APOD: Daphnis and the Rings of Saturn (2017 Feb 22)

[APOD Robot]

Daphnis and the Rings of Saturn

Explanation: What's happening to the rings of Saturn? Nothing much, just a little moon making waves. The moon is 8-kilometer Daphnis and it is making waves in the Keeler Gap of Saturn's rings using just its gravity -- as it bobs up and down, in and out. The featured image is a wide-field version of a previously released image taken last month by the robotic Cassini spacecraft during one of its new Grand Finale orbits. Daphnis can be seen on the far right, sporting ridges likely accumulated from ring particles. Daphnis was discovered in Cassini images in 2005 and raised mounds of ring particles so high in 2009 -- during Saturn's equinox when the ring plane pointed directly at the Sun -- that they cast notable shadows.

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[Fred the Cat]

Waves or another version of sand dunes of the solar system with gravity as the wind. Not gravity wind.

Guess it would depend on how fast they form and dissipate?

[FLPhotoCatcher]

This is a similar photo to the one posted on Jan 21st of this year, though this one gives more detail of the waves.
I am temped to call these waves gravity waves, like cloud waves are gravity waves, but are they? Aren't these 'waves' just kind of an allusion? They are just the combined orbits of ring particles that have had their orbits changed, with the 'crest' of the waves being the closest point in the particle's orbit, and the 'troughs' of the waves being the farthest point in the particle's orbit. Or do the particles also gravitationally disturb the surrounding particles to a significant degree?

[neufer]

This is a similar photo to the one posted on Jan 21st of this year, though this one gives more detail of the waves.
I am temped to call these waves gravity waves, like cloud waves are gravity waves, but are they? Aren't these 'waves' just kind of an allusion? They are just the combined orbits of ring particles that have had their orbits changed, with the 'crest' of the waves being the closest point in the particle's orbit, and the 'troughs' of the waves being the farthest point in the particle's orbit. Or do the particles also gravitationally disturb the surrounding particles to a significant degree?

We are dealing with a differentially rotating frame of reference where one's intuition may easily be deceived.

This is my take on it:

Daphnis's orbital eccentricity causes it to move radially ±4.5 km within the very narrow (42km wide) Keeler Gap. This produces 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 most recent apokrones of Daphnis. The short average orbital rotation period of ~14 hours Coriolis forces causes such horizontal extensions to quickly snap back into vertical extensions.

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

There may even be different ring layers superimposed: http://asterisk.apod.com/viewtopic.php? ... 83#p266383

[Catalina]

So is Daphnis "bobbing" in "time" with the waves, or are the waves formed just by the passage of the moon-let as would waves from a passing boat? Is Daphnis spinning like a football or tumbling or just bobbing in a regular pattern? Does a "video" of some sort exist that will actually show this movement?

[FLPhotoCatcher]

I'm quite sure that Daphnis would still make the waves if it were not in an eccentric orbit relative to the edges Keeler Gap, though the waves would look different somehow.

Also, it's interesting how the waves look much brighter than the relatively undisturbed rings. I would guess that it's due to the ring containing loosely held-together clumps that are then broken up by the tidal forces created by Daphnis, thus creating more surface area for the sun's light to reflect off of.

[neufer]

So is Daphnis "bobbing" in "time" with the waves,
or are the waves formed just by the passage of the moon-let as would waves from a passing boat?

• Because it's orbital plane is slightly different Daphnis is "bobbing" in "time" by ±1.5km.
  (Perhaps this is the main factor in the vertical waves as I originally thought).

Is Daphnis spinning like a football or tumbling or just bobbing in a regular pattern?
Does a "video" of some sort exist that will actually show this movement?

Daphnis is tidally locked to Saturn (as are many of Saturn's more distant moons.

(The time Cassini spends in closeups is too short to make such a video.)

[MarkBour]

Because it's orbital plane is slightly different Daphnis is "bobbing" in "time" by ±1.5km.
(Perhaps this is the main factor in the vertical waves as I originally thought).

I'm wondering about the tilt of Daphnis's orbit from the ring plane. I assume the maximum "vertical offset" you mention of 1.5 km happens at only two points of its orbit, but I don't know if there's any reason that these points would be at the apokronos and perikronos ... are they (which would give Daphnis a somewhat linear back-and-forth relative motion)? Or are they at other points of the orbit, say when Daphnis is more centrally located in the gap (which would give it a circular relative motion)?

By the way, at the right are two more easily-obtained views of the Keeler Gap.

That's our own moon, seen directly over Keeler Needle in California, which is apparently named after the same astronomer. In spite of appearances, there is no evidence that our Moon was responsible for the shaping of the peaks in this image. Anyway, the Keeler Gap is a name climbers use for the dip between Keeler Needle and the summit of Mt. Whitney, seen just to the right of the needle.






Here's a view looking down through the gap.

Hopefully, this will satisfy heehaw's desire for a close-up view?

I too, am anxious for the closest, best ring-particle picture Cassini will give us. This image today was a surprise, because it is still looking at the A-ring, while Cassini is getting very close to the F-ring in its current and next few orbits. With heehaw, I'm still on the precipice, waiting.

[neufer]

Because it's orbital plane is slightly different Daphnis is "bobbing" in "time" by ±1.5km.
(Perhaps this is the main factor in the vertical waves as I originally thought).

I'm wondering about the tilt of Daphnis's orbit from the ring plane. I assume the maximum "vertical offset" you mention of 1.5 km happens at only two points of its orbit, but I don't know if there's any reason that these points would be at the apokronos and perikronos ... are they (which would give Daphnis a somewhat linear back-and-forth relative motion)? Or are they at other points of the orbit, say when Daphnis is more centrally located in the gap (which would give it a circular relative motion)?

Now I really feel embarrassed .

Daphnis's vertical motion ±8.5 km is TWICE the horizon motion

This results in waves that are ±1.5 km.

<<The inclination and eccentricity of Daphnis's orbit are very small, but distinguishable from zero. Daphnis's eccentricity causes its distance from Saturn to vary by ~9 km, and its inclination causes it to move up and down by ~17 km. The Keeler Gap, within which Daphnis orbits, is about 42 km wide.

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's slight orbital eccentricity 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.>>

[MarkBour]

Now I really feel embarrassed .
Daphnis's vertical motion ±8.5 km is TWICE the horizon motion
This results in waves that are ±1.5 km.

No problem, thanks for correcting the numbers ... that makes them larger, which is nice. I'm still trying to picture its relative motion though. [From one viewpoint ...] does it come to apoapsis and pull particles "up and in" from the outer part of the ring, then does it go to periapsis and pull particles "down and out" from the inner part of the ring, I wonder.

And then that begs questions of what would ever have got it moving like this, and won't the other ring particles work to dampen its bobbing motion?

[Boomer12k]

Surf's UP!!

:---[===] *

[neufer]

Now I really feel embarrassed .
Daphnis's vertical motion ±8.5 km is TWICE the horizon motion
This results in vertical waves that are ±1.5 km.

No problem, thanks for correcting the numbers ... that makes them larger, which is nice. I'm still trying to picture its relative motion though. [From one viewpoint ...] does it come to apoapsis and pull particles "up and in" from the outer part of the ring, then does it go to periapsis and pull particles "down and out" from the inner part of the ring, I wonder.

After considerable thought (this time for a change) this is my take on it:

Daphnis's mere presence in the Keeler Gap induces a sinusoidal wake of horizontal ring motion because as it slowly passes particles in the out ring it forces these particles into slightly elliptical orbits.

Now: superimposed upon that (fundamental sine wave wake of horizontal motion) are sympathetic wakes induced by Daphnis's own ±8.5 km vertical and ±4.5 km horizon motions. These two modulations induce periodic vertical & horizon wakes in the outer ring whose phases follow the specific motions of Daphnis at the time the Cassini image was captured. These secondary wakes may or may not be in phase either with each other or with the fundamental sinusoidal wake ... thereby creating different wake structures even under the same lighting conditions.

(A similar process, of course, goes on with particles in the inner ring.)

And then that begs questions of what would ever have got it moving like this, and won't the other ring particles work to dampen its bobbing motion?

The damping of the three independent wakes (discussed above) is primarily a factor of the spread of ellipticities of the ring particles at the edge of the Keeler Gap. Each individual ellipticity will have a slightly different differential orbital period and thus a slightly different wavelength and these wavelengths quickly get out of phase with each other. Only much later will the resulting quasi random scattered particles slowly return to the ring.