[X] My Assistant

[dilo]

Most recent narrow-field images of Enceladus, reported in the "raw images"section of Cassini homepage, are very interesting but also pretty strange...
Images link are reported below and were taken on February 17 using the CL1 and CL2 with increasing exposure time.

http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=32337
http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=32338
filtershttp://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=32339

Looking in particular to first one, sky is bright while Enceladus darkside still black. Apparently, the only possible explaination is that background is the dark side of Saturn atmosphere, but the presence of a star trail below the satellite demonstrate that, incredibly, background is the sky (see first attached image, which is the sun the 3 images associated with RGB channels). I don't know encounter geometry and I will appreciate very much if anyone can help me...
Anyway, the interesting thing is the bright halo near the bottom of Enceladus, clearly showed in the second attached image (an elaboration made strating from first and last image). Could this be the plume produced by some geologic activity (like an iceberg/volcano)?
Regards...

Attached thumbnail(s)

 

[remcook]

it looks like the sun coming from behind enceladus. the 'plume' looks like part of the sun coming up.

[Mongo]

My guess is that Enceladus is being seen against the Milky Way, which should be bright enough to appear brighter than the dark side of the satellite. I have seen very similar 'blooms' on other photos, at the same locations relative to the illuminated objects, as well; I think that it may be caused by the ISS optics.

[erwan]

My guess is: there is three different Enceladus illuminated parts:
At left, Saturn illuminated: just a crescent, so Saturn is (left) behind: part of the F.O.V or not?
center :not illuminated
Below right: Sun illuminated, overexposed, beware of light diffusion artifacts of the very bright Enceladus.
It's a guess?

[ilbasso]

Very interesting lighting at work here!

The sun cannot be behind Enceladus in these pictures, because if it were, then we would not see any part of a crescent directly illuminated by sunlight. If moon is between us and the sun, then the sunlit part of the moon is entirely hidden from us (why we don't see a crescent moon during a solar eclipse).

In these images, the sun is shining from the bottom right. Is the reflected light that illuminates the left side of Enceladus coming perhaps from sunlight reflections off of Saturn's rings? What delights me in these images is the sharpness of the shadows along the terminator on the left side of the image. We can't see the terminator from Earthshine on our own Moon (because we're on the object casting the light onto the Moon)...I wonder if it casts shadows at the earthshine terminator?

I still think that Saturn must be the background of this picture. I don't think the background glow could be the Milky Way; it just doesn't look right to me. I wonder if the "star trail" is an artifact of the CCD. Or is it the trail of a previously unknown, very small moon passing between Enceladus and Saturn?

I'm sure that the viewing geometry data will answer all these questions! But it's sure fun to guess.

[erwan]

Ilbasso: If I understand what you wrote: Indeed we can see the entire moon illuminated on the first days moon (the new moon in french): look carefully in the evening to the new Moon, and you will see a dark entire globe left of the sun illuminated crescent: It's the reflect of the earth illuminating the moon!

[erwan]

Finally, I'm geometrically lost!

[erwan]

A link to the next images in "Latest 500 raw images", saturn.jpl.nasa.gov: http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=32338
Looks like the background is dark sky, voluntary overexposed (see the shorter star trail, bottom).

[ilbasso]

Erwan, what I was saying was that remcook's suggestion that the 'flare' being the sun peeking out from behind Enceladus wouldn't work. The "new" moon that we see on Earth is actually several days past new -- by the time we can see a crescent moon, the moon is more than 12-15 degrees away from the sun. When the sun is directly on the other side of the moon from us, we can't see the sun, and we can't see the moon either.

When the sun peeks out from behind the moon like remcook suggested was causing the 'flare' on Enceladus, we call that "the Diamond Ring Effect." It only happens at solar eclipses and it looks like this:
Diamond ring effect


The "ring" around the moon in the Diamond Ring effect happens because the sun and moon appear to be almost exactly the same diameter as seen from Earth...so, you're seeing a little of the sun peeking out when the moon isn't 100% covering it up. But the important thing to see in this picture is that you do not see an illuminated crescent of the moon, with details of craters, like you do in the picture of Enceladus.

[dilo]

Personally, i'm convinced that Sun is outside the field of view and illuminate Enceladus from lower/right side; the secondary light source on the left must be Saturn and it's rings while background is the sky (if you look carefully to the image, you'll see two more star trails above Enceladus, perfectly parallel to lower bright one).
Anyway, what about the halo/plume in the bottom? If is real (not due to camera/lens artifacts or bright surface reflections), source probably lies in large meridional fractured regions, as showed in this elaborated image (where I strongly enhanced colors variations and fine details from press image "845_1893_1").
Any comment/suggestion on this hypothesis?

Attached thumbnail(s)

 

[edstrick]

I've posted a pic and commentary on this pictue in the "icy-moons" / "Enceladus-t3 flyby" thread before spotting this thread here. There are multiple star trails, all parallel and proportional in length to the exposure time. Enceladus is seen against a brighter background with stars visible beyond a scattering medium. The background is probably the diffuse E-ring, forward-scattering sunlight at high phase angles.

I am inclined to believe the plume-like feature is real, not a camera artifact, for reasons given in that post, primarily because there is no trace of the "plume" on the night side adjacent to the sunlit crescent, though the "plume" extends several times further from the limb than the distance in the opposite direction to the darkside where there is no plume.

[remcook]

ibasso...I see! There seems to be another source of light. hmmm.... interesting indeed

[dilo]

I used space simulator (http://space.jpl.nasa.gov/) in order to reproduce the geometry of the scene (see attached). Based on the apparent size of Enceladus (near 5.5’, knowing the size of narrow field cam = 21’), image was taken about 12.15UTC, almost 9 hours after the flyby; simulated illumination from Sun and Saturn are consistent with image (phase angles of 153 and 157deg respectively) and the only possible explaination for the sky birightness is the presence of E-ring material, mostly located behind Enceladus (which is darker).
These conclusions are supported by informations reported in the final part of following article:
http://www.newscientist.com/article.ns?id=dn7029
Here is made also clear mention of “ice geysers” as probable cause of continuous surface changes. These ice crystal partially escape the weak gravity, forming the planet's tenuous E ring, as suggested by scientists. “They are looking out for volcanic plumes on the moon's horizon, where they would be easiest to spot against the inky blackness of space”. This is exactly what observed!

PS: Please, can someone explain me how to include an image directly inside the message post, instead to use attachments? Thanks…

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[Decepticon]

Use the IMG code below the FONT button.


Press IMG add the URL and then press the IMG again.

Than your ready to post the pic.

[dilo]

Thanks to Decepticon and compliments to edstrick, who reached the same conclusions in another thread... let's wait for NASA official announce on the probable Encelado geyser and, meanwhile, this is the image elaboration that I made:
[IMG]
http://mer.rlproject.com/index.php?act=Att...pe=post&id=5676[/IMG]
Regards...

[dilo]

Last trial, finally it seems to work!...

[Guest_BruceMoomaw_*]

Personally, I'm waiting to find out whether Cassini detected any signs of outgassing with its multiple instruments suitable for that purpose (UV, mass and plasma spectrometers; dust analyzer).

I do note that two of the new LPSC abstracts reveal that (to the surprise of their experimenters) the UV and plasma spectrometers have not detected any sign of a nitrogen torus in Titan's orbit, indicating that at this time it is losing little nitrogen -- but there is a strong nitrogen torus centered on the E Ring, suggesting that something in that area is emitting ammonia. Enceladus would seem to be the obvious suspect. It HAS already been announced that its VIMS surprised everyone during the close flyby by not detecting any signs of frozen NH3 or CO2 on Enceladus' surface, which seems to be almost pure water ice. Is it possible that the water/ammonia mixture expelled from Enceladus' vents is warm enough that all the ammonia and CO2 vaporize and escape from the moon completely, leaving just pure water ice behind?

[edstrick]

It's "normal" for objects in the solar system to be assymetric, if only due to random events like impacts. Tidally locked moons in synchronous rotation have to align with the axis of maximum rotational inertia pointing toward the planet and the axis of least inertia perpendicular to the orbit. The large craters on Mimas and Tethys, for example, are both roughly centered in the leading hemisphere near the equator, having probably removed enough material to put that intermediate axis perpendicular to Saturn. (Iapetus's dark hemisphere is probably somehow actually controlled by it's space environment and does appear to be "painted on" in some fashion)

What's odd about Enceladus is that it's main assymetry seems to be north-south. The really interesting looking, high contrast surface features are at high south latitude, while most of the craters are at middle to high north latitude. Whatever internal geologic processes are shaping Enceladus, they seem to have put the axis of least inertia through this region.

Voyager 1 saw the south polar features in a really marginal near-zero-phase-angle long-range picture: The disk was nearly featureless except for some "scribbles" at the south limb. The Voyager 2 image coverage was from above the equator and missed the south pole entirely. Let's hope future Cassini encounters manage to really cover this region well.

[volcanopele]

Thanks to Decepticon and compliments to edstrick, who reached the same conclusions in another thread... let's wait for NASA official announce on the probable Encelado geyser and, meanwhile, this is the image elaboration that I made:
[IMG]
http://mer.rlproject.com/index.php?act=Att...pe=post&id=5676[/IMG]
Regards...

Thanks to Decepticon and compliments to edstrick, who reached the same conclusions in another thread... let's wait for NASA official announce on the probable Encelado geyser and, meanwhile, this is the image elaboration that I made:
..

I hate to burst everyones bubble, but if you look at high phase images of Enceladus, Mimas, and Rhea from last month, you will find similar camera artifacts to this "plume" when you hard stretch the image. The reason you can see it in this image is because it was intentionally over-exposed. It's alway possible that it is real but that is becoming quite unlikely.

[tedstryk]

Jason: Do you know how the groundtrack for E4 will compare. During approach and on Tc, there seems to be a region "on top" of the terrain imaged this time that was just visble on the right hand limb of the last global narrow angle imagery this time.

[volcanopele]

We will get a nice mosaic of this hemisphere:

http://ciclops.lpl.arizona.edu/view.php?id=845

[scalbers]

Greetings,

Regarding the geometry and lighting effects of the Enceladus flybys, I have found the Celestia
software to be a very appealing and dramatic way to visualize this. For those who are unfamiiliar
with Celestia, it is a freeware software package and you can do Cassini point of view animated simulations, using detailed planetary maps. You can even update the maps with your own versions to improve upon the defaults.

The URL is http://216.231.48.101/celestia/


Steve Albers

[volcanopele]

nope, still an image artifact. If it were real, then when viewed from the same geometry, here are plumes over Mimas, Rhea, Tethys, and Titan over the same area of the crescent.