APOD: Tethys Behind Titan (2010 Jan 27)

[APOD Robot]

Tethys Behind Titan

Explanation: What's that behind Titan? It's another of Saturn's moons: Tethys. The robotic Cassini spacecraft orbiting Saturn captured the heavily cratered Tethys slipping behind Saturn's atmosphere-shrouded Titan late last year. The largest crater on Tethys, Odysseus, is easily visible on the distant moon. Titan shows not only its thick and opaque orange lower atmosphere, but also an unusual upper layer of blue-tinted haze. Tethys, at about 2 million kilometers distant, was twice as far from Cassini as was Titan when the above image was taken. In 2004, Cassini released the Hyugens probe which landed on Titan and provided humanity's first views of the surface of the Solar System's only known lake-bearing moon.

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[mwhidden]

That's no moon... It's a space station.

[neufer]

<<Sixty-one known moons orbit Saturn,
not counting hundreds of "moonlets" within the rings.>>

THe SYlvesTEr–Gallai theorem asserts that given
a finite number of points in the Euclidean plane, either

• 1. all the points are collinear; or
  2. there is a[t least one] line which contains exactly two of the points.

[pebble]

I've looked long and hard at these two images, and the leftmost image sure looks like someone's done a crude photoshop job on it.

How can the upper layer of Titan's haze become occluded by the shadow edge of distant Tethys? And why does the bright edge of Tethys cut a very artificial-looking notch into Titan's haze? These are optical and geometric impossibilities.

Rest assured I'm not one of those moon hoax people: I accept the moon landings as reality. Also, I'm not saying the image is a deliberate fake, I just don't know how to account for these unnatural artifacts which could not be present in such a photograph, unless it has been manipulated. I'm a visual effects artist with over a decade's experience in feature films, and some of my work has involved making believable spacescapes just like this one. If I produced a shot like this photo, the visual effects supervisor would bounce it back with a note saying "do it right".

Perhaps someone hand-adjusted brightness levels over Tethys, just to make a more pleasing exposure? But I'd expect the haze component of those RGB values to be boosted as well. A friend suggested maybe it's a halation effect, but that would take place in the camera, and any resulting glow would splash over everything, not respect the shadow edge of Tethys.

To my eye, someone has doctored this image to make it more dramatic. They certainly succeeded! But is it accurate? I'd really like to see the raw image. Any chance of that?

[geckzilla]

Perhaps someone hand-adjusted brightness levels over Tethys, just to make a more pleasing exposure? But I'd expect the haze component of those RGB values to be boosted as well. A friend suggested maybe it's a halation effect, but that would take place in the camera, and any resulting glow would splash over everything, not respect the shadow edge of Tethys.

I thought this when I first saw it this morning as well. There's just no way the black of Tethys's dark side would ever be distinguishable from the blackness of space, though there does seem to be some Saturnshine on Tethys on the right frame. It's not on Titan, though! I don't get it. The only info I can find is here: http://ciclops.org/view/6046/Tethys_Slips_Behind_Titan

I guess you have to ask Daiana DiNino since she's listed for doing the image preparation.

[pebble]

A quick addendum: I note what appears to be a slight darkening on the lower bright limb of Tethys, where Titan's haze would cross it. That totally makes sense, since Titan's haze would both reflect sunlight and absorb Tethys' transmitted light. This suggests Tethys was hasn't been repositioned for drama, like those camels over the pyramid, on that controversial National Geographic cover photo.

But the shadow seems artificially dark where there should be haze overlap, and that bright limb notch is really sloppy.

The best theory I can come up with is this: In the original photo Tethys might have been too bright, so someone made a mask over the moon and performed a slight darkening to make the overall levels more pleasing beside Titan. That could might account for the illusion of the shadow edge occluding the nearer atmposphere. The notch cut into the haze on the other side, where Tethys is bright, looks more like hand-painted correction.

It's just a theory. Seeing the raw image would tell me a lot.

[zbvhs]

Is it possible that the big crater on Tethys and the one on Iapetus are separation scars? I can visualize an early scenario in which the two were one body with spin rate sufficient to elongate it and eventually pull it apart into two separate bodies.

[geckzilla]

It's just a theory. Seeing the raw image would tell me a lot.

Found them.
http://saturn.jpl.nasa.gov/photos/raw/r ... eID=208202
http://saturn.jpl.nasa.gov/photos/raw/r ... eID=208201
http://saturn.jpl.nasa.gov/photos/raw/r ... eID=208200
http://saturn.jpl.nasa.gov/photos/raw/r ... eID=208199

http://saturn.jpl.nasa.gov/photos/raw/r ... eID=208195
http://saturn.jpl.nasa.gov/photos/raw/r ... eID=208194
http://saturn.jpl.nasa.gov/photos/raw/r ... eID=208193
http://saturn.jpl.nasa.gov/photos/raw/r ... eID=208192

It probably had something to do with making it appear in "true" color. As you can see, all of those images were taken using only two of many filters. So to create the RGB approximated "true" version they had to combine several images. Maybe they didn't have the same image in all three filters at once and had to fiddle with it like that.

[pebble]

Oh, man, those pictures are *awesome*. And they look exactly as I'd expect, no weirdness at all. Thank you so much, geckzilla.

And I think you're right, the resulting image is probably a combo of multiple exposures (with slightly different positions), requiring some photoshoppery. No harm, no foul, just...well, *I'd* have worked a little harder to preserve that backlighting on Tethys and that lovely gossamer haze crossing it.

[BMAONE23]

What I see is the apparent brightness of Tethys' day side, overpowering the brightness of Titan's upper atmosphere. Much like images of our moon through tenuous clouds makes it appear in front of them, the upper atmosphere of Titan is not sufficiently thick enough to block or distort Tethys so the brighter Tethys appears in front.
The "Notch" appears because the day side of Tethys is brighter than the tenuous upper atmosphere of Titan. If you look very closely at the full size image, you can discern Titan's tenuous upper atmosphere in a very slight band in front of Tethys.
A simple opdical delusion.

[pebble]

What I see is the apparent brightness of Tethys' day side, overpowering the brightness of Titan's upper atmosphere. Much like images of our moon through tenuous clouds makes it appear in front of them, the upper atmosphere of Titan is not sufficiently thick enough to block or distort Tethys so the brighter Tethys appears in front.
The "Notch" appears because the day side of Tethys is brighter than the tenuous upper atmosphere of Titan
A simple opdical delusion.

No, it's just a poor photoshop job. If you look at this raw grayscale image, you'll see how it appears in reality, with none of the weirdnesses in the color image:
http://saturn.jpl.nasa.gov/multimedia/i ... 147403.jpg

[neufer]

<<Astronomer Giovanni Domenico Cassini had the wooden Marly Tower, originally built as part of the Machine de Marly to lift water for the reservoirs and fountains at the Gardens of Versailles, moved to the grounds of the Paris Observatory. On this tower he mounted long tubed telescopes and the objectives of aerial telescopes made for him by the Italian optician Giuseppe Campani. In 1684 he used one of his aerial telescopes to find Dione and Tethys, two satellites of Saturn.>>


An engraving of The Paris Observatory in the beginning of the 18th century
with the wooden "Marly Tower" on the right.

http://antwrp.gsfc.nasa.gov/apod/ap091208.html
http://antwrp.gsfc.nasa.gov/apod/ap080624.html