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.