Oops! Thanks for the correction. Glad I got it here before I posted the wrong thing in the blog

--Emily

I noticed the "darkened" images of Iapetus and wondered what had caused that -- it never even occurred to me that it might be Saturn's shadow. Neat.

Btw, I once (2006-03-14) posted this:

Didn't realize the event of an eclipse at that time!

Bye.

Interesting . . .

Iapetus traversing the Saturnian shadow and the Saturnian magnetotail. Effluvia wafted from the Titanian atmosphere possibly lofting into Iapetus' space . . . .

This is probably an FAQ for the unhip (and please feel free to slap me if needed!), but will Cassini get any really good views of the dark/light border areas this September? Seems as if that's key for determining the origin of the Great Dark Splat...

Yes. Mostly on the side of Iapetus that faces away from Saturn.

Click to view attachment
We'll get a fairly good view of the most interesting 'White mountains' or 'Voyager mountains' at the western border of Cassini Regio to Roncevaux Terra...
CASSINI will follow the equatorial ridge from east to west first looking at a cresent Iapetus.

Btw, there will be two far encounters before:
Click to view attachment
April 15th

Click to view attachment
July 5th

Both will show some parts of the terra incognita including a huge bassin about 400-500 km in diameter...

Bye.

Thanks, TA & Rob.

Main thing is that we need to examine the border region with as much high-res as possible. The outer edges of the deposit should provide some clues on exactly how this stuff was laid down. Abrupt or very gradual borders could indicate a radiational origin as Tasp suggests, while streaky, irregular borders would suggest either an eruptive origin or infall from somewhere else.

We might note a correlation between degree of darkening, altitude, and longitude in the light/dark coverage.

Seems like a mathematical model or simulation analyzing sun exposure (insolation), local slope, altitude, longitude, and the Iapetan parameters of ~80 day long 'days' and introduction of a gaseous discoloring agent at aphelion could be made. Program needs to be recursive, the darkening modifies the local temperature and facilitates further darkening.

Interesting if someone comes up with some random topology to run through the program to see if it generates Cassini Regio like stains on the object.

Or tackling the problem the other way round, someone (more capable with math than I) confirms images of Cassini Regio darkening does correlate with altitude, local slope, longitude, and insolation . . .

Hi,
I just read that the Iapetuseclipse in february was actually caused by Saturn's rings:
Click to view attachment
Date: 2007-02-13
Distance: ~ 2.3 mio. km
Image scale: 14 km/pixel

Bye.

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.

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.

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

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!

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

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.

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

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.

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.

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...

Click to view attachment

Bye.

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

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.

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

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.

No, it really is November 2005. The January 2006 encounter was more distant, in fact I've got the entire sequence assembled into an image and an animation. It can be found here.
The distant approach images from November and January are pretty similar, but if you look closer the January images show more of the Snowman in sunlight.

That November 2005 image is interesting for another reason. Look at the 8 o'clock position on Iapetus' limb. You can see one of the large basins in profile. This one isn't normally as easy to pick out as some of the others.

It looks to me like the upcoming April 15th shots of Iapetus might have a chance of catching the big Roncevaux Terra basin in a similar profile.

Hi,
here some infos about the upcoming far-encounter:

april 15 -> 2 clear und 19 color images
april 17 -> 1 clear, 6 color and 3 WAC-images
april 18 -> 2 clear, 34 color and 2 WAC-images
april 21 -> 2 clear, 6 color and 2 WAC-images

Resolution (NAC) will be about 14 km/pxl.

Bye.

So,
NASA's Solar System Simulator shows this:
Click to view attachmentClick to view attachment
Click to view attachmentClick to view attachment

Bye.

More food, Iapetus hounds:
http://saturn.jpl.nasa.gov/multimedia/imag...iImageID=107531

For fans of Roncevaux Terra, the first set of Iapetus images from this very distant encounter are now up.

As I'd hoped, it looks as if the big trailing-side basin can be seen in profile as a "flat spot" on the moon's limb at about the 4:00 position. There seems to be a central peak visible.

There are also a couple of dark spots visible that may not have been seen before. And (once again) we get a tantalizingly fuzzy look at the Snowman crater system.

No sign of the bellyband on the moon's limb -- but 2.2 million kilometres is very long-range -- it may just not be that high at that particular point. We'll find out in a few months.

Wow. If there's one Iapetus image that's radically different from all previous Cassini images of this moon, it's this one.

Ahh,
there they are...

Here two takeouts, ~3x enlarged:
Click to view attachment
Distance: 2,258,272 km
Filters: BL1 and CL2

Click to view attachment
Distance: 2,258,232 km
Filters: UV1 and CL2

The trailing-side basin must have been a really big hit.
Interesting is also huge step/rim in the north polar region.
Is there a hidden basin? A tectonic fracture? Or what else?
Curious to see the moons almost bright side,
doesn't look like Iapetus...

Bye.

I noticed that too. I think it's probably a topographic feature that looks brighter than the surrounding area because it is tilted towards the Sun. We saw this kind of thing in the south polar region last year.

I remember such a structure from this saturnshine image:
Click to view attachment
Date: 2004-10-22
Distance: ~1.6 million km

Could be the same or a different one...
I'm not quite sure, wasn't this region observed during the Newyear's flyby 2005?
There were some saturnshine images...

This is a composite of four of the new images. I recall that stereo viewing of Voyager images of this northern bright area revealed apparently tectonic lineaments in this area - probably one of the old NASA TM reports on the geology and geophysics program.

Phil

Click to view attachment

If I'm not mistaken all these features show up very well on the Iapetus mosaic map on CiCLOPS. The big basin complex lower centre at about longitude 320 and the ones up north at about longitude 0. If I'm totally wrong please tell me gently.

Click to view attachment

Any idea for these structures? Real or artificial image errors?

Iapetus is a relatively small part of the image thus the image is more likey to get overstretched by the algorithm they are using. Thats what appears to have happened to the bright patches which are located in the area which is receiving the maximum sunlight.

Thanks for the composite, Phil.

In addition to the poorly known parts of Iapetus that are near its limb on this sequence of photos, we're also getting a much better look at some topography on the terminator. There are a couple of big craters on the northern terminator that have been hinted at in previous images but for one reason or another just haven't been very visible before now.

Over the course of the next few days these craters will become more visible. Cassini seems to be "chasing" Iapetus for the time being -- it will maintain about this distance for the next week or so. Perfect time for shooting stereo pairs!

Info on upcoming Iapetus imaging on Rev 43 'looking ahead' on Ciclops now.

More images appearing now.

So,
next series arrived...

Here two further takeouts, ~4x enlarged:

Click to view attachment
Date:2007-04-17
Distance: 2,310,887 km
Filters: CL1 and GRN

Click to view attachment
Date:2007-04-18
Distance: 2,334,981 km
Filters: CL1 and CB2

Interesting craters near the terminator in the northern hemisphere getting more and more visible now...
The huge step/rim in the north polar region is still recognizable - there even seems to be a kind of mountain near the pole at the disk rim there!
And still the bright patches at the sunlight equator region of Roncevaux Terra ...

Bye.

Iapetus, the Yin and Yang moon

I'd rather say 'tennis ball moon'- more three dimensional and less philosophical baggage.

TritonAntares I've just created a cross-eye stereo pair from two of your recent images by photocopying them at 50 and 42 percent to match the sizes. It took only a minute to do and works a treat.

Hi,
next pics are online...
A takeout, ~4x enlarged:

Click to view attachment
Date:2007-04-21
Distance: 2,442,013 km
Filters: BL1 and CL2

Interesting overlapping craters now visible at the left side near the terminator in the northern hemisphere.
Sadly the distance being so large once again -
what a sight this angle of vision would have offered from say 400.000 km... Click to view attachment

Bye.

My version of that latest set, 4 images merged, showing the giant basin on the terminator.

Phil

Click to view attachment

...and the next one...

Phil

Click to view attachment

Wow - those two make a beautiful stereo pair (but you have to place them diagonally).