APOD: A Season of Saturn (2023 Aug 25)

[APOD Robot]

A Season of Saturn

Explanation: Ringed planet Saturn will be at its 2023 opposition, opposite the Sun in Earth's skies, on August 27. While that puts the sixth planet from the Sun at its brightest and well-placed for viewing, its beautiful ring system isn't visible to the unaided eye. Still, this sequence of telescopic images taken a year apart over the last six years follows both Saturn and rings as seen from inner planet Earth. The gas giant's ring plane tilts from most open in 2018 to approaching edge-on in 2023 (top to bottom). That's summer to nearly the autumn equinox for Saturn's northern hemisphere. In the sharp planetary portraits, Saturn's northern hexagon and a large storm system are clearly visible in 2018. In 2023, ice moon Tethys is transiting, casting its shadow across southern hemisphere cloud bands, while Saturn's cold blue south pole is emerging from almost a decade of winter darkness.

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[Ann]

A Season of Saturn. Image Credit & Copyright: Andy Casely

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Interesting!

Saturn clearly looked its best (from the Earth's perspective, at top left) back in 2018, when its ring plane was most open. A fascinating storm could also be seen raging close to the north pole in 2018. But I also really like the 2019 image (second from top), when the north pole hexagon was strikingly visible, and one of Saturn's cloudtop belts was particularly bright.

There is nothing for it, Saturn looks less interesting when its ring system is more "closed" (as in the 2023 picture at bottom right).

This reminds me of the Voyager 1 pictures of Jupiter and Saturn. Voyager 1 reached Jupiter on March 5, 1979, and soon afterwards NASA began publishing pictures of Jupiter taken by Voyager 1. They looked something like this:

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Oh wow! I was ecstatic! Those Jupiter images were so so beautiful! They were amazingly colorful and totally gorgeous!

Yeah, well... NASA had processed the images so as to make Jupiter's colors too saturated. Because let's face it, Jupiter doesn't look like this. So later NASA quietly tried to back away from its colorism version of Jupiter and started watering down its hues - with some surprising and unattractive results! You sure haven't heard of the Great Yellow Spot of Jupiter, have you? Well, here it is:

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Yes, that's Jupiter and its Great Yellow Spot, in a "color-corrected" version of the flamboyant NASA 1979 chromatic splendor!

Anyway. This is supposed to be about Saturn. After NASA's color-fest at Jupiter, I most eagerly awaited its upcoming Saturn pictures. Saturn would look amazing too when photographed by Voyager 1, right? Right? Right?

But when the Saturn pictures arrived in November 1980, they looked like this:

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To say that I was deflated is putting it mildly! Look at that almost featureless boring yellow cloud cover of Saturn! And look at those ugly dark brown rings! (I was too upset to even notice orange Titan at upper right.)

NASA tried to make amends by dressing up Saturn in various cloud outfits that would have made the sixth planet fit for a Pride parade:

Click to view full size image

Click to view full size image

Click to view full size image

Click to view full size image

I had been so delighted by NASA's Voyager i pictures of Jupiter, but I was so terribly disappointed at its pictures of Saturn. And while I couldn't see through the color-primping of the Jupiter images, I certainly knew that NASA was "lying" to me about the colors of Saturn! I was furious!


Saturn is everyone's favorite planet. I like it too, these days, but it took me a long time to get over the Voyager pictures of it. Of course, when you have seen Cassini images of Saturn, how can you dislike the King of the Rings?

Saturn's icy moon Dione floats in front of the giant planet in this view from NASA's Cassini spacecraft on Sept. 22, 2005. The straight line is the ring plane. Credit: NASA/JPL/Space Science Institute

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On July 19, 2013, in an event celebrated the world over, NASA's Cassini spacecraft slipped into Saturn's shadow and turned to image the planet, seven of its moons, its inner rings -- and, in the background, our home planet, Earth. Image Credit: NASA/JPL-Caltech/SSI

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Saturn is a planet of so very many faces. But some of them do look better than others.

Ann

[Roy]

Lots of interesting facts I didn’t know on Britannica . They calculate the level on gas giants, at which pressure is one Bar, and use that as the putative surface for other calculations. Anyway, Saturn has a ten and a half hour day, an equatorial gravity of about two and one eighth g, and a mean density of 0.7 g/cc for the whole planet. It is therefore visibly oblate.
It has a very small moon, Methone, smooth and egg-shaped, which would serve as a Sci-fi large alien space station for UFO observers.

[dergolem]

I really appreciate the dedication of the photographer, Andy Casely. Taking that photo yearly lets us see the wonder of Saturn as it moves through space. This is the real thing, not an artist's interpretation or animation. What Galileo saw 413 years ago as fuzzy blobs are majestic rings. What might he have thought of that hexagon at the north pole and all the moons. A big thank you to the APOD author who put the photo in perspective, reminding me to always look at every post carefully. A decade of winter, a decade of summer. The universe is indeed awesome.

[Chris Peterson]

Lots of interesting facts I didn’t know on Britannica . They calculate the level on gas giants, at which pressure is one Bar, and use that as the putative surface for other calculations. Anyway, Saturn has a ten and a half hour day, an equatorial gravity of about two and one eighth g, and a mean density of 0.7 g/cc for the whole planet. It is therefore visibly oblate.
It has a very small moon, Methone, smooth and egg-shaped, which would serve as a Sci-fi large alien space station for UFO observers.

Saturn is the only planet that would float in water, if you had a big enough bucket.

[orin stepanek]

I like Saturn for its rings! Only Earth is more showy!

[johnnydeep]

Lots of interesting facts I didn’t know on Britannica . They calculate the level on gas giants, at which pressure is one Bar, and use that as the putative surface for other calculations. Anyway, Saturn has a ten and a half hour day, an equatorial gravity of about two and one eighth g, and a mean density of 0.7 g/cc for the whole planet. It is therefore visibly oblate.
It has a very small moon, Methone, smooth and egg-shaped, which would serve as a Sci-fi large alien space station for UFO observers.

Therefore oblate? What facts of those you listed would imply that?

[johnnydeep]

So, the northern hexagon is clearly visible in the first few pics. But these images are from a small Earth based telescope, are they not? I thought the hexagon was only discovered recently by Cassini (or some other probe)? Is it only a very recent phenomenon?

[johnnydeep]

Tethys and its shadow I presume:

[Ann]

Lots of interesting facts I didn’t know on Britannica . They calculate the level on gas giants, at which pressure is one Bar, and use that as the putative surface for other calculations. Anyway, Saturn has a ten and a half hour day, an equatorial gravity of about two and one eighth g, and a mean density of 0.7 g/cc for the whole planet. It is therefore visibly oblate.
It has a very small moon, Methone, smooth and egg-shaped, which would serve as a Sci-fi large alien space station for UFO observers.

Saturn is the only planet that would float in water, if you had a big enough bucket.

And yet Saturn has a big rocky and metallic (if diffuse) core:

Credit: Kelvinsong

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Wikipedia wrote:

In 2004, scientists estimated that the core must be 9–22 times the mass of Earth, which corresponds to a diameter of about 25,000 km. However, measurements of Saturn's rings suggest a much more diffuse core with a mass equal to about 17 Earths and a radius equal to around 60% of Saturn's entire radius.

Imagine the Earth floating in water:

Click to view full size image

It would take a very big bucket of water indeed to test the buoyancy of the Earth (though not as big as the bucket that could hold Saturn). I'd say the Earth hasn't passed its floating test, so it would need a really big lifesaver.

Credit GWEN SHOCKEY / SCIENCE PHOTO LIBRARY

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Ann

[pferkul]

You get a neat 3-D effect if you relax your eyes and focus on a more distant spot, like what you do for stereoscopic image pairs.

Click below to give it a try. It helps if you adjust your image size so that adjacent Saturns appear about 5-6 cm apart on your screen.

Click to view full size image

[johnnydeep]

You get a neat 3-D effect if you relax your eyes and focus on a more distant spot, like what you do for stereoscopic image pairs:

Click to view full size image

Sort of. But that also gives me headache.

( PS - your image link to SeasonSaturnapodacasely-rotated.jpg is broken. )

[pferkul]

Sort of. But that also gives me headache.

It helps if you adjust your image size so that adjacent Saturns appear about 5-6 cm apart on your screen.

[Roy]

Lots of interesting facts I didn’t know on Britannica . They calculate the level on gas giants, at which pressure is one Bar, and use that as the putative surface for other calculations. Anyway, Saturn has a ten and a half hour day, an equatorial gravity of about two and one eighth g, and a mean density of 0.7 g/cc for the whole planet. It is therefore visibly oblate.
It has a very small moon, Methone, smooth and egg-shaped, which would serve as a Sci-fi large alien space station for UFO observers.

Therefore oblate? What facts of those you listed would imply that?

Very large planet, less dense than water, (hydrogen & helium) rotating in such high spin rate, the centrifugal forces must be very high. Those facts.

Another thing: the Wikipedia article doesn’t make sense to me. How can measuring the diameter of the rings lead to a surmise that the core is less dense? All the descriptions of Saturn’s core are conjecture. No one knows if hydrogen ice is possible, not to mention helium ice, both under enormous pressure at unknown temperatures.

[Chris Peterson]

Lots of interesting facts I didn’t know on Britannica . They calculate the level on gas giants, at which pressure is one Bar, and use that as the putative surface for other calculations. Anyway, Saturn has a ten and a half hour day, an equatorial gravity of about two and one eighth g, and a mean density of 0.7 g/cc for the whole planet. It is therefore visibly oblate.
It has a very small moon, Methone, smooth and egg-shaped, which would serve as a Sci-fi large alien space station for UFO observers.

Therefore oblate? What facts of those you listed would imply that?

Very large planet, less dense than water, (hydrogen & helium) rotating in such high spin rate, the centrifugal forces must be very high. Those facts.

Another thing: the Wikipedia article doesn’t make sense to me. How can measuring the diameter of the rings lead to a surmise that the core is less dense? All the descriptions of Saturn’s core are conjecture. No one knows if hydrogen ice is possible, not to mention helium ice, both under enormous pressure at unknown temperatures.

Did you follow the footnoted references to the associated papers?

[johnnydeep]

Lots of interesting facts I didn’t know on Britannica . They calculate the level on gas giants, at which pressure is one Bar, and use that as the putative surface for other calculations. Anyway, Saturn has a ten and a half hour day, an equatorial gravity of about two and one eighth g, and a mean density of 0.7 g/cc for the whole planet. It is therefore visibly oblate.
It has a very small moon, Methone, smooth and egg-shaped, which would serve as a Sci-fi large alien space station for UFO observers.

Therefore oblate? What facts of those you listed would imply that?

Very large planet, less dense than water, (hydrogen & helium) rotating in such high spin rate, the centrifugal forces must be very high. Those facts.

I see, that makes sense! Thanks.

Another thing: the Wikipedia article doesn’t make sense to me. How can measuring the diameter of the rings lead to a surmise that the core is less dense? All the descriptions of Saturn’s core are conjecture. No one knows if hydrogen ice is possible, not to mention helium ice, both under enormous pressure at unknown temperatures.

Yeah, that didn't make much sense to me either, but the referenced paper presumably explains it. But I only have access to the abstract:

Abstract
The best constraints on the internal structures of giant planets have historically originated from measurements of their gravity fields1,2,3. These data are inherently mostly sensitive to a planet’s outer regions, stymieing efforts to measure the mass and compactness of the cores of Jupiter2,4,5 and Saturn6,7. However, studies of Saturn’s rings have detected waves driven by pulsation modes within the planet8,9,10,11, offering independent seismic probes of Saturn’s interior12,13,14. The observations reveal gravity-mode pulsations, which indicate that part of Saturn’s deep interior is stable against convection13. Here, we compare structural models with gravity and seismic measurements from Cassini to show that the data can only be explained by a diffuse, stably stratified core–envelope transition region in Saturn extending to approximately 60% of the planet’s radius and containing approximately 17 Earth masses of ice and rock. This gradual distribution of heavy elements constrains mixing processes at work in Saturn, and it may reflect the planet’s primordial structure and accretion history.

[johnnydeep]

Sort of. But that also gives me headache.

It helps if you adjust your image size so that adjacent Saturns appear about 5-6 cm apart on your screen.

Thanks! That worked. And I even had a hard time refocusing my eyes back to normal again.

[Chris Peterson]

Sort of. But that also gives me headache. :ssmile:

It helps if you adjust your image size so that adjacent Saturns appear about 5-6 cm apart on your screen.

Thanks! That worked. And I even had a hard time refocusing my eyes back to normal again. :ssmile:

Your mama warned you about those crossed eyes. Keep doing that and they'll be stuck that way forever!

[Ann]

Therefore oblate? What facts of those you listed would imply that?

Very large planet, less dense than water, (hydrogen & helium) rotating in such high spin rate, the centrifugal forces must be very high. Those facts.

I see, that makes sense! Thanks.

Another thing: the Wikipedia article doesn’t make sense to me. How can measuring the diameter of the rings lead to a surmise that the core is less dense? All the descriptions of Saturn’s core are conjecture. No one knows if hydrogen ice is possible, not to mention helium ice, both under enormous pressure at unknown temperatures.

Yeah, that didn't make much sense to me either, but the referenced paper presumably explains it. But I only have access to the abstract:

Abstract
The best constraints on the internal structures of giant planets have historically originated from measurements of their gravity fields1,2,3. These data are inherently mostly sensitive to a planet’s outer regions, stymieing efforts to measure the mass and compactness of the cores of Jupiter2,4,5 and Saturn6,7. However, studies of Saturn’s rings have detected waves driven by pulsation modes within the planet8,9,10,11, offering independent seismic probes of Saturn’s interior12,13,14. The observations reveal gravity-mode pulsations, which indicate that part of Saturn’s deep interior is stable against convection13. Here, we compare structural models with gravity and seismic measurements from Cassini to show that the data can only be explained by a diffuse, stably stratified core–envelope transition region in Saturn extending to approximately 60% of the planet’s radius and containing approximately 17 Earth masses of ice and rock. This gradual distribution of heavy elements constrains mixing processes at work in Saturn, and it may reflect the planet’s primordial structure and accretion history.

The information in the abstract is the kind of information that I expect I will be able to digest. I know that seismic waves are used to probe the interior of the Earth, and also the interior of the Sun, if I am not mistaken. The abstract told us (or so I think) that the rings of Saturn function as an "extension" of the "medium" through which seismic waves can pass in Saturn, and the waves in the rings can therefore be used to constrain the nature of the interior of Saturn. In the abstract, it is said that part of Saturn's deep interior is stable against convection (= mixing), i.e., the deep interior of Saturn appears to be a solid core.

Apparently convection processes are more and more constrained closer and closer to the core. Clearly there is more and more ice and rock mixed with gases the deeper one gets into the interior of Saturn, which is what is meant by a diffuse core. But the inner regions are also stably stratified.

I loved the suggestion that the distribution of heavy elements of Saturn may reflect the planet's primordial structure and accretion history.

Primordial pre-ring and gas Saturn? The illustration actually shows the largest rocky planet found so far, BD+20594b. Credit: JPL/Caltech.

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Ann

[johnnydeep]

Very large planet, less dense than water, (hydrogen & helium) rotating in such high spin rate, the centrifugal forces must be very high. Those facts.

I see, that makes sense! Thanks.

Another thing: the Wikipedia article doesn’t make sense to me. How can measuring the diameter of the rings lead to a surmise that the core is less dense? All the descriptions of Saturn’s core are conjecture. No one knows if hydrogen ice is possible, not to mention helium ice, both under enormous pressure at unknown temperatures.

Yeah, that didn't make much sense to me either, but the referenced paper presumably explains it. But I only have access to the abstract:

Abstract
The best constraints on the internal structures of giant planets have historically originated from measurements of their gravity fields1,2,3. These data are inherently mostly sensitive to a planet’s outer regions, stymieing efforts to measure the mass and compactness of the cores of Jupiter2,4,5 and Saturn6,7. However, studies of Saturn’s rings have detected waves driven by pulsation modes within the planet8,9,10,11, offering independent seismic probes of Saturn’s interior12,13,14. The observations reveal gravity-mode pulsations, which indicate that part of Saturn’s deep interior is stable against convection13. Here, we compare structural models with gravity and seismic measurements from Cassini to show that the data can only be explained by a diffuse, stably stratified core–envelope transition region in Saturn extending to approximately 60% of the planet’s radius and containing approximately 17 Earth masses of ice and rock. This gradual distribution of heavy elements constrains mixing processes at work in Saturn, and it may reflect the planet’s primordial structure and accretion history.

The information in the abstract is the kind of information that I expect I will be able to digest. I know that seismic waves are used to probe the interior of the Earth, and also the interior of the Sun, if I am not mistaken. The abstract told us (or so I think) that the rings of Saturn function as an "extension" of the "medium" through which seismic waves can pass in Saturn, and the waves in the rings can therefore be used to constrain the nature of the interior of Saturn. In the abstract, it is said that part of Saturn's deep interior is stable against convection (= mixing), i.e., the deep interior of Saturn appears to be a solid core.

Apparently convection processes are more and more constrained closer and closer to the core. Clearly there is more and more ice and rock mixed with gases the deeper one gets into the interior of Saturn, which is what is meant by a diffuse core. But the inner regions are also stably stratified.

I loved the suggestion that the distribution of heavy elements of Saturn may reflect the planet's primordial structure and accretion history.
...

Ann

So my understanding is that seismic waves are acoustic, and we measure them on Earth directly by sensors - seismometers - in the ground. And presumably the Sun's interior seismic activity can be measured indirectly by measuring fluctuations in the Sun's external gravitational field. Similarly, apparently waves - optically observed density waves? - in Saturn's rings were best - or partially? - explained by gravity waves that are best explained by "gravity-mode pulsations" originating in, and consistent with, Saturn having a large diffuse core.

[Chris Peterson]

So my understanding is that seismic waves are acoustic, and we measure them on Earth directly by sensors - seismometers - in the ground. And presumably the Sun's interior seismic activity can be measured indirectly by measuring fluctuations in the Sun's external gravitational field. Similarly, apparently waves - optically observed density waves? - in Saturn's rings were best - or partially? - explained by gravity waves that are best explained by "gravity-mode pulsations" originating in, and consistent with, Saturn having a large diffuse core.

Solar seismic waves are measured by direct observation of line-of-sight surface motion via Doppler shift.

[johnnydeep]

So my understanding is that seismic waves are acoustic, and we measure them on Earth directly by sensors - seismometers - in the ground. And presumably the Sun's interior seismic activity can be measured indirectly by measuring fluctuations in the Sun's external gravitational field. Similarly, apparently waves - optically observed density waves? - in Saturn's rings were best - or partially? - explained by gravity waves that are best explained by "gravity-mode pulsations" originating in, and consistent with, Saturn having a large diffuse core.

Solar seismic waves are measured by direct observation of line-of-sight surface motion via Doppler shift.

Cool! Did I get the rest of my superficial explanation about Saturn right?

[Chris Peterson]

So my understanding is that seismic waves are acoustic, and we measure them on Earth directly by sensors - seismometers - in the ground. And presumably the Sun's interior seismic activity can be measured indirectly by measuring fluctuations in the Sun's external gravitational field. Similarly, apparently waves - optically observed density waves? - in Saturn's rings were best - or partially? - explained by gravity waves that are best explained by "gravity-mode pulsations" originating in, and consistent with, Saturn having a large diffuse core.

Solar seismic waves are measured by direct observation of line-of-sight surface motion via Doppler shift.

Cool! Did I get the rest of my superficial explanation about Saturn right?

Don't know. Sounds reasonable. I think the important point is that the observations are optical.

[johnnydeep]

Solar seismic waves are measured by direct observation of line-of-sight surface motion via Doppler shift.

Cool! Did I get the rest of my superficial explanation about Saturn right?

Don't know. Sounds reasonable. I think the important point is that the observations are optical.

✓

[andyc]

Thanks for the kind words everyone, it's an honour to get a second APOD
@Ann, some very interesting info there - colour can be a thorny challenge in imaging and processing (the rings seem notably yellower in Cassini data than they do in amateur images for example, and clearly was a challenge for the space missions back in the day.
@dergolem, you're very welcome!
@johnnydeep the hexagon is relatively straightforward to image with modern techniques and equipment, but it was only discovered by Voyager in 1981. As only the north pole has a hexagon, and it's only in good view for maybe eight years of a Saturn orbit (roughly 2013-2020 on Chris Go's image archive for example), it doesn't come around so often. The last time it would've been well-placed was in the pre-digital imaging age (~1982-1990), when detail at Saturn's high latitudes would've been pretty hard to capture even for major observatories. Before Voyager, it would have been before about 1960 that it was possible to see it, and I guess resolving the hexagonal characteristic it was beyond the technology and observing skill of that era. It may not be permanent, but I suspect it has been around for longer than we've been able to make it out. We're so privileged to be able to do what we can now.
@johnnydeep yes, that is Tethys and its shadow - visible on captures in all channels (red, green, blue) that morning which was pretty unusual for me.
@pferkul - that is really neat! I'm a fan of cross-eye 3-D effects, and that's very clever, if a little mind-bending.