The unlit side of the rings Options

[Bjorn Jonsson]

Cassini has previously imaged the unlit side of the rings but now it is doing extensive observations of the unlit side. For the first time in its mission Cassini is spending a significant amount of time north of the ringplane - earlier it has done so only near periapsis but this orbit is different.

This is a quick RGB composite I did from wide angle images:



I made no attempts to correct the color - this is probably not very far from the true color of the rings. Large color variations are apparent, the C ring and the Cassini division appear much more bluish than the thicker rings (A and B ). It should be noted that these images were very probably downliked with the 12 -> 8 bit encoding which basically means that dark areas appear too bright in this image because I did not correct for this.

It's also interesting to compare this image to Voyager images:

Voyager 1: http://photojournal.jpl.nasa.gov/catalog/PIA02241

Voyager 2: http://photojournal.jpl.nasa.gov/catalog/PIA01955
Voyager 2: http://photojournal.jpl.nasa.gov/catalog/PIA01389

[ugordan]

Great composite. I think it's a tad on the too-blue side, but still it's very neat. Apart from the C ring and the Cassini division, the F ring is also notably blue - it shows nicely in your composite. It's a shame the raws were lossy compressed, otherwise this would make for a very crisp image. Good to see the rings again, even from above.

[ynyralmaen]

Beautiful... one really nice effect that's visible is the "terminator" coincident with Saturn's equator. The atmosphere south of the equator is strongly illuminated by ringshine, but the northern hemisphere significantly less so - neat!

[tasp]

I recall viewing pictures sent back by Pioneer 11 showing the unlit side of the rings.

It had never occured to me the sun only shown on one side at a time and it was pretty confusing.


I would like to think my general level of comprehension of this stuff has improved a bit . . . .

[Bjorn Jonsson]

Cassini's view of the unlit side of the rings is remarkably different from the one provided by Voyager 1 back in 1980:



This 'image' is based on intensity scans of Voyager 1 images (top) and Cassini images. The C ring is at left and the F ring at right. The Voyager 1 intensity scan has been corrected for radial distortion so ring features are at their correct relative locations. The Cassini intensity scan is rather crude and has not been corrected so there is some radial distortion, especially in the outer parts of the ring system (the location of major ring features has not changed). Also, since it is based on the JPG versions of the images, exact relative intensity differences are highly uncertain. Despite this it is obvious that there are large differences between the Voyager 1 and Cassini images. In the Voyager 1 images the C ring and the Cassini division appeared brightest whereas in the Cassini images the outer edge of the A ring is brightest with the Cassini division appearing rather dark and the inner part of the B ring appearing quite bright.

The differences in viewing geometry are that the Voyager 1 images were obtained at low phase angle and a low solar elevation angle. In contrast, for the Cassini images the solar elevation angle is high and the phase angle also high.

Interestingly, the Voyager 2 images of the unlit side appear somewhat similar to the recent Cassini images.

The highly varying appearance of the rings as a function of various geometric factors (phase angle, solar elevation angle etc.) is remarkable.

[alan]

Speaking of high phase angles, according to Solar System Simulator on September 15 Cassini will be pass through Saturn's shadow while at a distance of 2.163 million miles.
http://space.jpl.nasa.gov/cgi-bin/wspace?t...=1&showsc=1
I wonder what observations are planned for this.

[ugordan]

I wonder what observations are planned for this.

From an imaging standpoint, it looks like a very nice opportunity for a wide angle mosaic. 3 footprints ought to cover the width of the scene so it wouldn't be too expensive to acquire a multicolor set.
Earth should be visible as well -- if it's not drowned out by forward scattered light from the rings. This will be a very high phase angle situation so it's possible the E and G rings will stand out very nicely. The main rings (at least the C and F rings) might be brilliant as well.
Looking forward to it.

[Bjorn Jonsson]

The occultation lasts about 15 hours so there should be plenty of time to point the RSP towards Saturn without accidentally pointing it at the Sun. Sure looks like an interesting imaging opportunity.

The following animation showing the occultation starts at 2006-09-15 03:00:00 UTC and spans a period of 24 hours. The field of view is 8 degrees.

 sun_occult.avi ( 1.3MB ) Number of downloads: 544

[djellison]

Don't they use these sorts of obs for occultation measurements - VIMS I'd have thought would do well with it - or is there a large 'keep out' for instrument pointing around the sun.

Doug

[ugordan]

True, both VIMS and UVIS can stare at the sun. When these sorts of observations are planned precautions can be taken -- such as disabling the ISS subsystem. Accidental sun exposure could be dangerous to ISS cameras if they were in the middle of a planned imaging sequence and the shutter opened, allowing the sun to burn the CCDs.

[tallbear]

I wonder what observations are planned for this.
[/quote]

There are a number of ORS observations of the entire Ring system at this
very high phase viewing opportunity ... and even a high phase set of
observations of Saturn

There is also an RSS Occ observation of the Rings and Saturn
and a Solar Ring Occ observation as well.

In addition, the RSS 'beam' will pass within 10's of km of Enceladus' S Pole
but with DSS 63 down this will only be observed with Ka band

[alan]

The rings about a quarter of the way up form the bottom of this image look distorted.

http://saturn.jpl.nasa.gov/multimedia/imag...2/N00064487.jpg

[dilo]

Alan, I cannot see the distorsion... exactly, which rings and how they are distorted?

PS: thanks for great unlit side view and the beeatiful animation, Bjorn!

[helvick]

Alan, I cannot see the distorsion... exactly, which rings and how they are distorted?

I can see it but to me it looks as if the rings have quite a rough texture - it's almost as if they were drawn by someone with a fairly heavy brush action.

[dilo]

I can see it but to me it looks as if the rings have quite a rough texture - it's almost as if they were drawn by someone with a fairly heavy brush action.

Perhaps you're referring to the disuniformities visible as small hue variation is this RGB version (with enhanced saturation):
(start images N00064483/84/86)
Anyway, is something barely visible to me...

EDIT: just posted a short movie showing spokes transit in this region during previous day, the rough texture noted by alan seems to co-rotate with spokes...


I made the following mosaic of 3 images showing the A/F rings with Saturn shadow:
(start images N00064408/09/17)
I like it...

Finally, did someone noticed this very odd image?
http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=80270
Image explainations refers to "SATURN-ERING" but I suspect that this bright strip is another piece of main rings, partially eclipsed by saturn shape + shadow... other ideas?