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Posted by: ugordan Jan 2 2007, 10:47 PM

To wrap up the year, Cassini took a few Tethys low phase http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=94937 from about 400 000 km.
A stretched color (IR/G/UV) composite:

Posted by: elakdawalla Jan 2 2007, 11:44 PM

Do those color differences represent actual color differences, or are they just variations with phase angle?

--Emily

Posted by: alan Jan 2 2007, 11:55 PM

I believe they are real. I've noticed them before when looking at the maps on Steve Albers page.
http://laps.fsl.noaa.gov/albers/sos/sos.html
In fact Tethys isn't the only moon like this. Rhea and Dione appear darker on one hemisphere than the other, I believe it is the trailing hemisphere. Anyone seen an explanation for this?

Posted by: ugordan Jan 2 2007, 11:56 PM

Actual color differences. I http://www.unmannedspaceflight.com/index.php?s=&showtopic=2924&view=findpost&p=73630 a while ago that Tethys appears to have a large reddish splat covering a little less than one hemisphere, while the rest of the moon is apparently clean and fairly high albedo ice (IIRC, the second highest albedo at Saturn, after Enceladus). This viewing geometry seems to nicely capture the "best of both worlds".

EDIT: Note that this is different from the dark band visible in other imagery that is mostly grayish and runs approximately along the equator. I think it's located on the other side of the moon in this view.

Posted by: nprev Jan 3 2007, 03:00 AM

One thing that strikes me is that Tethys, Dione & Iapetus all have a number of relatively large craters (co-orbital small moon impacts?), and Rhea doesn't. However, Rhea looks considerably more battered at lower scales.

Sure would like to know the complete, unabridged story of the Saturn system...many tantalizing clues, but no grand unified theory seems evident.

Posted by: Ian R Jan 3 2007, 09:29 AM

Great image ugordan!

North is to the left in this view, with Penelope in the centre, Melanthius at roughly the 2 o'clock position, and Odysseus on the limb at 10 o'clock.

The 'yellow splat' shows up in several other Cassini and Voyager images of Tethys:

http://photojournal.jpl.nasa.gov/catalog/PIA01392
http://photojournal.jpl.nasa.gov/catalog/PIA08149
http://photojournal.jpl.nasa.gov/catalog/PIA06140

Ian.

Posted by: Ian R Jan 3 2007, 09:35 AM

Hmm... I wonder if the 'splat' is contemporaneous with the young terrain visible in this Voyager image?

http://photojournal.jpl.nasa.gov/catalog/PIA01397

Ian.

Posted by: Phil Stooke Jan 3 2007, 02:07 PM

Very nice color composite, ugordan.

The shading within Penelope shows up very well - darker on one side (east), light on the opposite rim (west) - this view has north at the left side. That shading looks like it's caused by illumination but it isn't - the sun is almost overhead and the pattern looks the same with the sun either side of the crater. Superficially, like the craters north of Cassini Regio on Iapetus.

Phil

Posted by: As old as Voyager Jan 3 2007, 07:11 PM

Rhea does have two fine impact basins; the 375 km Tirawa and a 500km basin alongside it. This second basin was identified by Phil Stooke in re-processed Voyager 1 images:

http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1074.pdf

Here's some nice pics of Tethys' stripe:

http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1553.pdf

Posted by: edstrick Jan 4 2007, 12:05 PM

Rhea, Dione and Tethys all have darkened trailing hemispheres, with the darkening peaking approximately at the center of the trailing hemisphere (270 deg longitude???). This is opposite Iapetus's shading. In the post Voyager days, it was suspected to be some combination of magnetosphere modification of the more irradiated trailing surfaces and greater impact disturbance of the leading hemispheres. This still seems plausible at an "arm waving" level, I don't know the current state of theory.

Posted by: ugordan Jan 5 2007, 10:23 PM

Here's an additional Tethys composite, showing roughly the same area as the first image. It's from the latest PDS release and is stretched color but was toned down to approximate natural color appearance. North is up on this one (the original image has north at left). Penelope is visible at center with I think Melanthius at 5 o'clock limb. The dichotomy is noticeable with the left hemisphere being slightly darker/orangish.
http://m1.freeshare.us/view/?126fs4739328.jpg

Posted by: nprev Jan 6 2007, 01:14 AM

Thanks for the enlightenment, AOAV, and belated congrats to Phil!

So, Rhea falls more nearly in line with the rest of the icy moons in general character as far as a history of major impactors. Does this imply that the Saturn system was riddled with large objects above the normal background level, and if so does this say something about the age of the ring system given the apparent antiquity of the craters?

Posted by: tedstryk Jan 6 2007, 05:14 AM

The stripe is very obvious in this image.

Posted by: As old as Voyager Jan 6 2007, 10:01 AM

Saturn's moons do sport some huge impact basins relative to their sizes. This could well be due a flux of errant objects in the Saturn system, perhaps the debris from the gigantic impact that shattered Hyperion. If the bulk of ring particles do originate from a shattered inner moon broken apart by one of these ancient impacts then the rings may be far older than thought. Alternatively, the rings may have originated from a tidally disrupted moon and may have formed far more recently.

Saturn's myriad moons are fascinating and are going to take a long time to understand.

Posted by: Bjorn Jonsson Jan 6 2007, 12:18 PM

Of all of the images I've seen of Tethys this one most clearly shows the fact that Penelope is far from circular - it is elliptical. I do not recall seeing an ellipitical crater as big as this one (at least relative to the planet/satellite).

Posted by: edstrick Jan 6 2007, 12:59 PM

TedStryk's voyager picture shows the most puzzling nature of the stripe quite well... it's a relatively uniform dark band with rather sharp edges, like it was painted on with a giant paintbrush in a single swipe. It just doesn't make much sense.

Posted by: ugordan Jan 6 2007, 01:15 PM

Here's http://farm1.static.flickr.com/113/255939582_2f8983f46e_o.jpg showing the band. It's not very precisely defined, more looks like it was sprayed, not painted on. The image also shows the boundary between two types of terrain -- the lower third of the disc is visibly less cratered than the rest of the moon.

Posted by: Bob Shaw Jan 6 2007, 02:00 PM

I see lots of linear terrain morphologies on Tethys!

Here's my take on it: it's a sub-scale Iapetus ridge effect, the same mechanism writ small (or large).

Impacts take place; debris is ejected. Some goes into orbit of Saturn. Said debris is perturbed into arcs, and a proportion impact the parent body - in arcs. Note that the bands all have reasonably low inclinations, too.


Bob Shaw

Posted by: ugordan Jan 6 2007, 02:16 PM

There was already a discussion about terrain types on Tethys in http://www.unmannedspaceflight.com/index.php?s=&showtopic=2924&view=findpost&p=62749. Ian R also made some mosaics that very nicely show the dark band, perhaps the best views yet.

Posted by: tasp Jan 6 2007, 02:33 PM

I'm having a little trouble figuring out the orientation of the band relative to the Tethysian equator.

If the band is parallel, we might be seeing an insolation effect.

Posted by: ugordan Jan 6 2007, 02:39 PM

It's roughly parallel to the equator, yes. Notice it in http://farm1.static.flickr.com/110/266209592_e66c4b34b2_o.jpg image, intersecting Odysseus. It's roughly centered on the leading hemisphere as well. It's too well-defined to be an insolation effect, plus why doesn't it go all the way?

Posted by: alan Jan 6 2007, 05:03 PM

Looking at the other thread Ugordon pointed out I notice that there appears to be a color difference between the dark band on the leading hemisphere and the dark trailing hemisphere, it shows well in the animation from the Sept 20 that he attatched to this post.
http://www.unmannedspaceflight.com/index.php?showtopic=2924&st=30