29-30 August 2007 Icy Satellites (rev 49), Last stop on the road to Iapetus Options

[Steve G]

The crentral peak area of Oydessus is slightly reminiscent of King Carter on the moon.

[ugordan]

Jason, I've seen this explained before but I keep forgetting the explanation. Can you explain the origin of the every-other-line truncation that appears in Cassini images that have lots of detail?

The truncation comes from the "LOSSLESS" compression scheme (obviously a bit of a misnomer since it can and does lose spatial information). The algorithm operates on line pairs and guarantees a compression ratio of at least 2. It's a variant of a Huffman encoder and if it figures out that the two lines it's currently encoding will turn out to be compressed more poorly than 2.0 ratio, it just stops and truncates the rest of the second line. In an absolute worst case scenario the second line could be completely truncated if the first line turned out to expand rather than shrink, which could happen since the algorithm uses fixed statistical encoding tables, they are not optimized for each frame.

I believe Voyagers also used something similar. Not sure if Galileo originally had it as well. It's a rather dumb and unflexible encoding scheme, but is computationally inexpensive so it's often used. The fixed 2.0 limit is probably due to ease of data policing on the onboard encoder - with LOSSLESS encoding you can guarantee that the frame will be at most half the original size so you can allocate space for each frame accordingly and not worry about losing the whole lower part of a frame in case your encoded frame turned out bigger than you predicted.

Here's another one for the photoalbum:

[DEChengst]

A rough 8 frame Rhea mosaic:

http://paranoid.dechengst.nl/saturn/Rhea-0...30-AUG-2007.jpg (1.8 MB)

It doesn't look too great with all the noise and stuff but a least it gives an impression.

[MarcF]

The floor of the "young" ray crater on Rhea is really interesting.

http://ciclops.org/view.php?id=3713

Eastern part shows many small craters, but it seems that giant landslides completely covered the western floor where no craters are visible anymore. The ray crater seems indeed young and the landslides even younger.

The landslides or "snow flows" remind me a little bit what is happening on Jupiter's moon Callisto.
Both Rhea and Callisto are considered as the boring moons in their respective satellite system. I think it is really not the case.
Marc.

[MarcF]

This WAC picture shows that the previously mentioned lobate flow was caused by an impact near the rim of the bigger crater (like on Callisto as I already mentioned).

http://saturn.jpl.nasa.gov/multimedia/imag...3/W00034593.jpg

Additional crater chains and other linear features are also visible.

Marc.

[nprev]

Both Rhea and Callisto are considered as the boring moons in their respective satellite system. I think it is really not the case.

Yeah...I think you're right, Marc. Rhea may look bland globally, but these close-ups are revealing an amazing amount of complexity at smaller scales. The sheer number of linear features is really remarkable, as UGordan observed. Beginning to wonder how many of the features that look like crater chains might actually be collapsed "cryolava tubes" dating back to Rhea's boisterious youth...

[ugordan]

A rough 8 frame Rhea mosaic:
It doesn't look too great with all the noise and stuff but a least it gives an impression.

Great mosaic stitchwork, DEChengst! Did you use any special software for this?
As for the noise, this showcases why histogram stretching for JPEG raws is basically a must for low contrast targets.

[DEChengst]

Did you use any special software for this?

Not really. Just used Photoshop to interpolate the missing lines (deinterlace filter), levels correction and cropping off the white borders. Stitching was done with PTGUI and Smartblend. The stitching is pretty much automatic with just some manual tinkering with the parameters.

As for the noise, this showcases why histogram stretching for JPEG raws is basically a must for low contrast targets.

Yep. The raw images look pretty bland. This is pretty evident in the filesize. They're only 40-60 KB a piece, while normally you expect something like 100-250 KB. I guess I'll have to wait for the PDS release as I lack the Photoshop skills to fix this.

[Gsnorgathon]

Would anyone here who's more familiar than I with the cratering rate at Saturn care to take a stab at estimating Big Ray's age?

[dvandorn]

The sheer number of linear features is really remarkable, as UGordan observed.

See, now, I'm beginning to wonder if I'm a ghost and no one sees my posts except for me, LOL... I have been trying to get a discussion going about the linear features and "crater chains" on Rhea for something like a year and a half, if not longer, and not a single person has ever responded to my comments. Now it's ugordan who's noticed these things?

I remember taking images of Rhea and using my primitive image-processing skills and tools to outline the large chains, and then pointing out how these same structures seem to persist all the way down to the smallest visible scales. If I remember right, this was some time in early 2006. I've revisited the topic pretty much every time we get more decent Rhea images. Am I actually going mad and remembering things I never did?

That said... ...I am very, very interested in whether these features are endogenic or exogenic. If endogenic, Rhea becomes an extremely interesting little moon. Even if exogenic, you have to explain how so many crater chains, of all sizes and varieties, show up on this moon while they show up only much more rarely on the other moons of Saturn.

-the other Doug

[ugordan]

Now it's ugordan who's noticed these things?

Ummm... I think both of you have the wrong guy here. I noticed what? The linear features? I'm most certainly not the first one to point them out. I'm not sure when exactly I mentioned them without someone else pointing them out earlier... And I'm most certainly not claiming credit to discovering them.

That being said, yes, I took some notice of them in higher res shots. There are too many of them and appear to be too narrow to be a simple impact chain, IMHO. I'd go with an endogenic origin of some sort. At least for some of them. Then again, I'm not a geologist.

[nprev]

The collective amount of insight & expertise on this forum is so massive that it's damn hard to keep track of who said & did what sometimes...apologies, oDoug & UG!

Regardless, Rhea's linear features are indeed fascinating. I can't recall the precise image now, but one of the recent set showed a manifest crater chain that had a relatively large, very pronounced teardrop-shaped terminal "crater"...hard to see that this could be anything less than the result of a hybrid of exo/endogenic original processes, with probability tilted towards the latter.

[JRehling]

Are any of the commercial photo packages (like Adobe) able to interpolate and fill in the truncated lines? eg by averaging the surrounding pixels?

It works pretty well to copy the area, paste it as a new layer, then nudge it one pixel up or down and select a filter that shows the max of the two layers' brightnesses. Basically, close the even numbered lines onto the odd ones.

[Gsnorgathon]

Maybe I just haven't been paying close enough attention, but it seems to me the crater chain/linear features all tend to be awfully narrow. I haven't noticed any really obvious crater chains like the ones that are all over Callisto. Assuming I have been paying close enough attention, this would incline me toward an endogenic origin. (And for what it's worth, dvandorn - I remember!)

[ugordan]

I've been casually looking through past Tethys images and there are similar linear features at a similar scale present there. They are, as you say, too narrow to be impact chains, but they do have that string-of-pearls appearance to them. Confusing...