APOD: Daphnis and the Rings of Saturn (2019 Nov 03)

[APOD Robot]

Daphnis and the Rings of Saturn

Explanation: What's happening to the rings of Saturn? A little moon making big 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 colored and more detailed version of a previously released images taken in 2017 by the robotic Cassini spacecraft during one of its 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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[Ann]

Click to view full size image

Daphnis attack! Photo: Cassini.

Daphnis looks like an old-fashioned spaceship out of a science fiction movie, attacking an evil ring of Saturn!

Ann

[Astronymus]

Interestingly the ridges show different angles. Looks like this moons orbit and axis changed a lot.

[Boomer12k]

I think it looks like a BUG, with its mouth open....

Great image...
:---[===] *

[neufer]

Click to play embedded YouTube video.

Click to play embedded YouTube video.

Daphnis looks like an old-fashioned spaceship out of a science fiction movie, attacking an evil ring of Saturn!

Daphnis attack! Photo: Cassini.

---

[orin stepanek]

To me it looks looks like a huge bolder in a river near a beach; and a desert with sand dunes!

[JohnD]

Who cares what it looks like? That only tells us about the obsessions and/or interests of the observer.

WHY does it look like that? OK, gravity, but plus tidal effects? And why do those effect the outer ring particles (?, lower in pic) but not those in the inner (upper) ring.
Is this an effect of perspective? The inner (upper) ring might be much further away than it looks if the picture was taken from only slightly above the plane of the rings.

John

[neufer]

To me it looks looks like a huge bolder in a river near a beach; and a desert with sand dunes!

---

<<Stone skipping (or stone skimming) is the art of throwing a flat stone across water in such a way that it bounces off the surface, preferably many times. The objective of the game is to see how many times a stone can bounce before sinking.

An early explanation of the physics of stone-skipping was provided by Lazzaro Spallanzani in the 18th century.

The stone generates lift in the same manner as a flying disc, by pushing water down as it moves across the water at an angle. Surface tension has very little to do with it. The stone's rotation acts to stabilize it against the torque of lift being applied to the back.

Research undertaken by a team led by French physicist Lydéric Bocquet discovered that an angle of about 20° between the stone and the water's surface is optimal. Bocquet and his colleagues were surprised to discover that changes in speed and rotation did not change this fact, it just allowed the stone to be in balance and to continue with a straight and uniform movement, due to gyroscopic effect. Work by Hewitt, Balmforth and McElwaine has shown that if the horizontal speed can be maintained skipping can continue indefinitely. Earlier research reported by Bocquet calculated that the world record of 38 rebounds set by Coleman-McGhee, unchallenged for many years, required a speed of 12 m/s, with a rotation of 14 revolutions per second.>>

[Chris Peterson]

Who cares what it looks like? That only tells us about the obsessions and/or interests of the observer.

WHY does it look like that? OK, gravity, but plus tidal effects? And why do those effect the outer ring particles (?, lower in pic) but not those in the inner (upper) ring.
Is this an effect of perspective? The inner (upper) ring might be much further away than it looks if the picture was taken from only slightly above the plane of the rings.

We care what it looks like because that's the first step in asking the question "why?" And now that we can see what it looks like, dynamicists have improved their models of the ring system and are able to create the same structures in the computer. What it looks like has instructed us on how gravity and tides have created this kind of structure.

[aildoux]

Really great details on this image. Does the mottled aspect of the ring plane show actual individual ring particles? Ring particles go up to some meters wide, so the larger ones will show up on the highest resolution images someday, for sure. Maybe some are visible here, looks like.

[neufer]

Really great details on this image. Does the mottled aspect of the ring plane show actual individual ring particles? Ring particles go up to some meters wide, so the larger ones will show up on the highest resolution images someday, for sure. Maybe some are visible here, looks like.

• No actual individual ring particles...possibly random clusters of ring particles.

Propeller moonlet Santos-Dumont from lit (top) and unlit sides of rings

---

<<The rings of Saturn consist of countless small particles, ranging in size from micrometers to meters.

In 2006, four tiny "moonlets" were found in Cassini images of the A Ring. The moonlets themselves are only about a hundred metres in diameter, too small to be seen directly; what Cassini sees are the "propeller"-shaped disturbances the moonlets create, which are several km across. It is estimated that the A Ring contains thousands of such objects.

In 2007, the discovery of eight more moonlets revealed that they are largely confined to a 3,000 km belt, about 130,000 km from Saturn's center, and by 2008 over 150 propeller moonlets had been detected. One that has been tracked for several years has been nicknamed Bleriot.>>

[sillyworm 2]

As with so many exotic Solar System vistas..what a treat it would be to closely observe this moon and it's wake of particle dispersion

[JohnD]

Who cares what it looks like? That only tells us about the obsessions and/or interests of the observer.

WHY does it look like that? OK, gravity, but plus tidal effects? And why do those effect the outer ring particles (?, lower in pic) but not those in the inner (upper) ring.
Is this an effect of perspective? The inner (upper) ring might be much further away than it looks if the picture was taken from only slightly above the plane of the rings.

We care what it looks like because that's the first step in asking the question "why?" And now that we can see what it looks like, dynamicists have improved their models of the ring system and are able to create the same structures in the computer. What it looks like has instructed us on how gravity and tides have created this kind of structure.

"It looks like an old-fashioned space ship" and "it looks like sand dunes" contribute nothing to 'why" unless the three tentacled Kj/lk;m people of Sagittarius 3 have arrived, or ice rings work like sand dunes (maybe?). I'll be grateful for an answer to why the other ring isn't affected.
John

[Chris Peterson]

I'll be grateful for an answer to why the other ring isn't affected.

Both edges of the Keeler Gap are affected. The amount varies over the circumference because Daphnis has both a nonzero eccentricity and a nonzero inclination. The waves are created by the way it moves slightly inwards and outwards in the gap, and the way it moves slightly above and below the gap. (Today's image is a fairly oblique viewpoint; the gap is about five times the width of the moon.)
_

[orin stepanek]

To me it looks looks like a huge bolder in a river near a beach; and a desert with sand dunes!

<<Stone skipping (or stone skimming) is the art of throwing a flat stone across water in such a way that it bounces off the surface, preferably many times. The objective of the game is to see how many times a stone can bounce before sinking.

An early explanation of the physics of stone-skipping was provided by Lazzaro Spallanzani in the 18th century.

The stone generates lift in the same manner as a flying disc, by pushing water down as it moves across the water at an angle. Surface tension has very little to do with it. The stone's rotation acts to stabilize it against the torque of lift being applied to the back.

Research undertaken by a team led by French physicist Lydéric Bocquet discovered that an angle of about 20° between the stone and the water's surface is optimal. Bocquet and his colleagues were surprised to discover that changes in speed and rotation did not change this fact, it just allowed the stone to be in balance and to continue with a straight and uniform movement, due to gyroscopic effect. Work by Hewitt, Balmforth and McElwaine has shown that if the horizontal speed can be maintained skipping can continue indefinitely. Earlier research reported by Bocquet calculated that the world record of 38 rebounds set by Coleman-McGhee, unchallenged for many years, required a speed of 12 m/s, with a rotation of 14 revolutions per second.>>

Interesting; When I was a kid; I could get a stone to skip about 3 times! Of course; I never had any Physicists study my techniques; so I never improved Actually; I never knew an interest in stone skipping even existed! I guess it isn't rocket science!

[owlice]

This image: kaPOW!!! Knocks me right out!!

[TheOtherBruce]

Interestingly the ridges show different angles. Looks like this moons orbit and axis changed a lot.

It's probably tidally locked to Saturn (just checked the Wiki page, yes it is) and I don't think there's anything in the theory that says this has to be a smooth progression down to zero. It's likely that, as the rotation got close to stopped, the moonlet had a bit of random tumbling. The larger inner moons of Saturn might have contributed to this; their gravitational effect is small, but it's there. Doesn't have to be much, just enough to give it more than one axis relative to the ring plane over a timescale of millions of years. Once the lock was complete, there would have been plenty of time for a sprinkling of ring particles to build up into the biggest ridge we see today.

Complicating things, remember that even the best known example, Earth's Moon, isn't completely one-side-only to Earth; it's orbit isn't perfectly circular, so it seems to rotate just a little bit back and forth every month. Daphnis has a much more circular orbit, but it's still not quite e=0.

[Chris Peterson]

Interestingly the ridges show different angles. Looks like this moons orbit and axis changed a lot.

It's probably tidally locked to Saturn (just checked the Wiki page, yes it is) and I don't think there's anything in the theory that says this has to be a smooth progression down to zero. It's likely that, as the rotation got close to stopped, the moonlet had a bit of random tumbling. The larger inner moons of Saturn might have contributed to this; their gravitational effect is small, but it's there. Doesn't have to be much, just enough to give it more than one axis relative to the ring plane over a timescale of millions of years. Once the lock was complete, there would have been plenty of time for a sprinkling of ring particles to build up into the biggest ridge we see today.

Complicating things, remember that even the best known example, Earth's Moon, isn't completely one-side-only to Earth; it's orbit isn't perfectly circular, so it seems to rotate just a little bit back and forth every month. Daphnis has a much more circular orbit, but it's still not quite e=0.

These structures form as the moon is passing, and do not persist. They are not structures that have been in place for a long period of time. They result because the orbit isn't perfectly circular and isn't perfectly in the plane of the rings.

[TheOtherBruce]

These structures form as the moon is passing, and do not persist. They are not structures that have been in place for a long period of time. They result because the orbit isn't perfectly circular and isn't perfectly in the plane of the rings.

I was talking about the ridge features on Daphnis, not the "wave" features on the ring edges.

[Chris Peterson]

These structures form as the moon is passing, and do not persist. They are not structures that have been in place for a long period of time. They result because the orbit isn't perfectly circular and isn't perfectly in the plane of the rings.

I was talking about the ridge features on Daphnis, not the "wave" features on the ring edges.

Ah. Missed that. It may be more of a rubble pile than a very solid, rigid body.

[JohnD]

I'll be grateful for an answer to why the other ring isn't affected.

Both edges of the Keeler Gap are affected. The amount varies over the circumference because Daphnis has both a nonzero eccentricity and a nonzero inclination. The waves are created by the way it moves slightly inwards and outwards in the gap, and the way it moves slightly above and below the gap. (Today's image is a fairly oblique viewpoint; the gap is about five times the width of the moon.)
_
PIA08319_Daphnis_in_Keeler_Gap.jpg

Thank you, Chris!
JOhn

[neufer]

I was talking about the ridge features on Daphnis...

Ah. Missed that. It may be more of a rubble pile than a very solid, rigid body.

I picture the ridge as more of a snow arête.