[X] My Assistant

[TritonAntares]

Hi,
CASSINI has transmitted 184 pics (!) over the last days.
Here five takeouts, 3-4x enlargement:



Date: 2009-09-06
Distance: 2.228.548 km
Filters: CL1 and CL2


Date: 2009-09-08
Distance: 3.215.284 km
Filters: P120 and GRN


Date: 2009-09-08
Distance: 3.216.610 km
Filters: P60 and GRN


Date: 2009-09-09
Distance: 3.390.271km
Filters: P60 and GRN


Date: 2009-09-09
Distance: 3.427.313 km
Filters: P120 and GRN

Maybe somebody is able to combine some of those images to show more details.

Bye.

[Rob Pinnegar]

They certainly took enough images this time around. There must be at least a hundred new ones.

[Edit: oops, 184; didn't read the above post carefully the first time.]

[Rob Pinnegar]

At the end of November, Cassini will be a bit over two million kilometres from Iapetus. That hardly even qualifies as a "distant encounter" -- it's really just plain "distant".

Nonetheless, looking at the Solar System Simulator for dates around November 25th, any images taken around this time will probably be nice to look at, because they should show a bit more of the trailing, light-coloured part of the Saturn-facing hemisphere, Roncevaux Terra, than we have seen before. (Of course, we've seen some of it relatively close-up in Saturnshine during the New Year's 2005 distant flyby.) Cassini will be near apastron then, so there should be a bit of free time for taking pictures of Iapetus.

We haven't seen much of this part of Iapetus in previous Cassini images. My impression (correct me if I'm wrong) is that this is due to the geometry of Cassini's orbit around Saturn, up to the present time. Whenever Cassini is well placed to photograph the trailing side of Iapetus, it will normally also be very close to Saturn, which means that other things would naturally take priority over getting fuzzy pictures of a moon three million kilometres away.

[angel1801]

I just used the Solay System simulator and I have the following:

Date: November 27, 2006
Time: 00 hours (UT)
Distance: 2.008 million km

A full disk view of the Saturn facing hemisphere of Iapetus at 12km/pxl

I'm sure with multiple images and super ehancement, we can fill in a imaging gap of Iapetus
in the high northern latitudes at better than the 9km/pxl that Voyager 2 achived in 1981.

[TritonAntares]

Hi,
here what you're talking about:

Should give some more additional information about northern Roncevaux Terra as well as a nice view of the 'Snowman'.

Bye.

[Decepticon]

I can't wait to see that!

[CAP-Team]

[something went wrong here]

[CAP-Team]

I created the same image with Xplanet and Steve Albers' map:



As you can see, it doesn't really cover any lands we haven't seen before..

[Rob Pinnegar]

As you can see, it doesn't really cover any lands we haven't seen before..

Maybe not, but it will give us a chance to see some familiar territory from a different lighting angle. This will also be the case during the three remaining 2-million-kilometre flybys (around Feb 11th, Apr 15 and Jul 5) before the close-up view a year from now.

I think it's during the February encounter that we will see sunrise on "Snowman" over the course of several days -- instead of sunset, which has usually been the case. That should be of some significance for people who are interested in the "moat" (even though, at that distance, we'll hardly be able to see any detail).

[tedstryk]

I think that super-resolution processing may be needed to pull any nice looking images out of this. But that would have to wait for the PDS release (Stacking the "raw" jpegs will help compensate for compression artifacts, but does litttle else to improve resolution.

[ugordan]

2-million-kilometre flybys

Now that just sounds silly!

I think that super-resolution processing may be needed to pull any nice looking images out of this. But that would have to wait for the PDS

So in 12 months time or so we'll get to unleash our image processing skills on a few 2 million km distant frames... By that time the 1500 km flyby will be over and noone will even look back at these puny frames!

[TritonAntares]

Maybe not, but it will give us a chance to see some familiar territory from a different lighting angle.
This will also be the case during the three remaining 2-million-kilometre flybys (around Feb 11th, Apr 15 and Jul 5)
before the close-up view a year from now.

I think it's during the February encounter that we will see sunrise on "Snowman" over the course of several days
- instead of sunset, which has usually been the case.
That should be of some significance for people who are interested in the "moat"
(even though, at that distance, we'll hardly be able to see any detail).

The april and july far-encounters will show some other details in different lightning angle:

The later one will probably show some parts of the mostly unknown great southern bassin.

Bye.

[TritonAntares]

Hi again,
new release in the NASA/JPL image gallery - 'Duotone Moon':

Date: 2006-09-06
Distance: ~2.2 mio km
Resolution: 13 km/pxl

Bye.

[CAP-Team]

I used NASA's space simulator a lot in the past, but I think their maps aren't quite as good as Steve Albers' maps.



The view of July is finally giving us a view of unexplored terrain, just prior to the close encounter of september 2007.
Really looking forward to that.

[tedstryk]

As TritonAntares said, the key is that the illumination conditions will be different. This "flyby" may be useful for albedo mapping.

[Rob Pinnegar]

The view of July is finally giving us a view of unexplored terrain, just prior to the close encounter of september 2007.
Really looking forward to that.

Yeah, that area was imaged in August 2004, but from so far away (10 million kilometres) that the only details visible were the basic outline of the Roncevaux Terra basin, and its central peak. So, although we *have* seen it, it's effectively unexplored.

The LPSC 2005 abstract by Denk et. al., "The first six months of Iapetus observations by the Cassini ISS camera", shows processed versions of these images. It's online and is easily located with a Google search. (Don't bother checking the raw images from August 2004 -- you can't see anything in those.) This gives a good idea of the size of that basin. It's about the same size as the huge one on the Saturn-facing side.

[TritonAntares]

Hi!

Yeah, that area was imaged in August 2004, but from so far away (10 million kilometres) that the only details visible
were the basic outline of the Roncevaux Terra basin, and its central peak.
So, although we *have* seen it, it's effectively unexplored.

I've looked through the images of this period and probably found some of those you meant.
Here a takeout:

Date: 2004-07-22
Distance: 3.489.136 km
Filters: CL1 and CL2
Difficult to say wether it's the western rim of the large southern bassin in Roncevaux Terra at the terminator.

This chart could be helpful:


The eastern rim of the bassin is nice in this pics from October 2004:


Bye.

[ugordan]

The lower left image is the same as the one I made a while ago, using stretched color, but toned down to better match the approximate true color. The contrast is stronger in my composite so darker details aren't very visible, though: Iapetus in color.

There's also the CICLOPS release showing the same image sequence in stretched color.

[TritonAntares]

Hi!

CASSINI took another few Iapetus images about 10 days ago,
here 3 of them:

Date: 2004-09-18, 20 and 21
Distance: 2.359.595 / 2.486.163 / 2.517.820 km
Filters: CL1 and CL2

Nice sunset out there...

Bye.

[nprev]

That recent small sequence is really striking to me; Iapetus looks a lot like a CD or DVD!

In fact...is this significant or coincidental? It looks like the dark area subtends a suspiciously well-defined angular area in this view (90 deg of the total spherical area of Iapetus?), which in turn would seem to argue for external deposition from some source. In any case, the geometry of the dark area is remarkable.

[tasp]

I suspect a thermo-reactive gas is introduced into the Iapetan environment at a fixed point in its' orbit about Saturn. Most likely when Iapetus traverses the Saturnian magnetotail.

The gas persists in the vicinity of Iapetus, and in equatorial to the midlatitudes the Iapetan surface temperatures are sufficient for the gas to 'tholinize' and coat the surface. The gas is used up (or dissipates into the void) in less than the ~80 days it takes for Iapetus to return to the place in it's orbit where the gas is replenished, thus, we do not see the darkening extending further around in longitude than we do.

That there is some 'ponding' of the gas in low areas seems apparent, there is some darkening outside of the main areal extent of Cassini Regio in the lower latitudes.

Additionally, the more directly perpendicularly sunward facing segments of crater bowls north and south of Cassini Regio also show the darkening, as expected, the local heating conditions being sufficient for the chemical staining reaction to occur.


The gas also appears to interact with Hyperion. Quantities of it appear to pond in the craters, and when the chaotic rotation of Hyperion causes the sun to shine perpendicularly into a given crater, the crater bowl reflects enough solar radiation onto the crater floor to make a local warm spot. The gas reacts, and we get dark crater bottoms all over Hyperion.

The same basic mechanism, a thermo-reactive gas being introduce into the Hyperionian and Iapetan environments, explains the dark areas of each moon.

[nprev]

That is both an interesting and highly creative hypothesis, Tasp. Skeptical questions must follow, though:

1. What is the nature of this thermoreactive gas? I presume that it is being scattered off of Saturn's upper atmosphere somehow.

2. Why do no other moons but Iapetus & Hyperion exhibit any apparent effects from this environmental condition? Probable minor compositional differences do not seem adequate. Additionally, this thermoreactive agent would presumably have a relatively high molecular weight, and therefore its transport throughout the region would not necessarily be confined to the outer limits of Saturn's magnetosphere in accordance with magnetic current flow patterns.

Sorry; I am not trying to put you on the spot at all...just trying to flesh out this most interesting thought of yours, which would have never occurred to me!

[tasp]

The gas comes from Titan's atmosphere. This implies only objects exterior to Titan's orbit will experience the darkening as there is no transport mechanism running from Titan inward.

The atmospheric gases of Titan contain carban and nitrogen and hydrogen. The thermosensitive chemical reaction (way out of my schooling here) might be a polymerization of those Titanian gases. I suspect the orange color of Titan and the rich brown of Iapetus are derived from similar compounds and elements.

I don't know if slow steady leakage of atmospheric gases from Titan is what is happening, or if big impact events on Titan liberate gases sporadically.

Maybe both processes have occured over the history of the solar system.

[TritonAntares]

Hi!

CASSINI took another few Iapetus far distance images 3 days ago,
here a takeout:

Date: 2006-11-05
Distance: 4.438.289 km
Filters: CL1 and CL2

View with Solar System Simulator:


By the end of this month we'll get a nice view from about 2 mio. km:


Finally a little change in the angle of view....

Bye.

[ugordan]

Here's the simulated pixel size Iapetus will appear on Nov 27. Obviously not that useful except for low phase observations.

[TritonAntares]

Hi once again,

CASSINI took another four Iapetus far distance images,
here a takeout:

Date: 2006-11-16
Distance: 3.128.691 km
Filters: CL1 and CL2

View with Solar System Simulator:


We're getting 'closer'...

Bye.

[TritonAntares]

Hi,

CASSINI took a series of 41 Iapetean far distance pics yesterday,
here a first takeout:

Date: 2006-11-27
Distance: 1.997.980 km
Filters: CL1 and CL2
There's an interesting dark feature east (right) of the 'Snowman'.

View with Solar System Simulator:


Part of Steve Albers map:

The mentioned dark structure contains at least one crater with a dark ground.

Bye.

[TritonAntares]

Hello again,

I always wondered about the dark patches in Iapetus' bright region Roncevaux Terra as seen on this map:

The left part is illuminated by saturnlight, the right part is made up of VOYAGER findings.
The mentioned dark feature east (right) of the 'Snowman' contains at least one crater with a dark ground.
But what about the dark markings on the bad resolved VOYAGER side?
Are these mostly shadowing effects like the craters in the northern hemisphere presume or real dark patches like the 'horseshoe' next to the border of the saturnlighted part or the black triangle and the spots around it?
Actually the triangle partly seems to overspread an underlying not so dark area...

Some ideas?

Bye.

[angel1801]

I would not know either. But we will know alot better after September 10, 2007 when Cassini will image this area with resolution as good as 200m per pixel!

[Phil Stooke]

The latest two Iapetus images, enlarged and merged.

Phil

[TritonAntares]

Hi,
nice pic Phil - I didn't notice the 2 originals on the JPL/NASA page at all.
It'll may help to measure the depth of Snowman A and the hight of its central peak...

Have you tried to combine some of the Nov.27th images, probably getting some colored ones?

Bye.

[TritonAntares]

Hallo,
one probably better resolved and enlarged pic of the last far-encounter is on the JPL/NASA page.

Date: 2006-11-27
Distance: ~2 mio km
Resolution: 12 km/pixel

Sadly no color composite...

Bye.

[ugordan]

Sadly no color composite...

Me, I've stopped expecting color composites altogether. It's as though they're taboo for the team. That said, they do manage to pull off a good sharpening/deconvolution job on the grayscales.

[Ian R]

Colour composite with Lightness channel consisting of 10 stacked images:

[Rob Pinnegar]

Is Roland the prominent crater at the 11 o'clock position?

[Ant103]

My version made from one RVB picture (saturation at 200%) and 10 b&w stacked and added pictures. 200% approx. size.

Attached thumbnail(s)

 

[ugordan]

Pink Iapetus, now that's something you don't see every day!

[TritonAntares]

Is Roland the prominent crater at the 11 o'clock position?

Iapetus northern hemisphere:



Seems to be Roland due to its position and the large central peak.
But be careful, there are 4 prominent craters in this region - one at the bright-dark border ...

Bye.

[Michael Capobian...]

Is Roland the prominent crater at the 11 o'clock position?

Yes. Here's the Voyager-based map showing the Saturn-facing hemisphere of Iapetus, which closely matches this image. Roland is at the top.

Michael

[Phil Stooke]

ugordan: "Pink Iapetus, now that's something you don't see every day!"


It's a Valentine!

Phil

[JRehling]

Pink Iapetus, now that's something you don't see every day!

This could turn into a drug-related discussion...

[ugordan]

This could turn into a drug-related discussion...

You mean something like: "There are no bad drugs. Just bad trips." ?

[nprev]

Huh...that "yin-yang" aspect makes Iapetus look like one of the hemispherical views of Fritz Leiber's The Wanderer...art imitating life in yet another subtle way, albeit decades in advance.

Just struck me also how deep & pronounced all these craters are in comparison to those on the inner large icy moons. Is this difference perhaps due to fallout from Enceladus on the latter, as discussed on another thread, or are we seeing evidence of a thicker crust on Iapetus?

[JRehling]

Huh...that "yin-yang" aspect makes Iapetus look like one of the hemispherical views of Fritz Leiber's The Wanderer...art imitating life in yet another subtle way, albeit decades in advance.

Just struck me also how deep & pronounced all these craters are in comparison to those on the inner large icy moons. Is this difference perhaps due to fallout from Enceladus on the latter, as discussed on another thread, or are we seeing evidence of a thicker crust on Iapetus?

Remembering that Iapetus is a startling outlier (literally and figuratively) in terms of its distance from its primary (only likely captured asteroids rival it among the natural satellites in the solar system), I suggest that its distance is the prime causative factor.

Look at the crusts of Mercury and the Moon and the Moon's maria. They are all three "old". The oldest crust there is not much more than 25% older than the youngest. But the rate of cratering is profoundly different: The cratering rate changed so sharply around 4 GYA that relatively minor differences in age led to tremendous differences in the preserved-for-all-time proportions of craters left behind.

I think the difference between Iapetus on the one hand and Rhea on the other (virtually identical size and distance from the Sun) is that the heat of proto-Saturn kept Rhea slushy just a bit longer, letting its primordial lumps from coalescing round out and its largest proto-impacts to fade into the slush. Iapetus cooled faster and has kept its original lumps in a way that no body outside the asteroid belt could have because all the planets are too large and all the other midsize satellites are too close to their primaries.

As a result, Ceres and Iapetus are less spherical than Rhea, Titania, and Oberon, and even some of the smaller icy satellites of Saturn and Uranus.

The large craters are just another manifestation of the big, early impacts having been left untouched by any general crustal slushiness during that period.

[tasp]

or are we seeing evidence of a thicker crust on Iapetus?

To extend this discussion, consider Iapetus accreting from the primordial chaos. Iapetus (or more accurately, the proto-Iapetus) pokes along in its' orbit at 5000 KPH (or was it MPH? no matter) and takes ~80 days to loop around Saturn. Its' accretion process took much longer than any other 'big time' satellite in the solar system. During this longer coalescense, the Al27 in the incoming materials had longer to radiate its' heat to space, and Iapetus had longer to radiate the collisional heating from impacts that formed it. Iapetus will be the satellite most accurately we can describe as cold formed. The crust will have enormous bearing strength. As we saw in the Cassini pix, the lumpy, facety surface of Iapetus exists to this day. The 'cat scratch' feature (IIRC, SW of 'landslide crater') might record an extremely deep penetration into the solid body of Iapetus by the impactor that made the 'landslide basin'.

Assuming a formation via external agency of the equatorial ridge structure long after Iapetus accreted, the structure rests on the surface, is 20 km high (60,000+ feet high!!!), and to date, in the Cassini photos, we see no faulting parallel to the ridge.

Additionally, to date, we see no signs of volcanism, past or present, no geysers, plumes, mud pots or warm spots. Internal heat sources have been inoperative at 'softening' the crust for a very long time.

Something else reducing the heating to the interior of Iapetus ids it's distance from Saturn. Granted, Iapetus is tide locked to Saturn today, but of all the major satellites in the solar system, tidal braking of Iapetus was uniquely slow. Any power dissipated this way had ample time to radiate away in the cryogenic environment, to no or little effect.

[TritonAntares]

Hi,
I've just checked the JPL raw images page for new iapetean pics - and was successful:
189 new images between Feb.12th and 16th (!)
Looks like there has been a shadow casting show by saturn during this time on Iapetus...

Here some takeouts:

Date: 2007-02-13
Distance: 2.269.440 km
Filters: CL1 and CL2
4x enlarged


Date: 2007-02-13
Distance: 2.267.233 km
Filters: CL1 and CL2
4x enlarged


Date: 2007-02-13
Distance: 2.264.177 km
Filters: CL1 and CL2
4x enlarged


Date: 2007-02-14
Distance: 2.248.671 km
Filters: CL1 and GRN
4x enlarged



Date: 2007-02-14
Distance: 2.248.666 km
Filters: CL1 and CL2
4x enlarged


Date: 2007-02-14
Distance: 2.248.655 km
Filters: CL1 and UV3
4x enlarged


Date: 2007-02-15
Distance: 2.322.227 km
Filters: P120 and MT2
4x enlarged

Have a view at the whole images series and you will recognize a migrating shadow on Feb.13/14th,
maybe somebody can affim an iapetean eclipse using some simulation program?!

Bye.

[alan]

Confirmed
http://space.jpl.nasa.gov/cgi-bin/wspace?t...=1&showsc=1

[ugordan]

No way! Mega cool!

Stretched color snapshots at the start and near maximum eclipse:


So... Saturn really is yellow. Who would have known!

[elakdawalla]

Way nifty. Here's an animation of the February 14 set, magnified 2x. Note that since this is during an eclipse, the ones that aren't in eclipse are necessarily pretty much zero phase.


Those of you who've worked more with making the raw JPEGs pretty, can you suggest improvements?

--Emily

[ugordan]

Actually, not necessarily zero phase. Remember this isn't the case where the observer is located on the body that's casting the shadow. I get a 14.5 degree phase from Cassini's vantage point at Feb 14, 02:00 UTC

[elakdawalla]

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

--Emily

[Rob Pinnegar]

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.

[TritonAntares]

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

And some pre-info about upcoming encounters:
2006 Mar 25 to Apr 18: Apr 11 - 602.412 km; 14-3.6 km/pxl; medium to high phase, southern hemisphere as crescent
2006 Jun 17 to 27: Jun 23 - 1.343.000 km; 14-8.1 km/pxl; medium to low phase, sub-Saturn hemisphere
2006 Sep 08 to 09: Sep 02 - 1.816.000 km; ~20 km/pxl; zero phase (3 to 0.05 deg), sub-Saturn
2006 Nov 26: 1.997.000 km; 12 km/pxl; very low phase, sub-Saturn
2007 Feb 12 to 15: Feb 14 - 2.249.000 km; ~14 km/pxl; sub-Saturn; very low phase and eclipse
2007 Apr 14: 2.256.000 km
2007 Jun 22: 1.817.000 km; ~14 km/pxl; med. phase, trailing side
2007 Sep 03 to 09: 8.6-0.7 km/pxl; very high phase, western Cassini Regio
2007 Sep 10: 480-10-540 m/pxl; targeted flyby, trailing side
2007 Sep 11 to 17: 0.7-7.1 km/pxl; low phase (~33 deg) trailing side
2007 Sep 27: 15 km/pxl, low phase sub-Saturn+trailing side
2007 Nov 26: 1.371.000 km; 8.2 km/pxl; very high phase, north pole, possibly graylight
2008 Feb 13: 2.045.000 km; 14 km/pxl; high phase, north pole, possibly graylight

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

Bye.

[tasp]

Interesting . . .

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

[nprev]

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...

[Rob Pinnegar]

... but will Cassini get any really good views of the dark/light border areas this September?

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

[TritonAntares]

... 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...

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:

April 15th


July 5th

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

Bye.

[nprev]

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.

[tasp]

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.

[tasp]

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 . . .

[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.

[ugordan]

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.

[Rob Pinnegar]

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.

[TritonAntares]

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.

[TritonAntares]

So,
NASA's Solar System Simulator shows this:



Bye.

[ngunn]

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

[Rob Pinnegar]

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.

[ugordan]

More food, Iapetus hounds

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

[TritonAntares]

Ahh,
there they are...

Here two takeouts, ~3x enlarged:

Distance: 2,258,272 km
Filters: BL1 and CL2


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.

[Rob Pinnegar]

Interesting is also huge step/rim in the north polar region.
Is there a hidden basin? A tectonic fracture? Or what else?

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.

[TritonAntares]

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:

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...

[Phil Stooke]

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

[ngunn]

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.

[TritonAntares]

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

[alan]

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.

[Rob Pinnegar]

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!

[ngunn]

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

[ngunn]

More images appearing now.

[TritonAntares]

So,
next series arrived...

Here two further takeouts, ~4x enlarged:


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


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.

[Pavel]

Iapetus, the Yin and Yang moon

Attached image(s)

 

[ngunn]

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

[ngunn]

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.

[TritonAntares]

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


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...

Bye.

[Phil Stooke]

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

Phil

[Phil Stooke]

...and the next one...

Phil

[ngunn]

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