[X] My Assistant

[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

[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.