mission.>>

Time passes, and little by little everything that we have spoken in falsehood becomes true. - Marcel Proust