June 12 2007 Icy Moons (rev 46) Options

[alan]

Rev 46 description available at CICLOPS

http://ciclops.org/view.php?id=3245

Some highlights:

Cassini will observe Mimas, the innermost of the mid-sized icy satellites of Saturn

June 12, Cassini makes one of its closest passes of the small, inner satellite Atlas, at a distance of only 38,000 km (24,000 mi). With an average diameter of only 31 km (19 mi), Atlas is one of the smallest moons of Saturn. Atlas will only appear to be 120 pixels across (at the equator). However, these images may still provide important clues about the formation of one of the most distinguishing aspects of the tiny satellite: its equatorial bulge. The bulge is thought to have been created by material from Saturn’s A ring being deposited preferentially along the equator of the satellite.

A number of observations are dedicated to observing some of Saturn's small moons, in order to refine scientists' estimates of their orbital paths. These sequences include observations of some of Saturn's outer satellites, such as Paaliaq, Hati, and S/2004 S13

I've seen a few of these small outer moons listed recently on the raw image page. They search tool only shows them from the last few orbits. Is this something new they are doing or have they been previously listed as Sky?

[jasedm]

I think it's a recent decision to attempt to pin down the orbital dynamics of some of the newly discovered moons. To my knowledge the mission so far hasn't included any observations of these captured rocks. But out at apoapsis, it strikes me as a good use of the spacecraft's time.
It's a shame no observations are planned on this orbit of Prometheus - the only named moon so far not imaged appreciably better than Voyager managed. I believe the spacecraft comes within 65,000km of Prometheus on June 12th - evidently better opportunities are available later on (including a sub 50,000km pass on June 28th this year).

[volcanopele]

Sorry, no dice AFAIK for Prometheus. The June 12 encounter unfortunately occurs at the same time as the Atlas encounter, and the Rocks/Rings folks chose to image Atlas. The June 28 encounter occurs a very high phase angle. Can't check to see if observations are still planned, but at C/A, you would only see a thin crescent.

EDIT: LOL, I found another good reason why there is no imaging during June 12 encounter: Prometheus is in eclipse at the time.

[jasedm]

Right decision I think to go for Atlas on this orbit VP, if that was the choice. There's only one closer pass to Atlas (including the EM), but many more opportunities for imaging Prometheus close up and at a decent phase angle.
Must be a difficult job being a mission planner sometimes!!

[Decepticon]

Some distant looks at Mimas. http://saturn1.jpl.nasa.gov/multimedia/ima...0&storedQ=0

[ugordan]

Atlas images are in. It doesn't look very saucer-shaped in this geometry.

http://saturn1.jpl.nasa.gov/multimedia/ima...iImageID=113545

Pretty noisy, too.

[MarcF]

Amazing !!
The surface is so smooth. Only few craters might be seen near the terminator.
It seems completely covered by "ring snow".
And where is the equatorial ridge ?
Marc.

[ngunn]

And where is the equatorial ridge ?

Presumably around the perimeter of that image.

[pat]

...A number of observations are dedicated to observing some of Saturn's small moons, in order to refine scientists' estimates of their orbital paths. These sequences include observations of some of Saturn's outer satellites, such as Paaliaq, Hati, and S/2004 S13

I've seen a few of these small outer moons listed recently on the raw image page. They search tool only shows them from the last few orbits. Is this something new they are doing or have they been previously listed as Sky?

The outer irregulars have been imaged before and yes the target was listed as 'SKY' meaning that the target was a RA/dec coordinate. One of the German group on the Imaging Team has recently begun observing the outer irregulars to get photometry, rotation curves etc and the observation designs for these are specifically targetted to the satellite of interest hence the name appearing as the target. I think that the Cassini observation design software may have only recently been updated to accept the outer irregulars by name as valid targets.

[pat]

Presumably around the perimeter of that image.

Looking at these images it may be that it isn't an equatorial ridge at all but that Atlas is pear shaped with bulges at the equator at roughly the sub-Saturn point and 180 degrees from the sub-Saturn point (anti-Saturn point?). The geometry in the latest images has us looking almost straight 'down' onto the 'bulge' facing away from Saturn. It reminds me of satellite imagery looking down on a mountain. Possibly the 'bulge' is cone shaped.

So maybe Atlas is roughly spherical with two large cones stuck on pointing towards and away from Saturn.

[volcanopele]

We are looking down on the southern hemisphere of Atlas. The equatorial bulge is the smooth periphery of the image. The south pole is within the rougher bump seen at the center of the disk.

[pat]

We are looking down on the southern hemisphere of Atlas. The equatorial bulge is the smooth periphery of the image. The south pole is within the rougher bump seen at the center of the disk.

Yep, I typed before I'd actually done some work on the geometry. Volcanopele is right, we're pretty much looking down on the south pole so the images are entirely consistent with an equatorial bulge of newer, smoother material ontop an older more eroded ellipsoid. The 'saucer' is back and the 'lemon' out the window. Ah well, it was nice while it lasted.

[MarcF]

OK, thanks. Now I see the perspective.
We can imagine that the actual moon corresponds to the rough core, and the smooth belt to catched ring particules.
Marc.

[Ian R]

Here's a version of the picture created from several stacked images. The noise has been removed from each indivdual layer:

[elakdawalla]

Why is there so much noise in the images -- is it just because we're real close to Saturn and there's lots of charged particles flying around?

I checked the tour page and it appears that periapsis this time around was at 2.7 Saturn radii. We haven't been this close to Saturn since Rev 14 (there was a nice Pandora flyby then, and I note those images were also noisy).

--Emily