[X] My Assistant

[angel1801]

I have been using the solar system simulator alot on the July 23 Tethys encounter and from what I see, Cassini will be able to image all of the remaining bits of Tethys' south polar region that until has been poorly imaged.

Is this true?

[volcanopele]

yepper. There is a great Tethys opportunity on that date. Closest approach looks to be around 120,000 km. Low-phase, best imaging of Odysseus that I am aware of Thanks for bringing this up.

[volcanopele]

Actually, Rev26 could be a good orbit for icy sats. Decent observations of Rhea and Dione are also planned, though not as good IMHO as the Tethys stuff. Like the Rev25 Enceladus observations where we missed the satellite, the Tethys observation is a ride-along observation. We should still hit the satellite, this time (fingers crossed)

[ugordan]

Like the Rev25 Enceladus observations where we missed the satellite, the Tethys observation is a ride-along observation.

Missed it? I'm curious of the ways that can happen. Is it merely a consequence of ride-along observations where you miss a slew or just bad Inertial-Vector-Propagator-thingies?

[Bjorn Jonsson]

Yes, a great Tethys flyby with closest approach at 120,000 km, subspacecraft lat=-39.7, subspacecraft lon=85.8, phase=37.5.

There is also 45,000 km Telesto flyby on July 24. I wonder if any imaging is planned for that flyby.

[alan]

One image down, nice view of the chasm
http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=79848

[Big_Gazza]

One image down, nice view of the chasm
http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=79848

Odd feeling of Deja Vu, check out this previous image from Feb 2005...

http://saturn.jpl.nasa.gov/multimedia/imag...fm?imageID=1353

[angel1801]

The raw images are available to view now.

[alan]

Check out the area to left of center in this image of Tethys, it appears to have fewer craters (is younger than?) the area it the right.
http://saturn.jpl.nasa.gov/multimedia/imag...2/N00064170.jpg

[David]

Check out the area to left of center in this image of Tethys, it appears to have fewer craters (is younger than?) the area it the right.
http://saturn.jpl.nasa.gov/multimedia/imag...2/N00064170.jpg

The boundary looks curved. Could it be a shallow impact basin that's been entirely filled in?

[Michael Capobian...]

The boundary looks curved. Could it be a shallow impact basin that's been entirely filled in?

It would have to be a very big one. There is a sharp border between the two terrains, coming right out of the side of Odysseus, but for most of the border there doesn't seem to be any topographic evidence of a basin rim.

It's interesting that the dark band area at the bottom right looks so similar to the adjoining, brighter terrain.

Michael

[MarcF]

Are you sure the big crater o the picture is Odysseus ?
It seems to be a little bit too small. I would say Melanthius or Antonous, one of the big craters localized near the south pole.

[volcanopele]

It's Melanthius.

[Phil Stooke]

There certainly do seem to be regional variations on Tethys... I don't think this boundary is a basin rim. The cratered side almost looks like a swarm of secondaries, or similar-looking craters, as if (say) a shattered co-orbital splattered across this area.

Phil

[David]

There certainly do seem to be regional variations on Tethys... I don't think this boundary is a basin rim. The cratered side almost looks like a swarm of secondaries, or similar-looking craters, as if (say) a shattered co-orbital splattered across this area.

I'm just guessing here, but wouldn't that produce a gradient in crater size (increasing toward the site(s) of impact and decreasing farther away) rather than a sharp(ish) boundary?

If not an impact basin, could it be a surface flow of some kind, caused by localized melting (or at least enough of a temperature increase to render the icy surface more plastic)?

[scalbers]

It's Melanthius.

I'm trying to get oriented still with image N00064170. That crater near the terminator looks like Melanthius. On the other hand, Melanthius is at longitude 204W degrees (trailing hemisphere) and this flyby near closest approach is at about 77W degrees longitude (leading hemisphere). Thus I would expect Melanthius to be invisible in this image unless my assumptions are off?

[volcanopele]

Central lat and lon for the first set of images is approximately 38 S, 104 W. For the second set, 39 S, 85 W. We can see Melanthius because of how far south we are.

[MarcF]

I guess these are the best views to date of the south polar region and of one of the Ithaca chasma ends.

[Michael Capobian...]

It's Melanthius.

Oops. Of course. Big sucker, but not -that- big. .

Michael

[Big_Gazza]

Odysseus is clearly visible in the following image to the right, but angle is severe and contrast poor.

http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=79861

In this one, the angle is even worse and the basin is virtually invisible, but if you look carefully on the right limb, you can just make out the basin rim in profile and a hint of the central peak. You can clearly see how the basin floor has relaxed under gravity and now matches the curvature of the moon. A hypothetical traveller standing on the central peak would see nothing of the basin rim and could be mistaken for thinking he was on an isolated cluster of low hills.

http://www.members.optusnet.com.au/ics_gs/...0064161_mod.jpg

Odysseus was always my favorite Saturnian basin. Its sheer size certainly made an impression when I first saw the Voyager images in 1980/81.

[karolp]

Is this a reflection of the Sun or a Saturnian moon?

[Bjorn Jonsson]

This is the opposition highlight. Cassini has imaged it before, for example take a look at this image.

[Ian R]

I've put together two VERY rough mosaics of Tethys from the latest flyby, and put them in a composite along with an image of the moon taken on December 24th 2005:



In each of the three pictures, the blue arrow points to the central peak of Melanthius, while the red arrow points to the central peak of Odysseus (the profile of Melanthius can just be discerned on the December image, right on the limb). The green line denotes the location of the boundary between the two types of terrain.

Not labelled, but still very obvious in all three pictures, is the "dust streak" in the northern hemisphere next to Odysseus.

Ian.

[Ian R]

Here are the original versions of the two rough mosaics featured in the previous post:



Ian.

[Michael Capobian...]

Questions:

Is the fact that the terrain boundary is approximately parallel to Ithaca Chasma significant? (Probably)

What is the age relationship between Melanthius and the event(s) that changed the terrain? Must it be younger to preserve the sharp boundary right up to the rim? If so, then why didn't the event(s) affect Melanthius in some way?

Very curious.

Michaelc

[dvandorn]

I think, perhaps, that the boundary between the more-cratered region (to the left in the image that started up this thread) and the less-cratered region looks to be solely defined by the distribution of a given-sized crater. This sized crater makes up several parallel arcs of crater chains, which defines the boundary.

If the less-cratered area was a resurfaced area, you would expect the boundary to show partially overlain or partially disrupted/destroyed craters. It does not. The crater chains that define the boundary are chains of whole craters, which aren't deformed or overlain by the less-cratered terrain.

I tihnk the crater chains are being caused by endogenous forces which manifest on the other side of Tethys as a chasm, and which manifest here as crater chains.

Yep -- I'm saying these might well be endogenic, like diatremes, and aren't impact features...

-the other Doug

[scalbers]

Greetings,

I've now added N00064141.jpg to my map of Tethys. I'm considering adding a few others as well. For now you can view this initial update at the following URL:

http://laps.noaa.gov/albers/sos/sos.html#TETHYS.

My newest revision has 3 images from the 7/23/2006 flyby.

This post has been edited by scalbers: Aug 14 2006, 05:43 PM

[Exploitcorporati...]

Hello,

Back from the Digital Grave again. Here's an alternate take on the 23 July Tethys mosaic:

[Exploitcorporati...]

Just for good measure, here's a vastly improved (over my embarassingly disjointed original) manual stitch of the regional observation from the September 2005 encounter. The large, deeply shadowed basin that appears at the terminator in the 23 July image is central to this mosaic. I love how the July image has the three big basins visible (Melanthius, Odysseus on the limb, and the near-vanished one entering darkness).

[Phil Stooke]

Beautiful mosaics.

In the past I have suggested another explanation for the equatorial dark stripe - endogenic - but that was never a very strong argument. In the face of more recent observations of the Saturnian system (equatorial ridges on Pan, Atlas and of course Iapetus) it looks much more likely that this is another manifestation of the same thing.

Maybe Tethys interacted with a diffuse ring to accumulate this darker material. The quantity would be insufficient to do more than dust the surface, so there's no ridge. Maybe this was a coorbital destroyed in a catastrophic impact, forming a temporary ring in the orbit of Tethys. Eventually it accretes on the leading side of Tethys. Some images show that the dark stripe has a brighter central division. This might indicate a later icy rather than dusty ring doing the same thing, but I think it's more likely that it reflects a slightly inclined ring. Either that, or the ring itself was double, like the Jovian ring associated with Thebe and reflecting its orbital inclination.

Phil

[ugordan]

Great mosaics, Exploitcorporations! Your mosaics such as the second one and your Europa work is so crisp and sharp that it leaves me wondering how you pull it off! Also glad to see you back again.

Here's a little three-frame approx. natural color animation showing Tethys rotating. The sequence was acquired by Cassini on 20/21 Sep 2005. The first two frames were taken at a distance of 1.38 million km, while the 3rd one at 1.58 million km. The phase angles were 50, 58 and 56 degrees, respectively. Sub-spacecraft latitude was practically the equator (less than 1 degree latitude in all three cases) and the corresponding longitudes are shown in the animation. The rotation is roughly 70 degrees between each frame and, obviously, it's prograde so features disappear on the sunlit limb.
Narrow-angle red, green and blue frames were composited and enlarged 2x after scaling the third image to the same scale. North is approximately up (to within 5 degrees).


The sequence nicely shows a stark difference in albedo and color between hemispheres. Since the phases are roughly the same, the brightness variation can be approximated to be mainly due to surface differences.

This post has been edited by ugordan: Oct 25 2006, 07:50 PM

[ngunn]

In the face of more recent observations of the Saturnian system (equatorial ridges on Pan, Atlas and of course Iapetus) it looks much more likely that this is another manifestation of the same thing.

Emplaced ring features becoming quite the norm! Wonderful. Now where is tasp . . . .

[ugordan]

Maybe Tethys interacted with a diffuse ring to accumulate this darker material.

Somehow these ring theories seem unconvincing to me. The dark band is just too precisely confined for it to be a ring interaction artifact. Tethys has an inclination of 1.12 deg to Saturn's equator, coupled with the semimajor axis of about 295 000 km this translates to vertical excursions of +/- 5800 km. If a ring were coorbital with Tethys, it would be quickly perturbed by Saturn and Tethys itself to a similar height scale. This is an order of magnitude greater than Tethys' diameter. What I'm getting at is that the ring particle impacts on the leading side would be quite smoothly dispersed over all latitudes, not being confined to a single narrow band.

The ridges on the ring moons I can buy as they are virtually at zero inclination and vertical excursions are on the order of a few km at most, compared to a few tens of km of their own dimension. As for Iapetus, I can say the same about a diffuse ring that orbits Saturn. A private ring might work (someone really should do a dynamic simulation on that!), but I find even that a bit of a stretch.

[Phil Stooke]

All very good points and probably quite enough to shoot the whole idea down!

Phil

[Michael Capobian...]

Hello,

Back from the Digital Grave again. Here's an alternate take on the 23 July Tethys mosaic:

Excellent. Your processing makes it clear that the terrain difference does postdate Melanthius, and propagates across it.

Michael

[tasp]

My formal training in the space sciences is sadly lacking. And in the process of going over the ridge system on Iapetus, I have done an excellent job of convincing myself that equatorially emplaced ring systems are not to be expected on satellites this close to Saturn. Perturbations of the proto-ring debris cloud (should we get that far 'round Tethys) by Saturn will bump up the orbital eccentricities of the individual particles and chunks, and I expect them to contact the Tethysian (Tethysusian?) surface prior to their collapse to the Laplacian plane.

--However--

The process might run to an 'intersting' extent prior to the materials all being deposited.


Additionally,

I am wondering if Cassini photos might be computer scanned with the idea being we are looking for small elongated secondary craters that resulted from grazing impactors. We would be looking for long axes that all point back to the parent crater. The areal extent of debris fields associated with specific impacts might be interesting for further study if the fields can be discerned. To study a field formed prior to Tethysian tide lock with Saturn would be interesting, we might be able to derive a rotation period for Tethys (or whatever moon) at the time of the impact.

[ugordan]

I am wondering if Cassini photos might be computer scanned with the idea being we are looking for small elongated secondary craters that resulted from grazing impactors.

The problem I see with that scenario is that the impactor needs to be coming in at a very shallow angle, any other angle will produce basically a circular impact feature as the impact energy is uniformly coupled to the ground. There were simulations like that mentioning this (might have been a Mars Express release on an elongated crater a while ago).
Now, it's arguable Tethys' gravity isn't strong enough to bring an ejecta fragment travelling at a significant speed as to cause a big impact crater back to the surface, instead of escaping to space. I'd expect slower fragments to fall back "gently", as an ejecta blanket while others just get launched into Saturn orbit. Keep in mind Tethys' escape velocity is 390 m/s, minimum orbit velocity is 280 m/s and that's not particularly energetic.

[scalbers]

Hello,

Back from the Digital Grave again. Here's an alternate take on the 23 July Tethys mosaic:

Glad to see you back among the digital living (and exploiting). Your mosaic is proving to be helpful in checking the navigation of recent images on my map, and in adding further images. Among other things, I'm starting to account for the non-spherical shape of Tethys. I have the triaxial numbers from a Voyager era paper, however I now gather there's an updated set from a paper (I think by Peter Thomas in Sept 2006). Has anyone taken a look at this update? Was this from the DPS conference?

[Rob Pinnegar]

I was just looking over the last couple of pages of this thread (most of which was posted several months ago) and wanted to float a late idea here.

This has to do with the dark band on Tethys. It was pointed out a couple of pages back that the notion of Tethys sweeping up a diffuse ring just doesn't work because of the satellite's orbital inclination -- the whole leading side should be darkened, not just the equator.

This is a super-long shot, but is it possible that the dark band might be the result of Tethys running into the dust cloud of a comet disrupted by a close pass by Saturn? Remember how Shoemaker-Levy 9 spread out into a long "line" of material -- that's what I've got in mind here.

If a comet were disrupted by a close pass by Saturn (perhaps including a pounding by the A and B rings), its remains might have been able to make it out to Tethys' orbit without spreading out too much. This could account for the poles not being "dusted".

I think this is unlikely, as the band's orientation is quite close to the equator, which just seems too coincidental. Also, this idea doesn't work at all for Iapetus. That said, it's worth mentioning.

[elakdawalla]

...however I now gather there's an updated set from a paper (I think by Peter Thomas in Sept 2006). Has anyone taken a look at this update? Was this from the DPS conference?

It was from LPSC. Here you go:
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1639.pdf

Tethys: a = 540.4; b = 531.1; c = 527.5 km; mean radius - 533.0 +/- 1.0 km
More interestingly, the density they calculate is 973 +/- 6 kg/m3; that's pretty low!

--Emily