APOD: Saturn from Above (2013 Oct 21)

Comments and questions about the APOD on the main view screen.
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APOD: Saturn from Above (2013 Oct 21)

Postby APOD Robot » Mon Oct 21, 2013 4:07 am

Image Saturn from Above

Explanation: This image of Saturn could not have been taken from Earth. No Earth based picture could possibly view the night side of Saturn and the corresponding shadow cast across Saturn's rings. Since Earth is much closer to the Sun than Saturn, only the day side of the ringed planet is visible from the Earth. In fact, this image mosaic was taken earlier this month by the robotic Cassini spacecraft now orbiting Saturn. The beautiful rings of Saturn are seen in full expanse, while cloud details are visible including the polar hexagon surrounding the north pole, and an extended light-colored storm system.

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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Beyond » Mon Oct 21, 2013 4:13 am

Yup! Saturn looks good from any angle. But it's especially nice when you can also see the north polar hexagon, even if it is a bit faint. :yes: :thumb_up:
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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Nitpicker » Mon Oct 21, 2013 4:22 am

That's just beautiful. Thank you APOD.

Is that "ringshine" I see on the far side of the planet? Fantastic!

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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Mactavish » Mon Oct 21, 2013 5:29 am

Another spectacular image from Cassini. What a magnificent performance over the past nine years!

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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Nitpicker » Mon Oct 21, 2013 6:38 am

I think I see three little moons near the outer edge of the rings along image top.

Can anyone confirm this and identify them?

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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Nitpicker » Mon Oct 21, 2013 6:50 am

Nitpicker wrote:I think I see three little moons near the outer edge of the rings along image top.

Can anyone confirm this and identify them?


To answer my own question without confirming anything, I'd guess that from left to right, we can see either Janus or Epimetheus (not sure which), then Pandora, then Prometheus.

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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Boomer12k » Mon Oct 21, 2013 8:57 am

Awesome!!!

Reminds me of the old Vinyl records....

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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Markus Schwarz » Mon Oct 21, 2013 9:00 am

Nitpicker wrote:That's just beautiful. Thank you APOD.

I second that! :yes:

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Re: APOD: Saturn from Above (2013 Oct 21)

Postby neufer » Mon Oct 21, 2013 10:40 am

Nitpicker wrote:That's just beautiful. Thank you APOD.

Is that "ringshine" I see on the far side of the planet? Fantastic!

Yep.
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Re: APOD: Saturn from Above (2013 Oct 21)

Postby wonderboy » Mon Oct 21, 2013 12:46 pm

I love the darkness that the shadow brings. you cant see a thing as no sunlight is refracting off of the ring fragments. awesome photo of saturn.
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Re: APOD: Saturn from Above (2013 Oct 21)

Postby gmPhil » Mon Oct 21, 2013 1:07 pm

Oh, wow... I'm in love!

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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Beyond » Mon Oct 21, 2013 1:56 pm

gmPhil wrote:Oh, wow... I'm in love!

Sorry, but Saturn is already spoken for. :mrgreen:
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Re: APOD: Saturn from Above (2013 Oct 21)

Postby rstevenson » Mon Oct 21, 2013 3:28 pm

Are there any moons visible in the image? So far I've found two small spots outside the rings, but I can't see anything I can unambiguously identify as a moon. Are they too small at this scale?

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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Chris Peterson » Mon Oct 21, 2013 3:37 pm

rstevenson wrote:Are there any moons visible in the image? So far I've found two small spots outside the rings, but I can't see anything I can unambiguously identify as a moon. Are they too small at this scale?

Size doesn't matter, except in terms of resolving objects. An image like this can show unresolved moons exactly as all our astrophotos show unresolved stars.

All that matters is that there's enough light coming from any moons to be above the noise floor of the camera. Indeed, it is generally easier to detect an unresolved body, since its light is spread out across fewer pixels.

Given the typical exposure times Cassini applies to Saturn, and the typical albedo of Saturn's moons, there is every reason to think that any moons captured in this field would be visible, whether or not they are optically resolved.
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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Anthony Barreiro » Mon Oct 21, 2013 5:12 pm

rstevenson wrote:Are there any moons visible in the image? So far I've found two small spots outside the rings, but I can't see anything I can unambiguously identify as a moon. Are they too small at this scale?

Rob

The pictures in this mosaic were taken on 10 October 2013. I took a look at Saturn on that date on Sky Safari. As the Nitpicker already said, from right to left across the top edge of the rings, you can see Prometheus just inside the F ring and Pandora just outside the F ring. He surmised that further to the left Janus or Epimetheus appears a little further outside the F-ring. But according to Sky Safari there's no moon anywhere near that location. I'm guessing that little speck of light might be a star. Magnitude 4.6 Iota Antlia could be the culprit.
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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Psnarf » Mon Oct 21, 2013 6:17 pm

Uh, WOW! We've come a long way during the past couple of decades.
Saturn somehow appears to be solid. Still brushing up on fluid dynamics to try and understand that wacky hexagonal flow going on near the pole. Perhaps the axis of rotation in the upper atmosphere does not coincide with the rotation of the core? Gotta be something gravitational, fluids don't behave that way...except on Saturn.

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Re: APOD: Saturn from Above (2013 Oct 21)

Postby geckzilla » Mon Oct 21, 2013 6:26 pm

Psnarf wrote:Uh, WOW! We've come a long way during the past couple of decades.
Saturn somehow appears to be solid. Still brushing up on fluid dynamics to try and understand that wacky hexagonal flow going on near the pole. Perhaps the axis of rotation in the upper atmosphere does not coincide with the rotation of the core? Gotta be something gravitational, fluids don't behave that way...except on Saturn.


There have been some experiments which were able to reproduce the hexagon in a lab here on Earth.
Click to play embedded YouTube video.
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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Chris Peterson » Mon Oct 21, 2013 6:38 pm

Psnarf wrote:Uh, WOW! We've come a long way during the past couple of decades.
Saturn somehow appears to be solid. Still brushing up on fluid dynamics to try and understand that wacky hexagonal flow going on near the pole. Perhaps the axis of rotation in the upper atmosphere does not coincide with the rotation of the core? Gotta be something gravitational, fluids don't behave that way...except on Saturn.

Actually, that is how fluids behave under certain conditions. This behavior has been modeled in vitro, and is even seen in the structure of Earth's polar air currents. It can be understood as a resonance phenomenon.
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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Anthony Barreiro » Mon Oct 21, 2013 6:55 pm

geckzilla wrote:There have been some experiments which were able to reproduce the hexagon in a lab here on Earth.
Click to play embedded YouTube video.

Here's the Science Magazine website article from which this video is excerpted.

Science Magazine wrote:Physicists Ana Claudia Barbosa Aguiar and Peter Read of the University of Oxford in the United Kingdom wanted to see if they could recreate the hexagon in the lab. They placed a 30-liter cylinder of water on a slowly spinning table; the water represented Saturn’s atmosphere spinning with the planet’s rotation. Inside this tank, they placed a small ring that whirled more rapidly than the cylinder. This created a miniature artificial "jet stream" that the researchers tracked with a green dye.

The faster the ring rotated, the less circular the green jet stream became. Small eddies formed along its edges, which slowly became larger and stronger and forced the fluid within the ring into the shape of a polygon. By altering the rate at which the ring spun, the scientists could generate various shapes. “We could create ovals, triangles, squares, almost anything you like,” says Read. The bigger the difference in the rotation between the planet and the jet steam—that is the cylinder and the ring—the fewer sides the polygon had, the team reports in this month's issue of Icarus. Barbosa Aguiar and Read suggest that Saturn’s north polar jet stream spins at a rate relative to the rest of the atmosphere that favors a six-sided figure, hence the hexagon.

Such polygonal formations have been observed in the center of major hurricanes on Earth, says Barbosa Aguiar, though they quickly dissipate. “Most planetary scientists are not aware of how ubiquitous these sorts of patterns are in fluid dynamics.”
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Re: APOD: Saturn from Above (2013 Oct 21)

Postby rstevenson » Mon Oct 21, 2013 8:47 pm

Anthony Barreiro wrote:
rstevenson wrote:Are there any moons visible in the image? So far I've found two small spots outside the rings, but I can't see anything I can unambiguously identify as a moon. Are they too small at this scale?

The pictures in this mosaic were taken on 10 October 2013. I took a look at Saturn on that date on Sky Safari. As the Nitpicker already said, from right to left across the top edge of the rings, you can see Prometheus just inside the F ring and Pandora just outside the F ring. He surmised that further to the left Janus or Epimetheus appears a little further outside the F-ring. But according to Sky Safari there's no moon anywhere near that location. I'm guessing that little speck of light might be a star. Magnitude 4.6 Iota Antlia could be the culprit.

Thanks Anthony, I can see them now. Two of them were the ones I mentioned, but didn't think of them as moons since they were outside the rings. Obviously I need to brush up on my Saturnian dynamics.

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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Nitpicker » Mon Oct 21, 2013 10:31 pm

Anthony Barreiro wrote:
rstevenson wrote:Are there any moons visible in the image? So far I've found two small spots outside the rings, but I can't see anything I can unambiguously identify as a moon. Are they too small at this scale?

Rob

The pictures in this mosaic were taken on 10 October 2013. I took a look at Saturn on that date on Sky Safari. As the Nitpicker already said, from right to left across the top edge of the rings, you can see Prometheus just inside the F ring and Pandora just outside the F ring. He surmised that further to the left Janus or Epimetheus appears a little further outside the F-ring. But according to Sky Safari there's no moon anywhere near that location. I'm guessing that little speck of light might be a star. Magnitude 4.6 Iota Antlia could be the culprit.


Anthony, your sky software is better than my sky software (which didn't show any of the above moons). I relied on Wikipedia. Did both Janus and Epimetheus show up in Sky Safari? They seem to be unique in the Solar System as "co-orbitals", swapping orbits every four years (one day I must check out what that means exactly). I'd be most impressed with software that accurately keeps track of them. Iota Antila is something I will certainly investigate later today, but at mag 4.6, it somehow feels too bright to be a match, at least before my morning coffee. And all these moons also orbit quickly, so establishing the precise time within a given 24 hour window can also pose a challenge.

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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Anthony Barreiro » Mon Oct 21, 2013 11:28 pm

Nitpicker wrote:
Anthony Barreiro wrote:
rstevenson wrote:Are there any moons visible in the image? So far I've found two small spots outside the rings, but I can't see anything I can unambiguously identify as a moon. Are they too small at this scale?

Rob

The pictures in this mosaic were taken on 10 October 2013. I took a look at Saturn on that date on Sky Safari. As the Nitpicker already said, from right to left across the top edge of the rings, you can see Prometheus just inside the F ring and Pandora just outside the F ring. He surmised that further to the left Janus or Epimetheus appears a little further outside the F-ring. But according to Sky Safari there's no moon anywhere near that location. I'm guessing that little speck of light might be a star. Magnitude 4.6 Iota Antlia could be the culprit.


Anthony, your sky software is better than my sky software (which didn't show any of the above moons). I relied on Wikipedia. Did both Janus and Epimetheus show up in Sky Safari? They seem to be unique in the Solar System as "co-orbitals", swapping orbits every four years (one day I must check out what that means exactly). I'd be most impressed with software that accurately keeps track of them. Iota Antila is something I will certainly investigate later today, but at mag 4.6, it somehow feels too bright to be a match, at least before my morning coffee. And all these moons also orbit quickly, so establishing the precise time within a given 24 hour window can also pose a challenge.

From the illustration in the wikipedia article on Saturn's Moons, Janus or Epimetheus would have been my first guess too. I played around with the time until Prometheus and Pandora were in approximately the right position, which was around 0300 UT 10 October 2013. At that time, if Pandora was approximately 1 o'clock in today's apod and Prometheus at approximately 2 o'clock, our mystery object is about 11 o'clock. Epimetheus was at 4 o'clock and Janus at 5 o'clock, on the opposite side of Saturn. I don't see anything at those locations in the apod -- perhaps the light wasn't reflecting toward Cassini?

By the way, I love sky safari. I have sky safari 3 pro on my ipad. When I'm using my computerized go-to telescope I can hook up the ipad to the telescope, tap an object on the screen, and the telescope will automatically slew to that object. It kinda takes some of the fun out of finding things in the sky, but it's great when I'm leading a group program and I don't want to spend time fiddling with the telescope.
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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Nitpicker » Tue Oct 22, 2013 12:51 am

Thanks Anthony, you may well be right about Iota Antila. It took me two coffees this morning before it dawned on me that Prometheus and Pandora (which both have an apparent mag of about 16) would appear much brighter to Cassini than to us, and hence would be much more comparable to a bright star like Iota Antila.

If you increase the contrast and brightness of this image to ridiculous levels, you can detect at least one vaguely convincing smudge at image bottom-right, which might or might not be Janus or Epimetheus, or a star. Of course, this spoils the image somewhat, as hidden in the dark pixels are what appear to be remnants of the image processing.

Maybe if I have another coffee and keep looking at this image, Janus and Epimetheus will dawn on me definitively.

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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Ann » Tue Oct 22, 2013 2:42 am

Anthony Barreiro wrote:
I'm guessing that little speck of light might be a star. Magnitude 4.6 Iota Antlia could be the culprit.


Iota Antlia is one of those dime-a-dozen K0III stars. For all intents and purposes it is a distant Pollux, pale yellow and not too bright. It's the kind of star that wouldn't stand out in any way, certainly not by visual inspection. It could certainly mimic one of Saturn's moons.

My only objection is - Antlia? That is definitely not where you would expect to see any of the major Solar system objects. Is the Saturnian system simply so huge that its outer parts, including its moons, spill into constellations far from the ecliptic?

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Re: APOD: Saturn from Above (2013 Oct 21)

Postby Nitpicker » Tue Oct 22, 2013 2:57 am

Ann wrote:
Iota Antlia is one of those dime-a-dozen K0III stars. For all intents and purposes it is a distant Pollux, pale yellow and not too bright. It's the kind of star that wouldn't stand out in any way, certainly not by visual inspection. It could certainly mimic one of Saturn's moons.

My only objection is - Antlia? That is definitely not where you would expect to see any of the major Solar system objects. Is the Saturnian system simply so huge that its outer parts, including its moons, spill into constellations far from the ecliptic?

Ann


Oops, I've been mis-spelling the constellation name. It is Antlia and the star is Iota Antliae. But this view from the Cassini spacecraft (i.e. the view in this APOD), is not at all aligned with the Ecliptic plane.


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