New Iapetian image series Options

[TritonAntares]

Hi,
I just read that the Iapetuseclipse in february was actually caused by Saturn's rings:

Darkness sweeps over Iapetus as the Cassini spacecraft watches the shadow of Saturn's B ring engulf the dichotomous moon. The image at left shows the unshaded moon, while at right, Iapetus sits in the shadow of the densest of Saturn's rings.

Date: 2007-02-13
Distance: ~ 2.3 mio. km
Image scale: 14 km/pixel

Bye.

[nprev]

Cool; didn't know that either, thanks!

Hmm. Anybody know exactly how much the illumination is cut down during these events? The total solar energy @ Saturn is only about 15 W/m^2. I can't tell how much loss happened during the Iapetus eclipse, but seems as if knowing that based on Cassini's imaging capabilities might provide some interesting data on the properties of the B-ring.

[scalbers]

There was also some pre-Cassini info on the B-ring attenuation from the occultation of 28-Sagitarii in 1989 visible from Earth. I watched this visually in my 6" telescope and could see the star fluctuate in brightness. Perhaps there are more accurate measurements that were made?

Also, I believe there were stellar occultations of the rings observed by Voyager and Cassini.

In any case it might be challenging to measure the densest portion of the rings.

[edstrick]

A Voyager watched a stellar occultation by the entire rings with the (partly crippled) Photo-Polarimiter instrument. With a very high data sample rate, it got radial sampling across the rings of a few ?10's? of meters... Astonishing structural detail. Cassini uses the UV instrument for this, I think. (Voyager also got UV occulation at lower resolution)

The radio science occultation data was limited ... INITIALLY ... to the kilometers-sized beam-spot of the diffraction pattern of the radio beam on the rings. .. That was until they processed the data with time/frequencey analysis much like the synthetic aperture radar processing and got nearly diffraction (and signal-to-noise) limited resolution in the rings. Note that the 2-frequency radio saw dramatic differences in ring opacity at S and X-band wavelengths.. directly due to different abundances of ring particles in the size range difference between the 2 wavelengths!

The B ring is highly opaque, and the shadow on Saturn is essentially truely black. Even multiple scattered light filtered-through the rings barely illuminates b-ring particles on the shadow side of the rings in the most opaque areas, even in long exposures

[CAP-Team]

Today I was looking at a rendering I created this morning, I noticed a weird spot on Saturn:



It apears to be a shadow of Iapetus, traversing Saturn's disk!

[ugordan]

Cool! This is no doubt on Cassini's imaging todo list.

[Rob Pinnegar]

You can see that using the Solar System Simulator too. Just enter "Iapetus seen from Saturn" for 6:50 UTC March 27th.

It looks like a near-miss in the Simulator, but that's just because it's off Saturn's centre a bit. Looks like roughly 10% of the Sun's light was blocked during this transit, at any particular spot under the penumbra.

[volcanopele]

I hate to burst everyone's bubble, but no images of that are planned. Sorry. Would have been cool though.

[ugordan]

From the latest PDS batch, here's an approx. natural color composite of Iapetus from June 2006 and alongside it a view from November 2005 showing almost the same hemisphere, but with different illumination:

Both images magnified 2x from original pixel scales.

[JRehling]

From the latest PDS batch, here's an approx. natural color composite of Iapetus from June 2006 and alongside it a view from November 2005 showing almost the same hemisphere, but with different illumination:
[image]
Both images magnified 2x from original pixel scales.

You know, I think this image pair (and/or others like it) could be incredibly useful to help answer the question of what is going on with Iapetus, but ordinary eyeball power isn't sufficient to help.

Basically, the second image gives us a lot of information about the relief of Snowman-A, while the first image gives us an albedo map. The nontrivial task of correlating the two could offer some important insights. This is one of the few places on Iapetus where the first-order model of Cassini Regio (an ellipse with fuzzy northern and southern borders) can be seen to break down on a macroscopic scale, in concert with relief. So what is happening? Is the eastern slope of Snowman-A's central peak light or dark? What about the inside rims? The outside rims? There are some possible answers here that would offer definitive rejection of some theories for the origin of CR. For example, if there's a relief feature with dark stuff on the eastern slope but white terrain on the western slope, then the theory that something is swept up as it strikes Iapetus's leading side is seemingly refuted.

[TritonAntares]

Let me just mention the far encounter later this month -
finally a chance to see another form of shadow casting at the Snowman as Iapetus left side is now in dark...



Bye.

[Michael Capobian...]

Here's a stereo view that gives a pretty good idea of the Snowman/Moat topography.

http://mysite.verizon.net/vzepeq2k/iapetus/id3.html

Michael

[tasp]

. . . (an ellipse with fuzzy northern and southern borders) . . .

Sorry I don't recall where the postings are, but someone here has informed the board that the tilt of Iapetus' orbital plane about Saturn varies over time. This seems to explain the more diffuse N-S boundaries compared to the E-W ones.

There is a consistent relationship to the degree of darkening on the Iapetan surface between elevation, local slope, latitude, and longitude referenced to the meridian directly aligned to the sun when Iapetus is furthest from the sun in it's orbit about Saturn.

There is a simple mathematical relationship correlating the degree of darkening to the above parameters. Sorry, my rudimentary mathematical skills aren't sufficient to write it out.

[CAP-Team]

Let me just mention the far encounter later this month -
finally a chance to see another form of shadow casting at the Snowman as Iapetus left side is now in dark...

Bye.

While a distance of 2,3 million seems fairly close, any pictures taken won't have the fine detail we all want to see. Fortunately the september flyby is only 5 months away

[scalbers]

From the latest PDS batch, here's an approx. natural color composite of Iapetus from June 2006 and alongside it a view from November 2005 showing almost the same hemisphere, but with different illumination:

Both images magnified 2x from original pixel scales.

Ugordan - nice PDS views of these images. The region around 0 degrees longitude at mid-far southern latitudes interests me in that it is somewhat tricky to navigate onto my map, so the more examples there are that show that area, the better. The second view though looks to me more like it's from January 2006, could that be the case? The November 2005 flyby was at a more northerly latitudes I think.