Did you see this? It's an artist's conception of how gravity from the tiny moon Daphnis creates ripples in Saturn's rings - created by Kevin Gill of NASA.
This image was pretty popular here, and elsewhere on the web - but people often don't come out and say from the start that it's not a photo. The actual photos are less beautiful but... hey, they're real! And the ripples look different in the photos. Let's take a look.
(1/n)
This 2005 photo, taken by the Cassini probe, was the first time anyone actually saw Saturn's moon Daphnis! It's only 8 kilometers across.
This gap in Saturn's A ring was first discovered by Voyager, and it was named the Keeler Gap. It's 35 kilometers wide. I guess this gap let people guess the existence of a moon, and later the ripples in the A ring let people guess where the moon must be! I don't really know the history here.
(2/n)
There's a larger gap in the A ring called the Encke cap, created by a larger moon called Pan, which you can see clearly here.
To the left you see the smaller Keeler gap. If you look very closely you can see the ripples near the Keeler gap... and if you look *very* closely you can see, or at least imagine, the moon Daphnis.
(3/n)
Now here is a really *great* actual photo of Daphnis and the ripples it creates in Saturn's rings!
It was taken by the Cassini probe and released in February 2017. It was taken in visible light using Cassini’s narrow-angle camera. Cassini was 28,000 kilometers away from Daphnis, and the image scale is 168 meters per pixel.
https://www.esa.int/About_Us/ESAC/Saturn_s_moon_Daphnis_in_the_Keeler_Gap
What other really good photos can we find?
(4/n)
Here's a nice photo of Daphnis in the Keeler gap in real color! It was taken by Cassini on July 5, 2010 - taken in red, green, and blue and then recombined.
https://www.planetary.org/space-images/daphnis-in-keeler-gap
(5/n)
And here's an excellent image of Daphnis taken by the Cassini spacecraft on one of its ring-grazing passes on January 16, 2017 - the closest to Daphnis it's gotten so far, I believe!
NASA says:
"Material on the inner edge of the gap orbits faster than the moon, so the waves there lead the moon in its orbit. Material on the outer edge moves slower than the moon, so waves there trail the moon. The waves Daphnis causes cast shadows on Saturn during its equinox when the sun is in line with the plane of the rings."
(5/n)
Finally, here's a really crazy picture by Kevin Gill - a view you could only see if you sailed through the Keeler gap!
Someday I hope humanity does this.
You can see more images by Kevin Gill here:
https://www.flickr.com/photos/kevinmgill/
(6/n, n = 6)
Explore Kevin Gill’s 9,991 photos on Flickr!
@johncarlosbaez A cruise around the Rings of Saturn is a bit more interesting that Katy Perry's 10 minute hop into 'almost Space'
@johncarlosbaez
Great pictures, interesting thread with interesting commentary from readers and you responding, and you provided links to interesting material.
I don't usually say such things, but today I get the impression that you *do* like such feedback instead of your readers being briskly business-like and terse.
@johncarlosbaez
Ah; I should have guessed that. Maybe I misunderstood because *I* wildly overestimated my expertise in some interaction with you, and suffered the consequences. I don't recall such a thing specifically, but it's believable! 🙂
I will try to make a point of giving you (friendly and hopefully intelligent) feedback from now on.
@johncarlosbaez There's a connection with Ford circles too. If you have one moon then you get gaps when the orbital period of the debris is a rational fraction of the moon's. Simpler fractions making larger gaps, so the base of the larger circles are the larger gaps.
So the thickest rings should be the periods hardest to approximate by a rational multiple of the moon's, such as phi. They're least resonant.
Orbital radius having a simple relationship with orbital period.
@penguin42 - btw, Kevin Gill doesn't just do space art, he also creates photographs for NASA. (It takes work to make them look good.) Check out his Flickr account!
Explore Kevin Gill’s 9,991 photos on Flickr!
I remember a physics colloquium in the 80s (maybe the late 70s?! prob early 80s, though) when we saw some of the first images of the sheer *complexity* of the rings, and how much is going on in the "gaps".
That was the first time I heard the phrase "shepherd moon". Nobody, as I recall, predicted them. But the moment you saw them, it was clear they were inducing waves and even braids.
Absolutely gorgeous.
(Though a quick search didn't find any images of braided rings, so maybe that memory is unreliable.)
@weekend_editor - I could spend a lifetime studying the rings of Saturn! So rich in physics structure!
Here NASA says the F ring is "braided", though it's hard to see that.
@xgebi - you're right, I found some truly beautiful ones. I would still love one taken from the viewpoint of the artist's conception, but I don't think Cassini will get that close to the rings.
It's like the difference between an actual photo of someone you love and an AI-generated picture. Reality is better because it's real.
@kristinHenry - yes, it's great how starting by seeing a mysterious gap in the A ring and trying to figure it out, people eventually built a space probe that could see what's going on in gorgeous detail. And someday I hope we see it even better.
Look at that little faint trail of white material near Daphnis!
@benjohn - I'm pretty sure that Kevin Gill's images were *not* generated from simulations. Here's the only paper I've found on modeling this sort of system:
https://arxiv.org/abs/2403.03012
The math and physics are tough but some of the pictures are great!
Observations by the Voyager and Cassini spacecrafts have revealed various striking features of the gap structure in Saturn's ring, such as the density waves, sharp edge, and vertical wall structure. In order to explain these features in a single simulation, we perform a high-resolution (N~10^6-10^7) global full N-body simulation of gap formation by an embedded satellite considering gravitational interactions and inelastic collisions among all ring particles and the satellite, while these features have been mostly investigated separately with different theoretical approaches: the streamline models, 1D diffusion models, and local N-body simulation. As a first attempt of a series of papers, we here focus on the gap formation by separating satellite migration with fixing the satellite orbit in a Keplerian circular orbit. We reveal how the striking gap features - the density waves, sharp edge, and vertical wall structure - are simultaneously formed by an interplay of the satellite-ring and ring particle-particle interactions. In particular, we propose a new mechanism to quantitatively explain the creation of the vertical wall structure at the gap edge. Inelastic collisions between ring particles damp their eccentricity excited by the satellite's perturbations to enhance the surface density at the gap edge, making its sharp edges more pronounced. We find the eccentricity damping process inevitably raises the vertical wall structures the most effectively in the second epicycle waves. Particle-particle collisions generally convert their lateral epicyclic motion into vertical motion. Because the excited epicyclic motion is the greatest near the ring edge and the epicycle motions are aligned in the first waves, the conversion is the most efficient in the gap edge of the second waves and the wall height is scaled by the satellite Hill radius, which are consistent with the observations.
I did see this! But, I didn't know if it was a reconstruction and if it was one how accurate it was so I didn't share it. I thought "I ought to find out if that really happens..." so I'm delighted to see this thread.
Sometimes cool things really do exist!
are there clues here about how we could shepherd our Low Earth Orbit debris?
@benh - great question! I bet some experts on celestial dynamics and/or spacecraft could tell us if people have thought about that.
It may be that gravitational shepherding is a rather slow process and people are too impatient to use that method.
www.universetoday.com/130351/new-jpl-visualization-waves-...
@Yuras - you're right, the inner rings rotate faster.
You wrote: "Why ripples dissipate though?"
Great question! I'm a physicist but the answer is not obvious to me. It would require actual calculations, I think. People do such calculations:
Observations by the Voyager and Cassini spacecrafts have revealed various striking features of the gap structure in Saturn's ring, such as the density waves, sharp edge, and vertical wall structure. In order to explain these features in a single simulation, we perform a high-resolution (N~10^6-10^7) global full N-body simulation of gap formation by an embedded satellite considering gravitational interactions and inelastic collisions among all ring particles and the satellite, while these features have been mostly investigated separately with different theoretical approaches: the streamline models, 1D diffusion models, and local N-body simulation. As a first attempt of a series of papers, we here focus on the gap formation by separating satellite migration with fixing the satellite orbit in a Keplerian circular orbit. We reveal how the striking gap features - the density waves, sharp edge, and vertical wall structure - are simultaneously formed by an interplay of the satellite-ring and ring particle-particle interactions. In particular, we propose a new mechanism to quantitatively explain the creation of the vertical wall structure at the gap edge. Inelastic collisions between ring particles damp their eccentricity excited by the satellite's perturbations to enhance the surface density at the gap edge, making its sharp edges more pronounced. We find the eccentricity damping process inevitably raises the vertical wall structures the most effectively in the second epicycle waves. Particle-particle collisions generally convert their lateral epicyclic motion into vertical motion. Because the excited epicyclic motion is the greatest near the ring edge and the epicycle motions are aligned in the first waves, the conversion is the most efficient in the gap edge of the second waves and the wall height is scaled by the satellite Hill radius, which are consistent with the observations.
@duncan_blues - I don't know how well the calculations match the observations, but people are working on it:
Observations by the Voyager and Cassini spacecrafts have revealed various striking features of the gap structure in Saturn's ring, such as the density waves, sharp edge, and vertical wall structure. In order to explain these features in a single simulation, we perform a high-resolution (N~10^6-10^7) global full N-body simulation of gap formation by an embedded satellite considering gravitational interactions and inelastic collisions among all ring particles and the satellite, while these features have been mostly investigated separately with different theoretical approaches: the streamline models, 1D diffusion models, and local N-body simulation. As a first attempt of a series of papers, we here focus on the gap formation by separating satellite migration with fixing the satellite orbit in a Keplerian circular orbit. We reveal how the striking gap features - the density waves, sharp edge, and vertical wall structure - are simultaneously formed by an interplay of the satellite-ring and ring particle-particle interactions. In particular, we propose a new mechanism to quantitatively explain the creation of the vertical wall structure at the gap edge. Inelastic collisions between ring particles damp their eccentricity excited by the satellite's perturbations to enhance the surface density at the gap edge, making its sharp edges more pronounced. We find the eccentricity damping process inevitably raises the vertical wall structures the most effectively in the second epicycle waves. Particle-particle collisions generally convert their lateral epicyclic motion into vertical motion. Because the excited epicyclic motion is the greatest near the ring edge and the epicycle motions are aligned in the first waves, the conversion is the most efficient in the gap edge of the second waves and the wall height is scaled by the satellite Hill radius, which are consistent with the observations.
The first and second (5/n) were my favorites. I love real ;)
Althoug (6/n) is great because you can actually feel the movement as in a dance.
This is a really nice thread. Thank you very much für sharing it with us!
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