Starship Asterisk*

Ringside with Titan and Dione

Explanation: Orbiting in the plane of Saturn's rings, Saturnian moons have a perpetual ringside view of the gorgeous gas giant planet. Of course, while passing near the ring plane the Cassini spacecraft also shares their stunning perspective. The rings themselves can be seen slicing across the middle of this Cassini snapshot from May of last year. The scene features Titan, largest, and Dione, third largest moon of Saturn. Remarkably thin, the bright rings still cast arcing shadows across the planet's cloud tops at the bottom of the frame. Pale Dione is about 1,100 kilometers across and orbits over 300,000 kilometers from the visible outer edge of the A ring. Dione is seen through Titan's atmospheric haze. At 5,150 kilometers across, Titan is about 2.3 million kilometers from Cassini, while Dione is 3.2 million kilometers away.

<< Previous APOD

Discuss Any APOD

Next APOD >>

[/b]

This is stunning. It's because of photos like this that I hate to see APOW and APOM go away. This is the sort of thing that could be an artist's conception, except that it's an actual photograph.

Amazing! "Titan is about 2.3 million kilometers from Cassini". Its hard to imagine a million kilometers. OK a million pounds, or dollars, is easy to imagine (!), but not kilometers... Have to congratulate all at NASA and especially the APODians... Many thanks for sharing these images.

bystander wrote: http://asterisk.apod.com/viewtopic.php?f=29&t=26268

I'm having problems with bystander's link, as well as with all the APOD links that have anything to do with today's image. That is to say, I can't open the links "this Cassini snapshot". "features Titan" or "and Dione". While I can go to the page bystander links to, the Cassini images doesn't open. In fact, the Cassini images on the page that bystander links to has never opened for me. I have managed to see them anyway by using the address that bystander gave us, NASA/JPL-Caltech/Space Science Institute/CICLOPS. However, as I tried to use it again, I got to a text page, but I couldn't see any pictures.

The reason why I wanted to check up the links was that I wanted to find out what filters were used to produce the image, which is always of the utmost interest to me.

Well, I have been unable to find out about the filters, so I will have to comment on the colors of today's APOD anyway. I enjoy the pale yellow color of Saturn, which is almost exactly the hue I see when I watch Saturn through my binoculars. I appreciate the gray-white color of Dione, too, but it leaves me wondering about the surface of Dione. It is icy, right? But the ice can't be perfectly fresh and untarnished, can it? Well, theoretically it can. The light of the Sun is faint where Saturn and its moons are located, and if a picture is taken where the exposure is too short, then even a perfectly white and shiny surface might look gray.

However, I managed to find a page I could open to read about this picture, http://www.skyandtelescope.com/communit ... g/newsblog, posted by Kelly Beatty, December 27, 2011. Kelly Beatty wrote:

Dione reflects about 60% of the sunlight striking its icy surface, whereas Titan reflects only 20% and consequently looks darker.

Really interesting. Dione has a bright surface, but certainly not as white and shiny as the surface of Enceladus. Presumably the surface of Dione is made of slightly, but not very, dirty ice. Note the neutral color of Dione, suggesting that it reflects all the (white) light of the Sun equally. However, that is probably not the case, since the "dirt" that can darken the surface of Dione from a perfect white is probably somewhat reddish. Even Dione, then, should preferentially reflect the yellow and red light of the Sun, thus taking on a slightly reddish tint. But of course, compared with the colors of Saturn and Titan, Dione will look perfectly icy gray.

Note, by the way, that the rings of Saturn appear to be a perfectly neutral shade of gray, too, although they look a little darker than Dione. But the rings are not perfectly white or gray, since they are darkened by slightly reddish "dirt" and thus reflect more of the Sun's red light than its blue light.

What is almost astonishing to me in this picture is the dark appearance of Titan. It doesn't look orange at all to me, but rather a dark yellow-gray. The only thing that looks at all orange in this picture is the haze of Titan. I find it interesting that the haze of Titan is so orange, since I thought it was blue. But admittedly this can be an effect of the filters used to produce the image.

But, as Kelly Beatty wrote in his blog, Titan reflects only 20% of the sunlight that hits it. That doesn't sound impressive, although it is bright indeed compared with the Earth's Moon, which only reflects 7% of the sunlight that hits it. Still, I wonder if the comparative darkness of Titan has anything to do with its orange (or yellow-gray) color. Or maybe not. My impression of Pluto is that this minor planet is at least as orange as Mars, yet it is at least as shiny and reflective as Dione.

Ann

I'd seen this great pic of Saturn and some others at Life Unbounded.

WOW!!!!!!!!!!!!!!!!
This is one of the most spectacular photos yet!

Fantastic picture !!

Sergio

"WOW" is right !!!

I was fascinated by the links in the APOD writeup. Frustratingly, I couldn't find anything more specific about what the Huyguns probe found when it landed on Titan but the phrase "muddy". Is there a website that describes more about its atmosphere and surface?

Ann wrote:The reason why I wanted to check up the links was that I wanted to find out what filters were used to produce the image, which is always of the utmost interest to me.

Green, red and blue were combined to make todays APOD. It didn't say anything else about the filters that i could find.

Awesome! Imagine what it would be like if one could live on Dione! The night sky would have a sky almost full of Saturn when Dione was on the day side of Saturn. When on Saturn's night side Dione would be without sun until it orbited around from behind the giant planet!

agulesin wrote:Amazing! "Titan is about 2.3 million kilometers from Cassini". Its hard to imagine a million kilometers. OK a million pounds, or dollars, is easy to imagine (!), but not kilometers... Have to congratulate all at NASA and especially the APODians... Many thanks for sharing these images.

I don't know... I've got a half million kilometers on my car. Doesn't seem so hard.

Seriously, though, I'm always interested in the sort of values that people are able (or unable) to visualize. When we first encounter a new one, we try converting it to something we are familiar with- like the walk to the local supermarket. Of course, when you try expressing the distance between a planet and its moons in that light, it's hard to imagine. Even worse for the distances between galaxies! But once you get comfortable working with these distances, the need to convert goes away. You might think of a moon being not a few million kilometers from its parent, but three times further away than some other moon. Three is easy to visualize.

Big numbers become easy once we stop trying to actually count them.

"...Titan is about 2.3 million kilometers from Cassini, while Dione is 3.2 million kilometers away..."
How can that be? Dione appears to be between Titan and Saturn in the picture.

deneiltopan wrote:"...Titan is about 2.3 million kilometers from Cassini, while Dione is 3.2 million kilometers away..."
How can that be? Dione appears to be between Titan and Saturn in the picture.

Yeah... so what exactly is the problem?

Dione is ~0.4 million km above Saturn, Titan is ~1.2 million km above Saturn, and Cassini is ~3.6 million kilometers above Saturn. So we see Saturn in the background, then Dione, then Titan (partly blocking Dione), and all the distances add up just about right.

Has anyone else noticed the dark arc in Titan's atmosphere, appearing to come off the surface at about 20-30 degrees from north, on both sides? At first I thought it was a shadow, but it's darkening Saturn, and it's on both the day and night side. Could this be a plume from a volcano that's located on the distant side of the moon? Or just a more common cloud formation higher up in the atmosphere that just appears to be an arc from this angle (the atmosphere does appear slightly darker across the face at that latitude as well)? I do a lot of planetary animations, and this atmospheric effect jumped out at me.


He Who Hasn't Had To Animate Titan, Yet...

Ann wrote:

bystander wrote: http://asterisk.apod.com/viewtopic.php?f=29&t=26268

I'm having problems with bystander's link, as well as with all the APOD links that have anything to do with today's image. That is to say, I can't open the links "this Cassini snapshot". "features Titan" or "and Dione".

Here is an alternative link to "this Cassini snapshot" from CICLOPS. In case you can't open it either (don't know why you are having problems), here is an excerpt:

Saturn's third-largest moon, Dione, can be seen through the haze of the planet's largest moon, Titan, in this view of the two posing before the planet and its rings from NASA's Cassini spacecraft.
...
This view looks toward the sides of Titan (3,200 miles or 5,150 kilometers across) and Dione (698 miles or 1,123 kilometers across) facing away from Saturn. North is up on the moons. This view looks toward the northern, sunlit side of the rings from just above the ring plane.

Images taken using red, green and blue spectral filters were combined to create this natural color view. The images were obtained with the Cassini spacecraft narrow-angle camera on May 21, 2011, at a distance of approximately 1.4 million miles (2.3 million kilometers) from Titan and 2 million miles (3.2 million kilometers) from Dione. Image scale is 9 miles (14 kilometers) per pixel on Titan and 12 miles (19 kilometers) on Dione.

Here is an alternative link to "this Cassini snapshot" from CICLOPS.

Thanks for that link. The cleaner image shows that this does indeed look like a cloud band. ON closer examination, this band thins as you go south, but continues to the south pole, creating that dark 'ring' in the upper atmosphere. The atmosphere to the north of the band is free of this dark rim, so I guess it's just a cyclonic/coriolis effect.

He Who Looks Forward To More Photos Like This.

Thanks for the filter info, Beyond, and thanks for the alternative link, bystander!

Ann

The ring shadows and the limb of Saturn appear to be curving the wrong way. I thought this might be an optical illusion, but using a piece of paper against my screen as a straightedge, I confirmed that they curve slightly downward at the ends. At the distances sighted, I can't conceive that it's a wide-angle lens artifact - so I'm left to ponder why they curve the way they do. Anybody got an explanation?

It's because of the shape of Saturn. Find a sphere and look at how shadows cast against its surface act. Here's some more photos of Saturn's ring shadows.

RickM wrote:The ring shadows and the limb of Saturn appear to be curving the wrong way. I thought this might be an optical illusion, but using a piece of paper against my screen as a straightedge, I confirmed that they curve slightly downward at the ends. At the distances sighted, I can't conceive that it's a wide-angle lens artifact - so I'm left to ponder why they curve the way they do. Anybody got an explanation?

The shadows are being cast on a sphere, which is what's causing them to curve downward. The image shows so little of Saturn that it just looks weird. Here's a few more distant shots that show the same effect with better context.
http://apod.nasa.gov/apod/ap110308.html
http://apod.nasa.gov/apod/ap100215.html
http://apod.nasa.gov/apod/ap081020.html
http://apod.nasa.gov/apod/ap050722.html


I particularly like this image:
http://apod.nasa.gov/apod/ap051231.html

He Who Hopes Those Links Don't Cause Problems.

This image of Titan and Dione is very good.
Saturn, my predilect planet is beautiful and it has many new things for we.