Very, very little in comparision to Enceladus, but there may be some small-scale current activity. Article here.

So, they indeed were looking for a plume at Dione when they took images at high phase angles !!
Marc.

How can we be sure that it's just not ejecta from impacts? Sounds like very small amounts of materials leaving the moon.

By taking similar measurements for Tethys and Rhea.

Dione and Enceladus are actually in an orbital resonance... I guess that what we are seeing here is the warmth at the other end of the hot poker, so to speak.

Enceladus and Dione are the densest of the icy Satrunian moons

Enceladus at 1.61

Dione at 1.44

See this CHARM presentation for predictions of Dione outgassing.
http://saturn.jpl.nasa.gov/multimedia/prod...ARM_Krishan.pdf

More AL26 in their early compositions?

Craig

Nothing to do with outgassing, but here are the latest pics of Dione - I have merged the two and fiddled with the brightness of the terminator and saturnshine area.

Phil

Click to view attachment

I don't know whether it may have something to do with this, or whether this phenomenon has been explained in some way, but I remember that the Voyagers discovered that radio emissions from Saturn are further modulated with a period matching that of Dione.

Nice, Phil!

Hate to say it, but I don't think there's any way in hell to visually spot "plumes" who's average output is about 6g per second. Hopefully we can get some targeted hi-res IR along the floors of some of those canyons...

Phil - nice image, where might that basin be? The best candidate I can spot on my map would be near 210 E longitude and 50S.

It's the same one we have often seen before in these southern hemisphere views, Steve, just a bit west of the Voyager 1 area. There's only one near the South pole.

Phil

Yikes - here it is again. This new sequence has a very long exposure image... Carolyn had better get onto it before someone else starts publishing latin anagrams.

Phil

Click to view attachment

I'm having trouble orienting myself here...would one of you image magicians be so kind as to post a polar projection of Dione's south pole?

--Emily

This observation covers the southern leading hemisphere. The big crater at upper left is Dido, the fractures cutting from upper right to lower left across the middle of the image are the southern part of Palatine Linea, and the large crater to the left of the big basin is Sabinus.

Where is the south pole on this image? That's what I'm having trouble figuring out.

--Emily

...yeah, I remember that too! Does Dione have an associated torus of water & dissociate products like Enceladus, albeit much more attenuated?

Here is an orthographic view of the southern hemisphere from directly below using Steve Albers' map:

Click to view attachment

And the approximate viewing geometry:

Click to view attachment

The subspacecraft latitude is approximately -74° and the subspacecraft longitude 278°.

Original from Steve Albers.

Phil

Click to view attachment

Thank you very much, Bjorn and Phil. I was having trouble figuring out what those lineaments were doing as they approached the south pole.

Dione is no Enceladus; while there are some fractures wandering around the south polar region, you can see from all the craters that we're looking at a reasonably ancient surface. I suppose there's no reason to assume that Dione would necessarily be a one-plume body with the plume at the south pole, either. If I were forced to guess where outgassing might be happening, I'd guess the "blue" walls of the tectonic fractures formerly known as wispy terrain.

Hmm...I wonder if there is anything interesting hidden in all the information that's been lost to stretching and JPEG compression...we'll have to wait and see...

--Emily

Just for fun, an animation of five short-exposure clear-filter images taken at equal intervals, about 3,700 km or 10 minutes apart:



--Emily

Dione simmering....

http://www.space.com/scienceastronomy/0704...dione_moon.html

Craig

The problem of course with imaging any venting from Dione is whether 6 grams of water vapor is enough to have much, or any for that matter, entrained ice grains, which we could observe with ISS, for example. The obvious candidate location for venting are the bright fracture walls in the wispy terrain (maybe sapping slowly releasing water vapor...)

Would it be difficult to tell whether there is frost deposition on the fracture walls? Even without frost deposition, those walls should be very reflective. It'd be the difference between "bright" and "brighter", I guess.

Latest pics are up. Look at this one:

Click to view attachment

Irregular nested craters containing large irregular positive relief features, lying on a fracture belt within a generally smoother plains region. If one didn't know better one might interpret it as a volcanic complex.

Phil

They remind me Ali Baba and Aladin craters on Enceladus.
Marc.

A lower resolution view from a year ago or so shows generally the same area, but in lower resolution. That particular image (which has south approximately at the bottom) appears to be rotated 180 degrees w/respect to this one.

Quick-and-dirty mosiac:

Click to view attachment


Best daylight view of the cross-section craters from the targeted pass as well.

Nice! What kind of sharpening do you use, it doesn't bring out JPEG artifacts at all?

In this instance it was simply unsharp masking at 100% and a radius of one pixel with a theshold of 0 levels on the full-sized mosaic. Seems to work really well with these images...positional accuracy and limb fit are another matter entirely...definitely a compromise.

I'm sure this has likely been brought up somewhere before, but do the broad fractures visible around the big unnamed basin near the south pole in many views of the region seem to radiate from it?

Probably just an artifact and a coincidence, but the way the faint glow beyond the terminator peaks near the nested craters certainly is interesting.

I brightened the terminator area to make near-terminator topography more visible - which I always do in my processing - and this will seriously mess up the dark space beyond the terminator. Don't read anything into it.

Phil

Exploitcorporations: " I'm sure this has likely been brought up somewhere before, but do the broad fractures visible around the big unnamed basin near the south pole in many views of the region seem to radiate from it?"

I would rather interpret those broad gouges as being like Vallis Bouvard, Vallis Schroedinger, and indeed Vallis Rheita on the Moon, coalescing secondaries and/or gouges cut by ejecta emplacement. The old idea (first half of 20th Century) that impact basins would produce radial fractures (Imbrium sculpture and so on) is largely discredited.

The best Voyager mosaic of Dione showed a set of ridges at the southern terminator, plus a very subdued trough parallel to them, all now revealed to be related to that big basin.

Phil

This just appeared on the ESA website.

See the Editor's Summary ("In the shadow of Saturn") on the Nature website for links to the paper.

I can understand Dione maybe ejecting particles to make up a plasma torus, but Tethys? Where is the energy coming from? It certainly isn't radioactive heating, there are more silicates in my backyard than there are inside Tethys. You also suffer from the same problem with tidal heating, you are going to run into problems generating enough heat without something other than water. If these are sputtering or micrometeorite generated, why doesn't Rhea have a similar torus (or does it and I missed that memo...)?

I expected some activity from Dione..... this moon shares a higher density, along with Enceladus, than the other mid-sized moons.

But Tethys?????

Curioser and curioser!!!!!!

Craig