[X] My Assistant

[Decepticon]

Here's what extrapolating the current SPICE kernels yields:

IAPETUS 2005 NOV 12 14:34:49 185078153 415,451 km arcmin=12.1
IAPETUS 2006 JAN 25 21:13:08 191495653 873,345 km arcmin=5.7

IAPETUS 2006 APR 11 17:16:28 198047853 602,419 km arcmin=8.3
IAPETUS 2007 SEP 10 12:31:28 242699553 1,259 km arcmin=2186.8

Posted by jmknapp

This time a little further out.

Jan. 2006 - 879,000 km - 5.3 km/pixel - eastern Cassini Region and transition zone, part ofthe "moat" posted by VP


If anyone has made a animation of the encounter please post it!

[Decepticon]

If possible can a MOD fix my date I put 15th instead of the 25th. Thanks



Times like this I wish I was MOD.

[volcanopele]

Fixed

[Decepticon]

Sorry I sould have said in topic heading, the encounter is on

JAN 25,06

Titan is on the 15th.

[volcanopele]

Sorry I sould have said in topic heading, the encounter is on

JAN 25,06

Titan is on the 15th.

okay, NOW it's fixed...sorry about that.

[jmknapp]

If anyone has made a animation of the encounter please post it!

It's not the most dynamic animation because the flyby is fairly distant and the range doesn't change very much for a couple days around closest approach. From what I can tell from the public science plans, they're going to make these observations of Iapetus during Jan 24-26:

begin: 2006JAN24 21:04 UTC
end: 2006JAN25 02:03 UTC
range: ~910,000 km
CIRS_020IA_FP3GLOBAL002_PRIME
Global_FP3_map

Long turns (33+34 min.) 2-scan FP3 mosaic of 4 mrad at 4urad/sec (00:34); then FUV, NAC, FP3, FP1 stares (00:49 each). One support image for each stare. Center detectors, 3 cm-1 resolution, shutter closed 00:00-00:05 and 02:45-02:50.

begin: 2006JAN24 21:04 UTC
end: 2006JAN25 02:03 UTC
range: ~910,000 km
CIRS_020IA_FP3GLOBAL002_SI CIRS_020IA_FP3GLOBAL002_SI

Four support images for pointing confirmation. Timing flexible (+/- 10 min or so), with images at approximately NACs at 01:35, 02:20, 03:05, and WAC at 03:55 into observation.

Also the ORS platform will evidently be pointed at Iapetus during the following times, but I can't find a corresponding science plan entry:

Jan 25 11:46 - Jan 25 12:21, range 880,209 - 879,382 km
Jan 26 12:30 - Jan 26 12:51, range 890,464 - 891,246 km

times approximate

Here are the approximate views for these three vantage points:







The Moat is only in Saturnshine for those observations.

They seem to be paying a little closer attention to Iapetus on Jan. 20 with these specific science plan items:

Iapetus limb topography and geodesy observation

ISS_020IA_LIMBTOPOA001_PRIME

begin: 2006JAN20 12:13 UTC
end: 2006JAN20 12:43 UTC

Turn -Y to Iapetus; do 1x1 mosaic 3 NAC clear filters; turn to WP


ICYLON: Longitude / Phase Space Coverage
UVIS_020IA_ICYLON002_ISS

2006JAN20 12:43 UTC
2006JAN20 13:33 UTC

Target to icy satellite. Measure uv albedo across longitude / phase space. Opportunities selected by SOST-ORS consortium. Slit orientation is flexible.


Iapetus global color observation

ISS_020IA_GLOBCOLA001_PRIME

begin: 2006JAN20 12:43 UTC
end: 2006JAN20 13:33 UTC

Do two times 1x1 mosaic 24 NAC color + 9 NAC polarizer + 4 WAC polarizer filters; turn to WP

Here's the view during those observations:



Looks like the Moat will be in sunlight, but the disk pretty small in the NAC view.

[Bjorn Jonsson]

If anyone has made a animation of the encounter please post it!

This and several Iapetus flybys were discussed in a different thread several weeks ago.

This is from that thread and contains a link to an animation I did of the upcoming distant flyby:

http://www.unmannedspaceflight.com/index.p...opic=1627&st=98

[TritonAntares]

Here the planned ISS-observations between january 20th and 28th:
Request ID ______________________ Start UTC ___________ Duration ____ Images
ISS_020IA_LIMBTOPOA001_PRIME ___ 2006-020T12:13:00 ___ 000T00:30 ___ 1 clr, 7 long exp.
ISS_020IA_GLOBCOLA001_PRIME ____ 2006-020T12:43:00 ___ 000T00:50 ___ 4 clr, 25 color, 13 polarizer
ISS_020IA_FP3GLOBAL001_CIRS _____ 2006-022T21:34:00 ___ 000T01:00 ___ 4 clr, 8 color, 9 pol, 8 long exp, a few SI
ISS_020IA_LIMBTOPOD001_PRIME ___ 2006-023T14:01:00 ___ 000T00:15 ___ 2 clr, 4 color, 3 pol, 7 long exp
ISS_020IA_SPECPHOTE001_PRIME ___ 2006-024T12:08:00 ___ 000T00:15 ___ 1 clr, 8 color, 6 pol, 1 long exp
ISS_020IA_GLOBMAPF001_PRIME ____ 2006-025T12:08:00 ___ 000T00:15 ___ 1 clr, 4 color, 6 pol, 2 long exp
ISS_020IA_SPECPHOTG001_PRIME ___ 2006-026T12:38:00 ___ 000T00:15 ___ 1 clr, 4 color, 3 pol, 2 long exp
ISS_020IA_SPECPHOTH001_PRIME ___ 2006-027T04:38:00 ___ 000T00:15 ___ 1 clr, 4 color, 6 pol, 2 long exp
ISS_020IA_SPECPHOTI001_PRIME ___ 2006-028T11:53:00 ___ 000T00:15 ___ 2 clr, 4 color, 6 pol, 1 long exp

17 clr, 61 color, 52 pol, 30 long exp, altogether 160 pics...

Information was given by Tilmann Denk in a German forum.
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Is anybody able to create/post an animation for this flyby based on the latest japetus-map:


Also interesting would be an animation similar to this one of the november encounter:


Bye...

[TritonAntares]

Some additional information:

Start UTC ___________ Distance _______ Diameter
2006-020T12:13:00 ___ 1,950 Mio.km ___ 2,5'
2006-020T12:43:00 ___ 1,942 Mio.km ___ 2,5'
2006-022T21:34:00 ___ 1,242 Mio.km ___ 4,0'
2006-023T14:01:00 ___ 1,100 Mio.km ___ 4,5'
2006-024T12:08:00 ___ 0,958 Mio.km ___ 5,2'
2006-025T12:08:00 ___ 0,880 Mio.km ___ 5,6'
2006-026T12:38:00 ___ 0,891 Mio.km ___ 5,5'
2006-027T04:38:00 ___ 0,943 Mio.km ___ 5,2'
2006-028T11:53:00 ___ 1,116 Mio.km ___ 4,4'

[scalbers]

Speaking of Iapetus maps, I'm fine-tuning an updated map with an additional Saturn-shine image. I'm reading this from the JPL raw images page for now, though I hope to use this occasion to learn how to read it from the PDS. The current map version can be seen here:

http://laps.noaa.gov/albers/sos/sos.html#IAPETUS

[Decepticon]

Very Nice!

[Michael Capobian...]

Yes, this map shows the dotted line north of the moat much better than the official one. I'd be interested in seeing an exogenous explanation for it.

Michael

Very Nice!

[JRehling]

Yes, this map shows the dotted line north of the moat much better than the official one. I'd be interested in seeing an exogenous explanation for it.

Michael

I agree. In fact, I'd be interested in seeing a purely endogenous explanation, too!

(I'll interject here, high in my message: continued great work by Steve Albers!)

A purely endogenous explanation (eg, the ridge is a Europa/Enceladus style rift that sprayed the dark stuff out over hundreds of km, painting the surface) is going to have a very hard time explaining the patchy irregularities in eastern CR. One possibility (but a reach) is that the terrain is so extremely rugged that at the furthest reaches of the spray, when the material is coming in almost tangential to the geoid, that pronounced and bizarre topography could allow almost arbitrary complexity in the boundary. Really, any "geyser"ish explanation demands that. And sure enough, we know that Iapetus has, in general, appreciably nonspheroidal topography. But we won't be able to characterize the topography of eastern CR within the primary mission -- that would have to remain an ad hoc explanation. Even that seems to strain credulity at the Snowman craters, where the topography can only be so irregular (is a crater still vaguely bowl-shaped or not)? We can similarly (almost!) reject the exogenous explanations that have Iapetus having passed (in one event or many) through a cloud of material. The bizarre boundary of eastern CR is implausible in that case.

An endogenous explanation holding that the dark stuff flooded upwards can already be rejected. The northern boundary clearly shows that the dark stuff was emplaced by ballistic delivery from the south.

I think we're going to be left to conclude that the material sprayed outward from the surface of Iapetus, within what is now CR, in a fashion much more energetic and irregular than geysers could plausibly explain, and that some sort of impact event led to the staining of Iapetus.

The remaining flybys are going to show us western CR at very high resolution, and the southern boundary at decent resolution. We will not see the eastern/Snowman region in very high resolution (and sunlight rather than saturnshine) at all during the primary mission -- excepting decent coverage as Cassini departs the Sept. 2007 flyby and eventually the Snowman will rotate in.

For the sake of discussing whether or not Iaptus merits more flybys in the extended mission, I think not. The remaining flybys are going to show so much of the hardly-seen boundaries of CR at higher resolution that I can't see dedicating another flyby to finish the almost-complete map. ALTHOUGH, the one portion of the border that will not be mapped may be the single most intriguing! The big Snowman crater will be seen at about 1.3 million km distance in Sept 2007. A flyby to see that same region at higher resolution seems like a big cost for a small gain. In the absence of other reasons to have apoapsis be so far out, I would expect the extended mission to pull the apoapsis way in, so Titan encounters happen much more frequently, and the RADAR mapping can add a swath or two per month instead of a swath every few months.

[Guest_BruceMoomaw_*]

Well, during the planning for the primary mission Iapetus was officially listed as the third most interesting moon, right after Enceladus (Dione, for some reason, was fourth). I have trouble seeing them omitting at least a second Iapetus flyby, although I doubt we'll see a third one.

[alan]

Yes, this map shows the dotted line north of the moat much better than the official one. I'd be interested in seeing an exogenous explanation for it.

Michael

This abstract may explain some of the dark features on the trailing side

....The Phoebe-dust model is very compelling because it naturally explains why the leading face of Iapetus, the side that is receiving dust from Phoebe, is dark. The model has not gained universal acceptance, however, primarily due to the following dynamical problems: i) the distribution of dark material on Iapetus does not precisely match predicted contours of constant dust flux from Phoebe, ii) there are dark-floored craters in Iapetus' high-albedo hemisphere...These problems are greatly reduced with the realization that Iapetus took nearly a billion years to become tidally locked to Saturn. I suggest the following scenario for the origin of the black/while dichotomy on Iapetus. Phoebe was probably captured early in the Solar System's history, well before Iapetus' spin slowed to its present synchronous rate. While Iapetus was spinning rapidly, dust from Phoebe accumulated at all longitudes on Iapetus uniformly..... After Iapetus became tidally locked, its trailing side was shielded from Phoebe dust, and volatile ice accumulated there burying the dark Phoebe material. Large impacts on the trailing side have dredged up some ancient Phoebe debris, creating the dark-floored craters..... Since the distribution of Iapetus' dark material depends on the flux of Phoebe dust over 4.5 billion years, it is not surprising that the distribution does not precisely match predictions which consider only the present dynamical configurations of the two satellites.

http://adsabs.harvard.edu/cgi-bin/nph-bib_...43b1f26c2909851

The mote around the snowman crater and the linear features may be dark material uncovered by landslides along the crater rim and Dione like fractures

[jmknapp]

The remaining flybys are going to show so much of the hardly-seen boundaries of CR at higher resolution...

The interior of CR also contains much information as to the direction of the emplacement, where the crater walls facing away from the equator have been shielded from any deposits. This color image is from Tilmann Denk's 1/25/05 CHARM presentation:

Viewgraph 38:


Viewgraph 39:

Pete: Tillman, this is Pete Goldie from San Francisco.  I have several questions.  The bright edge of the crater walls relative to the distance from the albedo change, is that oriented from the equator?  Did you have enough images to see whether there was any relation between the bright walls and the equator or just the bright walls and the edge of the albedo change?

Tillman: These are facing away from the equator, that is right.  So, if you are referring to viewgraph number 38, I think--this is the first one with color image--then the equator is somewhere to the left of the image.  Just outside the lower left edge of the image and it's running approximately parallel to the boundary between the dark and the bright terrain.  So this means that the bright crater walls are indeed facing towards the northern hemisphere – the North Pole – and away from the equator.  Is this what you wanted to know?

Pete: Yes.  Is that true south of the equator?  Were you able to tell that?

Tillman: The New Year’s Eve flyby did not observe the area south of the equator, but from low resolution images that are a little bit further to the south where you can really see the craters that are in the dark terrain, also it's possible near the edge.  There it's also very obvious in the same way.  Let's say the bright faces, yes, these are obvious somewhat.  What's even more obvious is that the dark crater walls facing within the dark terrain; this was also very obvious.

How can the material NOT have come from the equatorial ridge, given this pattern within the CR craters?

[Michael Capobian...]

Very interesting explanation. I'm not convinced, but I can at least see how such a mechanism might work. It should be easy to test this with higher resolution views of equatorial Roncevaux Terra.

Michael

This abstract may explain some of the dark features on the trailing side

http://adsabs.harvard.edu/cgi-bin/nph-bib_...43b1f26c2909851

The mote around the snowman crater and the linear features may be dark material uncovered by landslides along the crater rim and Dione like fractures

[volcanopele]

Another reasonable explanation for bright, poleward facing crater walls is frost migration. This effect is seen on a number of different worlds like Callisto, Ganymede, and Rhea. In this case, water ice is preferentially sublimated from equator-ward facing slopes (receive more solar energy), and is preferentially deposited on pole-ward facing slopes. Because of additional dark material in the region, this effect may have been enhanced as frost made several "jumps" from Cassini Regio to the poles and Roncevaux Terra. In this case, the only frost remaining in Cassini Regio is on poleward facing slopes. So the slight amount of dark material in this region became even more concentrated and the region darker as water ice sublimated and was deposited on these slopes and in Roncevaux Terra.

[JRehling]

The interior of CR also contains much information as to the direction of the emplacement, where the crater walls facing away from the equator have been shielded from any deposits. This color image is from Tilmann Denk's 1/25/05 CHARM presentation:
[...]
How can the material NOT have come from the equatorial ridge, given this pattern within the CR craters?

Clearly, the material here came generally from the south, but there are some quirks. If you draw an arrow pointing from central peaks through the middle of the white rim crescents, you find different headings. This could be because of nonisotropy in the downfalling material, or because of irregularities in the local terrain (these are not just bowls in a perfect sphere). Maybe even landslides widening the white area on one end or another, arbitrarily. Whatever the cause, it means that we've got some error bars on the vectors. Is that enough to allow the possibility that the stuff came from south by southeast, and a possible origin in eastern equatorial CR?

Or: Could a really large impact create complex phenomena of rain-out of the debris it creates? Think of this as not just a bigger version of Tycho on the Moon, but as a smaller version of the impact into Earth that *created* the Moon. Wherein, the material that was spalled out failed to attain orbital velocity, but fell very heavily downrange on the order of 1/4 the global circumference in distance. There might even be significant secondary impacts. We certainly see enough big craters in and near CR to suggest that something big (or many somethings big) happened. The ridge might be the upheaval caused by the shock wave of an off-center impact that was almost big enough to break Iapetus apart. The white mountains in western CR might be the place where the wave moving through the solid interior of Iapetus arrived just after the downfalling dark "snow" completed.

I agree that the crater "shadow" vectors do point pretty well back to the ridge, though. That is somewhat damning for an "impact splash" origin, whereas the features east of CR, and the overall shape of CR (rounder in the east, pointy in the west) is somewhat damning for a "big venting ridge" endogenous origin. The answer may be a "third way" somehow: an impact event that begat an endogenous upheaval, or impact events in the east (Snowman region) displacing deposits that had an endogenous origin.

[jmknapp]

Another reasonable explanation for bright, poleward facing crater walls is frost migration.  This effect is seen on a number of different worlds like Callisto, Ganymede, and Rhea. 

Do you have any references to that regarding the effect with crater walls? I found a few items on large scale migration for Triton, Pluto and Ganymede, but that's about it.

BTW, are the CHARM conferences continuing? Seems like they might have skipped a month (Dec.).

[elakdawalla]

I listened to a couple of talks at DPS that had bearing on this subject. Here are the notes, from my blog:

Tilmann Denk performed some color mapping of Hyperion and Iapetus. He found interesting patterns of colors on Iapetus' surface. While the dark terrain is generally on Iapetus' leading side, it does also wrap around to trailing side. He found that the leading side dark material is redder than the trailing side, which is relatively greener in color. He suggested that the greenish stuff could be "primordial," endogenous to Iapetus, while the dark red material on the leading side is exogenous. And he wondered out loud whether there may be as many as "three more or less independent processes responsible for the formation of the extreme hemispheric albedo asymmetry on Iapetus?"

John Spencer used CIRS maps to study Iapetus. Because of the dark material and the very long day, Iapetus is "probably warmer than any other surface in the Saturnian system," John said. While Iapetus has a thermal inertia very similar to Phoebe's, the thermal wave from each daytime round of heating penetrates much deeper into Iapetus' skin than it does on Phoebe because of the longer day, 3 centimeters on Iapetus as opposed to 2 millimeters on Phoebe. John covered the usual arguments for why the shape of the albedo dichotomy is strange: "most simple exogenic models [that is, models in which the dark stuff comes from outside Iapetus] darken the leading hemisphere, but Iapetus' bright material extends over the poles, dark stuff extends around the equator [to the trailing side], and pole-facing slopes are bright." So John created a model in which all of Iapetus is covered in a very thin layer of typically dirty ice. The warm days on Iapetus will sublimate some of the ice, which will eventually refreeze -- preferentially so at cooler spots, like the poles. Then, he said, in his model he darkened the leading side symetrically about the apex, which is what you would get from ballistic emplacement of exogenous material. If he runs the model forward in time, "in only about 10,000 years you start to burn off frost on the leading side. In 100 million years you start to burn stuff off on the trailing side. You can actually make this look pretty similar to the current albedo pattern. If you try to do it with a thicker frost it doesn't work."

[volcanopele]

Do you have any references to that regarding the effect with crater walls? I found a few items on large scale migration for Triton, Pluto and Ganymede, but that's about it.

BTW, are the CHARM conferences continuing? Seems like they might have skipped a month (Dec.).

Ganymede: J.R. Spencer, L. Prockter, R. Pappalardo, J. Head, J. Moore, and Galileo SSI team. Local Volatile Migration on Ganymede: Galileo SSI Images, PPR Radiometry, and Theoretical Consideration. http://scholar.google.com/url?sa=U&q=http:...98/pdf/1149.pdf

I'll look for others

[The Messenger]

I listened to a couple of talks at DPS that had bearing on this subject.  Here are the notes, from my blog:

If I understand the Spencer model, He has a thin white layer on top of the dark layer...with perhaps a thick dirty white layer underneath?

Is there a reason the dark layer could not be the moon's surface, with no 'white inside'?

Could the structure be akin to Tempel 1?

[tasp]

I listened to a couple of talks at DPS that had bearing on this subject.  Here are the notes, from my blog:

{sorry if I messed up the quote here, I can't tell as I put this together}

The part I was replying to is the dark region of Iapetus as being apparently the warmest area in the Saturn system.

I suspect there are area's just as warm on Hyperion.

The deep bowl craters sides will 'focus' the distant suns energy onto their floors when ever Hyperions chaotic rotation has the sun directly overhead.

Eventually, all craters on Hyperion will have the sun directly overhead, and each crater floor (in the appropriately shaped craters) will experience additional solar energy relected onto it from the crater walls.

I am suspecting the dark spots on Hyperion will have compositions strongly consistent with the dark coating of Iapetus' Cassini Regio.

I am also thinking the dark material is only formed at the higher temperatures prevalent in the crater bottoms of Hyperion and Cassini Regio. The darkening material may be kerogen related and formed from methane or a mixture of methane and nitrogen.

The source of the gaseous material would be Titan's atmosphere, and the transport mechanism to each of these two satellites would be via Saturn's magnetotail. The leading hemisphere of Iapetus will be preferentially darkened as Iapetus moves from beyond Saturn (relative to sun) when the gas is deposited, and around to the sunward side of Saturn. The gas is depleted in the roughly 40 days this takes, and the the darkening does not form generally on the trailing hemisphere of Iapetus.

Additionally, on Iapetus, as one moves north or south of the equator, the solar insolation decreases until at around 45 degrees N or S, and the dark marerial production is less efficient. Except for the poleward slopes of craters. Their local angle to the incident solar radiation is the key factor in producing the local warm temps needed for the dark material to form.

A color variation from east to west across the Cassini Regio could be explained by the gas mixture (presumably N and CH4) varying in ratio as the thermo-reaction proceeds.

Gas transport from Titan to Hyperion and Iapetus would be expected to occur slowly from the 'leakage' of Titan's atmosphere and perhaps in 'spurts' in response to extermely large cratering events on Titan. I am not sure what the 'Teller treshold' is for Titan, but assuming it to be in the 100 to 500 megaton range, it should be possible to estimate gas quantities transported away from Titan via the Saturn magnetotail.

[scalbers]

Even that seems to strain credulity at the Snowman craters, where the topography can only be so irregular (is a crater still vaguely bowl-shaped or not)? We can similarly (almost!) reject the exogenous explanations that have Iapetus having passed (in one event or many) through a cloud of material. The bizarre boundary of eastern CR is implausible in that case.

It looks like two of the Saturn-shine images looking from different angles at the Snowman crater could be reprojected in such a way to provide an interesting stereo pair. In the course of looking at experimental map images, I had switched between the two and a surprisingly deep Snowman crater popped out at me. Perhaps at some point I can try to provide two such images unless someone else beats me to it. The large angles involved may actually exaggerate the stereo effect.

[TritonAntares]

It looks like two of the Saturn-shine images looking from different angles at the Snowman crater could be reprojected in such a way to provide an interesting stereo pair. In the course of looking at experimental map images, I had switched between the two and a surprisingly deep Snowman crater popped out at me. Perhaps at some point I can try to provide two such images unless someone else beats me to it. The large angles involved may actually exaggerate the stereo effect.

Stereo images sounds quite interesting ...

I always wondered about the 'missing' central peak in the Saturn-shine images of the Snowman ...

...there should be something very huge...


Low contrast does explain this disappearance, but it also shows how misleading these Saturn-shine images can be.
Thinking about the equatorial ridge, I wouldn't be surprised if it is hidden in this low contrast pic.
A white (and following dark) linear structure beginning southwest of the largest Snowman crater could be a hint.

I'm very keen on seeing these stereo-images....

[Rob Pinnegar]

The notion posted above, regarding Hyperion's deep craters acting as crude "mirrors" that could focus sunlight on the dark spots, is a very interesting one.

We need to keep in mind that the crater walls are not smooth surfaces and this will of course decrease any such effects. Also, Hyperion's craters seem to have cone-like shapes, which suggests to me that they might preferentially reflect sunlight straight back into space (much like the laser reflectors left behind by Apollo).

I haven't looked at any shots of Hyperion recently -- are there any for which the Sun was almost directly "behind" Cassini when the picture was taken? If so, it might be interesting to look at the apparent brightness of the craters, to see if they are reflecting an inordinate amount of light back towards the Sun. If they aren't, this notion of ray-focussing on the dark crater floors might really have something to it.

I know this is a bit off topic for the Iapetus thread -- however, it's perhaps arguable that _anything_ regarding dark material on Saturn's icy moons, is at least partially on topic where Iapetus is concerned.

[ugordan]

It seems to me thermal conditions on Hyperion's dark stuff aren't that similar to those on Iapetus. While both have dark, heat-absorbing materials, keep in mind that Iapetus has a several times longer day and any material is going to be more efficiently "baked" than the stuff on the bottom of Hyperion's craters.

As far as the craters on Hyperion acting as crude mirrors, that idea reminds me somewhat of the chicken-and-the-egg problem. You'd need dark stuff so it melts through the craters, leaving them bowl-shaped. And yet, this idea suggests that their bowl-shaped appearance is responsible for the darkening in the first place. Lastly, I reckon the craters should be pretty steep not to reflect the majority of the diffuse light back into space.

[JRehling]

As far as the craters on Hyperion acting as crude mirrors, that idea reminds me somewhat of the chicken-and-the-egg problem. You'd need dark stuff so it melts through the craters, leaving them bowl-shaped. And yet, this idea suggests that their bowl-shaped appearance is responsible for the darkening in the first place. Lastly, I reckon the craters should be pretty steep not to reflect the majority of the diffuse light back into space.

A starting point in the data reduction, would be to survey, for imaged portions of Hyperion's surface, the various concavities that are visible, and whether or not dark stuff appears at the bottom. If it were possible (and stereo-pair DEMs might), careful tabulation of slope, etc., as an independent variable, would be highly useful.

When I eyeball the images, without giving it a formal treatment, I reach some intermediate conclusions:

1) The darkness is surprisingly all-or-nothing. There is not a lot of continuous shading between the light and dark, or at least, the intermediate albedo zones are very narrow boundaries between light and dark. The transition zones do exist, but are shaper than one might imagine.

2) Some larger concavities have no dark stuff at all, while some small concavities have dark floors.

3) Individual dark floor patches are rarely circular. Likewise, the concavities in Hyperion's surface are not very circular in most cases. Serious deformation has taken place since most impacts were emplaced.

4) There is a strong correlation between whether or not a concavity's neighbors have dark floors and whether or not the concavity itself does. Stepping back, we see clusters of adjacent pits with dark bottoms while in other areas, no concavities, large or small, have them. These clusters are multiple concavities across.

5) The dark stuff is seemingly never exposed on steep slopes. It is always on floors. It is not seen on horizontal surfaces atop ridges.

6) The dark stuff is rare or nonexistant in concavities located well above mean radius. It is common in lower-altitude concavities.

7) The dark stuff can be intruded upon by very sharp-edged peninsulas that are lighter, where ridges seem to descend slopes down into it.

8) There are some isolated very small dark dots scattered here and there.

I keep wondering if the dark stuff rose up from below, consistent to a certain constant level of gravity, like a water table on a nonspherical world. This cannot explain the entire pattern, however. It doesn't seem like any simple explanation offered so far fits all 8. (???)

[Tom Tamlyn]

A purely endogenous explanation (eg, the ridge is a Europa/Enceladus style rift that sprayed the dark stuff out over hundreds of km, painting the surface) is going to have a very hard time explaining the patchy irregularities in eastern CR.

Forgive me, but I'm having a little trouble following this discussion, despite checking some other threads and TPS" Iapetus page. What is "CR"? What are "the Moat" and the "Snowman Crater" Perhaps someone could point me to a reference map of Iapetus with the features under discussion identified.

Thanks!

TTT

[alan]

Forgive me, but I'm having a little trouble following this discussion, despite checking some other threads and TPS" Iapetus page. What is "CR"? What are "the Moat" and the "Snowman Crater" Perhaps someone could point me to a reference map of Iapetus with the features under discussion identified.

CR = Cassini Regio = dark area of Iapetus
Snowman Crater = Three large adjacent craters visible on the right in large nightside image in TritonAres post
Mote = dark material surrounding largest of the snowman craters

[JRehling]

Stereo images sounds quite interesting ...

I always wondered about the 'missing' central peak in the Saturn-shine images of the Snowman ...  

Low contrast does explain this disappearance, but it also shows how misleading these Saturn-shine images can be.
Thinking about the equatorial ridge, I wouldn't be surprised if it is hidden in this low contrast pic.
A white (and following dark) linear structure beginning southwest of the largest Snowman crater could be a hint.

I'm very keen on seeing these stereo-images....

A major issue here is that the albedo contrast (which is the whole subject here!) confounds single-image efforts to determine topography via shape-from-shading. To be specific, when I see the saturnshine images, I think I see dark material emplaced from the EAST onto the western interior rim of Snowman A (the biggest one). If that's true, it spells doom for certain, simpler theories of CR's origin, since other craters near east CR show emplacement from the WEST. The same pattern seems to hold for Snowman B.

http://ciclops.org/media/ir/2005/708_1409_1.jpg

However, maybe I am greatly misinterpreting where I think the topographical crater is. Perhaps what I think is the western interior wall of Snowman A is actually the western exterior wall, "tarred" from the west.

I doubt this. At least, I think the trompe l'oeil is partial at best -- the smudge on Snowman B really does seem to be, from perspective (the apparent thickness of the smudge) on the inside of Snowman B. So either material came from the east just to hit Snowman B, and not the terrain around it in any direction, or the Snowman craters have somehow overlapped in time of origin with the dark stuff. And those craters look quite ancient.

If that reasoning holds, then either the dark stuff was not delivered ballistically, or it was already part of the crust in considerable quantity, and the Snowman craters plowed it up a bit, or the Snowman craters were formed as part of the same event that brought the dark stuff elsewhere.

[jmknapp]

The notion posted above, regarding Hyperion's deep craters acting as crude "mirrors" that could focus sunlight on the dark spots, is a very interesting one.

We need to keep in mind that the crater walls are not smooth surfaces and this will of course decrease any such effects.

Rather than reflection per se, maybe the idea is that the that the crater surfaces absorb sunlight and heat up, and then radiate infrared which is then focussed so to speak more in some places than others.

[JRehling]

Rather than reflection per se, maybe the idea is that the that the crater surfaces absorb sunlight and heat up, and then radiate infrared which is then focussed so to speak more in some places than others.

Interesting thought -- except that at these temperatures, there wouldn't be a lot of infrared. Microwaves, etc. I suspect the thermal radiation is pretty low, even if that stuff's at albedo 0.0. The solar input is about 1% that of the Moon...

[tasp]

In regards to Hyperion, given the chaotic rotation (is this absolutely confirmed yet?) every crater will eventually have the sun directly overhead at some time or another.

Additionally, the "focusing effect", doesn't have to be terribly efficient should the 'normal' temperature of Hyperion be close to the temperature needed to initiate the darkening reation in the (hypothetical) traces of Titanian N and CH4 that periodically waft by Hyperion.

Once even a small 'spot' develops, it will warm further and 'spread'.

An interesting (if frowned upon) experiment would be to blast a fresh crater in Hyperion and/or Iapetus (like DI) and then just watching what happens.

Any kind of atmospheric sensitive scans that Cassini could do at Iapetus while close to Iapetus at the time Iapetus is in the magnetotail of Saturn would be interesting. Keeping in mind though, that such N and CH4 neccessary for darkening over the life of the solar system would be vanishingly small at any particular instant.

[Michael Capobian...]

I spent an hour trying to make a stereo image from the Iapetus Saturnshine sequence, without success. I used a program called Stereophotomaker, and it seemed to have all the necessary tools, but I couldn't align the images correctly enough to make them "pop."

I hope someone with more experience will take a crack at this.

Michael

A major issue here is that the albedo contrast (which is the whole subject here!) confounds single-image efforts to determine topography via shape-from-shading. To be specific, when I see the saturnshine images, I think I see dark material emplaced from the EAST onto the western interior rim of Snowman A (the biggest one). If that's true, it spells doom for certain, simpler theories of CR's origin, since other craters near east CR show emplacement from the WEST. The same pattern seems to hold for Snowman B.

http://ciclops.org/media/ir/2005/708_1409_1.jpg

However, maybe I am greatly misinterpreting where I think the topographical crater is. Perhaps what I think is the western interior wall of Snowman A is actually the western exterior wall, "tarred" from the west.

I doubt this. At least, I think the trompe l'oeil is partial at best -- the smudge on Snowman B really does seem to be, from perspective (the apparent thickness of the smudge) on the inside of Snowman B. So either material came from the east just to hit Snowman B, and not the terrain around it in any direction, or the Snowman craters have somehow overlapped in time of origin with the dark stuff. And those craters look quite ancient.

If that reasoning holds, then either the dark stuff was not delivered ballistically, or it was already part of the crust in considerable quantity, and the Snowman craters plowed it up a bit, or the Snowman craters were formed as part of the same event that brought the dark stuff elsewhere.

[JRehling]

In regards to Hyperion, given the chaotic rotation (is this absolutely confirmed yet?) every crater will eventually have the sun directly overhead at some time or another.

Additionally, the "focusing effect", doesn't have to be terribly efficient should the 'normal' temperature of Hyperion be close to the temperature needed to initiate the darkening reation in the (hypothetical) traces of Titanian N and CH4 that periodically waft by Hyperion.

Once even a small 'spot' develops, it will warm further and 'spread'.

An interesting (if frowned upon) experiment would be to blast a fresh crater in Hyperion and/or Iapetus (like DI) and then just watching what happens.

Any kind of atmospheric sensitive scans that Cassini could do at Iapetus while close to Iapetus at the time Iapetus is in the magnetotail of Saturn would be interesting.  Keeping in mind though, that such N and CH4 neccessary for darkening over the life of the solar system would be vanishingly small at any particular instant.

Chaotic rotation does not mean that Hyperion is whizzing around like a garden hose. Since it is in an orbit roughly planar with the Sun and Jupiter and Saturn and Titan, I would expect the poles to be fairly well fixed, with variations in the east-west direction only.

Notice that there are frequently clusters of craters that share dark bottoms, but with light rims separating them. Other areas have lots of craters with no dark bottoms. And many craters with dark floors look no different than other craters elsewhere that lack dark floors. But in the dark-bottom clusters, the dark is not creeping up over the edge and down into the neighbors. There is some causal link going on, and "spreading" appears not to be it, unless the dark stuff is spreading *through* the light rims, the way that water on a rocky beach could rise and fall through angular boulders.

Alternately, if radiative heating is a factor, the clusters could be defined by portions of Hyperion's surface where the regional mean slope, if coupled with the slopes of craters, leads to higher radiation than in similar craters on more convex regions of Hyperion. This sounds ad hoc, though.

Take a look at the Earth's Moon at roughly 90 East. It looks a lot like Hyperion, and the reason why is dark lava flowed onto basin and crater floors. In fact, this similarity is rather profound, except Moon craters are mainly round, not deformed.

[Tom Tamlyn]

CR = Cassini Regio = dark area of Iapetus
Snowman Crater = Three large adjacent craters visible on the right in large nightside image in TritonAres post
Mote = dark material surrounding largest of the snowman craters

Thanks, that helps a lot.

TTT

[Guest_BruceMoomaw_*]

Chaotic rotation does not mean that Hyperion is whizzing around like a garden hose. Since it is in an orbit roughly planar with the Sun and Jupiter and Saturn and Titan, I would expect the poles to be fairly well fixed, with variations in the east-west direction only.

Don't necessarily count on that -- there is some theorizing that the very slight differences in orbital plane between Jupiter and Saturn are what has given Saturn its large axial tilt. Given the much stronger tidal forces operating on Hyperion and its much greater irregularity of shape, the same thing could very easily happen to it.

[Phil Stooke]

The supposed chaotic rotation of Hyperion was only a first impression based on Voyager images. In this reference:

Thomas, P.C., Black, G.J. and Nicholson, P.D., "Hyperion: Rotation,
Shape and Geology from Voyager Images", ICARUS 117: 128-148, 1995.

a much more thorough analysis gives a very different kind of rotation, a "slow long axis" rotation mode like that modelled for the nucleus of Halley's Comet and asteroid Toutatis. This is a mode where the object rotates slowly around its long axis instead of its short one, but precesses quite rapidly at a steep angle to that axis. It looks a bit chaotic but it isn't.

If you look at each of the Hyperion image sequences back in past threads, the slow rotation about the long axis is very apparent. It really does not appear to change. It's long past time to lay the 'chaotic rotation' story to rest.

Phil

[jmknapp]

Ganymede: J.R. Spencer, L. Prockter, R. Pappalardo, J. Head, J. Moore, and Galileo SSI team. Local Volatile Migration on  Ganymede: Galileo SSI Images, PPR Radiometry, and Theoretical Consideration. http://scholar.google.com/url?sa=U&q=http:...98/pdf/1149.pdf

I'll look for others

Thanks! Spencer gave the Iapetus CHARM talk along with Denk & it's nice to see his thoughts. Referring to Ganymede he says "Poleward-facing slopes at high latitudes will be colder than their surroundings and will be preferred sites for ice accumulation..."

Is this relevant to Iapetus, where the craters in the image above are at a relatively low 30-40 degrees north latitude? Also, Ganymede has very little tilt against the ecliptic (2 degrees), accentuating any effect at high latitudes. Iapetus has 15 degrees of seasonal variation.

According to Spencer:

At low latitudes, horizontal surfaces in depressions or other concavities (such as the bases of slopes) receive thermal radiation from surrounding slopes, and will thus tend to be warmer than horizontal surfaces on topographic highs.

So the craters become a sort of IR radiation oven.

To explain the bright peaks on Iapetus, maybe this quote is of interest:

Topographic highs will be relatively cold because they do not receive radiation from their surroundings, and will thus be sites for ice deposition and will be bright.

[Rob Pinnegar]

A couple of points:

(1) Has anyone given much consideration to estimating the thickness of the dark layer in Cassini Regio? One would think that, if it is only a very thin layer, there should be at least one small crater somewhere that would show signs of having punched through it. (I realize frost migration could cover up such craters eventually, but it would take time.)

If the layer turns out to be a couple of kilometres thick, or even several hundred metres, this would have major ramifications. Do we have any idea?

(2) Not to kick a dead horse too much, but I'm going to throw in (one of) my old half baked hypotheses back into the ring. This one has to do with Hyperion's origin.

From what I recall, it's currently thought that Titan's atmosphere was originally something like 30 times more massive than it is now, based on isotope data. Considering the atmosphere's current height, one would therefore expect that it must have originally extended outwards quite a bit further than it does now.

This means that, back in the "old days" (i.e. the Hadean Eon) Titan's atmosphere would've made up a pretty good chunk of its total effective cross-section for collisions. From that, it's not too much of a stretch to imagine that, from time to time, large planetesimals would have come blazing through Titan's atmosphere without necessarily colliding with Titan itself.

Some of these planetesimals could have been broken up through a combination of atmospheric drag and the "disintegrative capture" process that was proposed to explain the origin of the Earth's Moon a few decades ago. Both of these effects will slow down the disrupted body, perhaps enough to allow its fragments to be captured into orbit around Saturn. I honestly don't know whether that is dynamically possible, but let's suppose for the moment that it is. (Incidentally, there are some papers on disintegrative capture in the "Origin of the Moon" conference volume from around 1986, if anyone is interested. These papers include some results of simulations, which are the source of some of the assertions I'm making here.)

Anyways, disintegrative capture tends to break the disrupted body into pieces that propagate outwards in a "spray". Since the pieces interact with each other gravitationally after disruption, they don't follow ballistic trajectories, so they wouldn't necessarily end up in Titan-crossing orbits. It wouldn't be surprising if some of the debris ended up in the 4:3 Titan resonance, where one would expect it to coalesce into a small moon. Whether or not we could get something as big as Hyperion out of this process is of course a big question.

If by some miracle this totally conjectural hypothesis were to turn out to be workable, then it would imply that the dark material on Hyperion might not be similar to what we see on Iapetus. Instead, it might be organic material from Titan's early atmosphere, that got assimilated into Hyperion, and hasn't boiled off yet because it's too deep inside the moon.

Any comments?

[ugordan]

From what I recall, it's currently thought that Titan's atmosphere was originally something like 30 times more massive than it is now, based on isotope data. Considering the atmosphere's current height, one would therefore expect that it must have originally extended outwards quite a bit further than it does now.

Personally, I don't think it means the atmosphere was that much denser in the past. If the atmosphere indeed originated by means of crustal venting/cryovolcanism it could imply simply that the atmosphere is ancient. Atmospheric loss could have been balanced by continuing replenishment from the crust and the only sign of the major loss during the aeons might be the nitrogen ratio, heavier nitrogen being more likely to stay behind. It doesn't neccessarily mean the entire atmosphere came to be at once and has since been slowly eroding away.

Could that be possible?

[JRehling]

A couple of points:

(1) Has anyone given much consideration to estimating the thickness of the dark layer in Cassini Regio? One would think that, if it is only a very thin layer, there should be at least one small crater somewhere that would show signs of having punched through it. (I realize frost migration could cover up such craters eventually, but it would take time.)

If the layer turns out to be a couple of kilometres thick, or even several hundred metres, this would have major ramifications. Do we have any idea?

(2) Not to kick a dead horse too much, but I'm going to throw in (one of) my old half baked hypotheses back into the ring. This one has to do with Hyperion's origin.

From what I recall, it's currently thought that Titan's atmosphere was originally something like 30 times more massive than it is now, based on isotope data. Considering the atmosphere's current height, one would therefore expect that it must have originally extended outwards quite a bit further than it does now.

This means that, back in the "old days" (i.e. the Hadean Eon) Titan's atmosphere would've made up a pretty good chunk of its total effective cross-section for collisions. From that, it's not too much of a stretch to imagine that, from time to time, large planetesimals would have come blazing through Titan's atmosphere without necessarily colliding with Titan itself.

Some of these planetesimals could have been broken up through a combination of atmospheric drag and the "disintegrative capture" process that was proposed to explain the origin of the Earth's Moon a few decades ago. Both of these effects will slow down the disrupted body, perhaps enough to allow its fragments to be captured into orbit around Saturn. I honestly don't know whether that is dynamically possible, but let's suppose for the moment that it is. (Incidentally, there are some papers on disintegrative capture in the "Origin of the Moon" conference volume from around 1986, if anyone is interested. These papers include some results of simulations, which are the source of some of the assertions I'm making here.)

Anyways, disintegrative capture tends to break the disrupted body into pieces that propagate outwards in a "spray". Since the pieces interact with each other gravitationally after disruption, they don't follow ballistic trajectories, so they wouldn't necessarily end up in Titan-crossing orbits. It wouldn't be surprising if some of the debris ended up in the 4:3 Titan resonance, where one would expect it to coalesce into a small moon. Whether or not we could get something as big as Hyperion out of this process is of course a big question.

If by some miracle this totally conjectural hypothesis were to turn out to be workable, then it would imply that the dark material on Hyperion might not be similar to what we see on Iapetus. Instead, it might be organic material from Titan's early atmosphere, that got assimilated into Hyperion, and hasn't boiled off yet because it's too deep inside the moon.

Any comments?

The layer is clearly either very thick (at least in some places) or fairly recent (or recently replensihed). An interesting possibility would be if the ridge is simply the thickest portion of a layer that is at least 10 to 15 km thick along that central line, with the depth tapering off to the edges. That's just a quirky possibility, but one that would exclude any of the explanations relying solely on some sort of cloud/debris that Iapetus is sopping up as it passes through it's orbit.

Your model is a two-parter: Titan's atmosphere as a source of dark stuff, and "sopping up from intersatellite space" as the means of delivery of the dark stuff onto Iapetus and Hyperion both.

I see trouble with the means-of-delivery part.

I can't see why Hyperion would have many large craters with no dark stuff (more or less inbetween ones that do have it) if "sopping up" were the means of delivery there. Or why intricate topographical features intruding into crater floors remain bright. Or even the whole of the non-crater-floor surface.

Why would Iapetus just happen to have a big ridge running down the middle of the area where fluffy dark stuff descended upon it? If Iapetus gathered many km of this stuff, how did, eg, Rhea end up with none? How are there bright mountains within western CR? How did dark stuff get to ring the western rims of the Snowman craters which are on the trailing side?

Perhaps Titan's atmosphere did end up as a source of interesting dark material, but if the means of delivery isn't there, then we wouldn't have any dark stuff thus transported to explain!

[TritonAntares]

Here some info about the transmission times - named 'SP_020EA' - for the upcoming encounter:

Request name _________________ Start UTC ________ Duration ___ End UTC
ISS_020IA_LIMBTOPOA001_PRIME ____ 2006-020T12:13 ___ 00:30:00 ___ 2006-020T12:43:00
ISS_020IA_GLOBCOLA001_PRIME _____ 2006-020T12:43 ___ 00:50:00 ___ 2006-020T13:33:00
SP_020EA_G34BWGNON021_PRIME ___ 2006-021T02:55 ___ 08:55:00 ___ 2006-021T11:50:00
CIRS_020IA_FP3GLOBAL001_PRIME ___ 2006-022T21:34 ___ 04:54:00 ___ 2006-023T02:28:00
SP_020EA_G34BWGNON023_PRIME ___ 2006-023T02:53 ___ 09:00:00 ___ 2006-023T11:53:00
ISS_020IA_LIMBTOPOD001_PRIME ____ 2006-023T14:01 ___ 00:15:00 ___ 2006-023T14:16:00
SP_020EA_G70METSEQ024_PRIME ____ 2006-024T02:33 ___ 09:05:00 ___ 2006-024T11:38:00
ISS_020IA_SPECPHOTE001_PRIME ____ 2006-024T12:08 ___ 00:15:00 ___ 2006-024T12:23:00
CIRS_020IA_FP3GLOBAL002_PRIME ___ 2006-024T21:04 ___ 04:59:00 ___ 2006-025T02:03:00
SP_020EA_G70METSEQ025_PRIME ____ 2006-025T02:33 ___ 09:05:00 ___ 2006-025T11:38:00
ISS_020IA_GLOBMAPF001_PRIME _____ 2006-025T12:08 ___ 00:15:00 ___ 2006-025T12:23:00
SP_020EA_G70METSEQ026_PRIME ____ 2006-026T02:33 ___ 09:05:00 ___ 2006-026T11:38:00
ISS_020IA_SPECPHOTG001_PRIME ____ 2006-026T12:38 ___ 00:15:00 ___ 2006-026T12:53:00
SP_020EA_M34HEFSEQ026_PRIME ____ 2006-026T19:03 ___ 09:00:00 ___ 2006-027T04:03:00

Images should be seen in the JPL Raw Image Archive on early evenings in Europe, when work has just started in Pasadena...

[Rob Pinnegar]

I can't see why Hyperion would have many large craters with no dark stuff (more or less inbetween ones that do have it) if "sopping up" were the means of delivery there. Or why intricate topographical features intruding into crater floors remain bright. Or even the whole of the non-crater-floor surface.

For the most part, I'd intended this disintegrative-capture/atmospheric drag hypothesis as a wild, but possibly workable, idea, that might explain where Hyperion came from, and why it is a rubble pile. The idea that it might have "sopped up" some of Titan's atmosphere was something that I added in at the last minute as I was typing it out. Admittedly, I didn't think that part through very well. The points you've raised here are certainly good ones against it being true.

By the way, I didn't say anything about Titan's atmosphere being a source for Iapetus' dark material. I agree that at first glance there doesn't seem to be any way THAT could be true.

[tasp]

By the way, I didn't say anything about Titan's atmosphere being a source for Iapetus' dark material. I agree that at first glance there doesn't seem to be any way THAT could be true.

Possibly slow steady leakage from Titanian atmosphere would be 'spirited' away down Saturn's magnetotail to 'bathe' Hyperion and Iapetus over the age of the solar system.

That Enceladus can briefly retain atmosphere suggests that a fraction of the N and CH4 wafting past Hyperion and Iapetus would stay around them briefly too.

The lack of fresh crater 'punch throughs' in CR may suggest that whatever makes the dark coating is still doing so today.

That Iapetus may 'entrain' some N and CH4 while on the far side of Saturn from the sun, and then perhaps retain it for roughly 40 days or so would explain the geographic distribution of the dark CR material east and west.

Should the dark CR material be a solar UV or temperature catalyzed, the symetrical distribution north and south of the Iapetan equator may also be explained.


Additionally, largish cratering impacts on Titan (apparently above the equivalent of a 100 megaton nuke) would seem to be capable of expelling Titanian atmosphere above the impact site and positing it in Saturn space in quantities where the magnetospheric action would again move it preferentially anti-sunward and able to 'paint' Hyperion and Iapetus.

The temp or UV catalyzing of the dark material on both bodies is suggested by the distribution of the dark material on both bodies. On Hyperion, seemingly a great many (latest thinking seems to be not all, though) craters will eventually have the distant sun directly overhead so the crater bowl can 'focus' (for lack of a better term) somewhat more radiation onto the crater floor to trigger the darkening reaction.

And on Iapetus, the necessary temp or UV exposure is achieved by the very slow rotation of Iapetus.

[Rob Pinnegar]

Possibly slow steady leakage from Titanian atmosphere would be 'spirited' away down Saturn's magnetotail to 'bathe' Hyperion and Iapetus over the age of the solar system.

Nice idea, but I don't buy it, not for Iapetus at least. The amount of material intercepted by Iapetus would be minimal -- and, besides that, it would tend to collect on the Saturn-facing (i.e. Titan-facing) hemisphere, rather than the leading hemisphere.

[hubdel11]

Let's go !
http://saturn.jpl.nasa.gov/multimedia/imag...storedQ=1168571

[Decepticon]

Looks like something whizzed by the camera on this shot.

http://saturn1.jpl.nasa.gov/multimedia/ima...7/N00048883.jpg

[ugordan]

Looks like something whizzed by the camera on this shot.

Cosmic ray hit.

[TritonAntares]

Two new Iapetus images released.

Here is one:

Date: 01/22/06
Distance: 1.280.080 km

Remarkable is the 'white mountain' in the southeastern part of the disk.
Probably the centralpeak of a 'rotten' bassin.

There are also some former unknown craters visible at the terminator.
The rifts - visible in this attached november picture...

Date: 11/10/05
Distance: 639.972 km
...- aren't clearly visible until now.
But some similar structures are recognizable in the bright region east of CR.

Bye.

[TritonAntares]

Another series of 49 Iapetus images has been released.
More here...

Four takeouts:

Date: 01/22/06
Distance: 1.226.480 km
Filters: CL1 and CL2


Date: 01/22/06
Distance: 1.226.482 km
Filters: CL1 and CL2


Date: 01/23/06
Distanz: 1.098.263 km km
Filters: P120 and GRN


Date: 01/23/06
Distanz: 1.098.263 km km
Filters: CL1 and CL2

Enormous is the hight of the centralpeak of Snowman A...
...probably 15-25 km !!!

The mentioned 'white mountain' is also detectable in Jan.23 images.

There are some valleys/rifts/graben visible running from NW to SO east of CR.
Also two overlapping craters may be south of Snowman A.
You can recognize two parallel dark markings, the shadows of the western crater rims.
--------------------------------------------------------------------------------------------------
Additionally a simulation with data for this far-encounter:


Unfortunately Solar System Simulator v4.0 is only using a poorly painted Iapetus-map...
Someone should update this...

Bye.

[ugordan]

Check out this Jan 24 image from 954 000 km. What topography can produce a feature at the bottom of the disk, along the terminator? One of the huge impact basins?


That sure is one lumpy moon.

[TritonAntares]

Check out this Jan 24 image from 954 000 km. What topography can produce a feature at the bottom of the disk, along the terminator? One of the huge impact basins? ...

Date: 11/24/06
Distance: 910.020 km
Filters: CL1 and CL2

This 'white mountain' is really curious;
it's still in sunlight, where other parts at this longitude are in shadow.
Doesn't really look like the centralpeak of a larger crater...
I wonder what happened there...

~ 3x enlarged

...That sure is one lumpy moon.

For sure...

Bye.

[TritonAntares]

13 new Iapetus images released;
one takeout:


Date: 11/25/06
Distance: 879.656 km
Filters: CL1 and IR1

This 'white mountain' is still visible...
I've never seen such a deformed terminator on a planetary body without atmosphere of this size...
Look at the shape at the left side of the disk, there is clear detectable bulge over CR.

Bye.

[Decepticon]

Can that area be associated with the Basin imaged partly in Oct, 15 2004 ???

http://photojournal.jpl.nasa.gov/catalog/PIA06145

[Rob Pinnegar]

Can that area be associated with the Basin imaged partly in Oct, 15 2004 ???

I don't think so. It looks like this new "mystery area" is pretty much right on the south pole. That's not the case for the big Roncevaux Terra basin. This is something new.

[TritonAntares]

Can that area be associated with the Basin imaged partly in Oct, 15 2004 ???

No, please compare image and Steve Albers map:

The bassin you mean is far more east...
The concerning region must be somewhere in the white area in the lower left corner of this map.
There might be another bassin SW of the one you think about as the map suggests.
These structures could also be a series of two or more craters...
Unfortunately, the map is difficult to interpret at such high latitudes; a polar projection would be more useful here...

What I believe is that the bassin imaged partly in Oct.2004 - probably the second largest on Iapetus - could be the driving force for the unround shape of the part of the southern hemisphere we are talking about.

Bye.

[jmknapp]

13 new Iapetus images released;
one takeout:


Date: 11/25/06
Distance: 879.656 km
Filters: CL1 and IR1

This 'white mountain' is still visible...  

Here's web page showing the orientation of the disk with lat/lon lines: Iapetus file 61859

In polar projection, the red circle below marks the approximate location of the "white mountain" region:

[TritonAntares]

In polar projection, the red circle below marks the approximate location of the "white mountain" region:

Cool projection...
Now the map is much easier to interpret at those high latitudes,
but there is some 'terra incognita' left to speculate...

Many Thanks, Joe.

11 new Japetus images released...

Two takeouts:


Date: 11/26/06
Distance: 890.982 km
Filters: CL1 and IR3



Date: 11/26/06
Distance: 891.194 km
Filters: CL1 and CL2

Really deformed terminator...

Bye.

[Decepticon]

Hogland is gonna have a Hay Day with this.

Last week he had the nerve to say NASA switched the Stardust Capsule.

[nprev]

Hogland is gonna have a Hay Day with this.

Last week he had the nerve to say NASA switched the Stardust Capsule.

...OT here, but I've never really paid any attention to that guy. Is he actually "seeing" alien artifacts all over the Solar System now (instead of just pyramids on Mars) and accusing NASA of vast conspiracies? Sheesh...

[nprev]

Cool projection...
Now the map is much easier to interpret at those high latitudes,
but there is some 'terra incognita' left to speculate...

Many Thanks, Joe.

11 new Japetus images released...

Two takeouts:


Date: 11/26/06
Distance: 890.982 km
Filters: CL1 and IR3



Date: 11/26/06
Distance: 891.194 km
Filters: CL1 and CL2

Really deformed terminator...

Bye.

Is that flattened area on the sunlit side at about 10 o'clock Odysseus in profile? If so, wow!

Iapetus looks like it got cold & stayed cold pretty rapidly...hard to believe all this surface relief could have ever survived otherwise.

[Toma B]

Here are images from latest non-targeted Iapetus flyby.
6 images from 20thJan to 26thJan.
Hope you will like this...

189kB.

[Decepticon]

Cool! I was waiting to see something like that.

[jmknapp]

Now the map is much easier to interpret at those high latitudes,
but there is some 'terra incognita' left to speculate...

Even the terra cognita is a bit lacking in detail. In April there's an opportunity to get a bit of the south polar region, but as far as I can tell from the info in the SPICE kernels, they do not plan to image Iapetus then. Here's a predicted view:

[scalbers]

Thanks to Toma in post #65 for that great montage. That is a big help for me in deciding on mapping options. I may try the 956000km range image first. That along with some additional Saturn-shine ones I'm wrapping up on navigating should really help to illustrate this light/dark boundary area.

While the activity is busy with watching the current flyby, I quietly posted an initial version of the promised stereo imagery for the Iapetus Snowman craters in a separate thread.

Will also be interested to see any further comments on whether the April NT can or should include any imaging.

[tasp]

Iapetus looks like it got cold & stayed cold pretty rapidly...hard to believe all this surface relief could have ever survived otherwise.

I think you've nailed it. Iapetus, due to its' large distance from Saturn accumulated leisurely from the 'primordial chaos'. It may never have warmed significantly to 'iron out the lumps'. Or if it had a molten epoch, it ended prior to the cessation of the accretion bombardment. The lumpy surface is a relic from a time in the solar system no other sizable body has been able to record.

The rigidity of the icy crustal materials at the expected Iapetan temperatures and the low surface gravity (~2 1/2% e) are sufficient to maintain the 'unsphereocity'
seen by Cassini.


Additionally, in view of the distance to Saturn, the tidal braking effects would have been operative over a longer time frame than the other Saturnian satellites prio to tide lock. Internal heating from this process would have dissipated to space, rather than maitaining a longer duration molten period.

The 'scratches' feature may be related to the crust 'inflating' due to injection of materials via an impact underneath the feature, or displacement of the materials underneath the scratches due to an oblique (ish) impact to the northwest.

[nprev]

Tasp, you're too kind..thanks!

Well, if the "primordial icebox" conjecture is true, then the dark material almost has to be both relatively recent and exogenic. Wonder if a small outer moon similar to Phoebe in composition (but much closer to Saturn) might've gotten pulverized in a high-energy collision & the smithereens subsequently swept up by Iapetus...

[TritonAntares]

Another 13 new Iapetus images released...

Two takeouts:

Date: 11/27/06
Distance: 942.437 km
Filters: CL1 and GRN


Date: 11/27/06
Distance: 943.293 km
Filters: CL1 and CL2

... In April there's an opportunity to get a bit of the south polar region, but as far as I can tell from the info in the SPICE kernels,
they do not plan to image Iapetus then. ...

Really sad to read this....
But somehow I cannot believe it...
... the minimum distance will be about 602.000 km, 277.000 km lees than this month and partly unknown south polar regions will be observable.
Why should this opportunity be omitted?
There are no other targets to observe - despite of giant Saturn - in nearer vicinity in apoapsis!
Probably they'll put some OPNAVs; and these are not listed as far as I know...

[Rob Pinnegar]

But somehow I cannot believe it...
... the minimum distance will be about 602.000 km, 277.000 km lees than this month and partly unknown south polar regions will be observable.
Why should this opportunity be omitted?

Any chance we could be wrong about the SPICE kernels? It seem counterintuitive that _any_ opportunity to look at Iapetus close-up would be left out -- after all, when Cassini is close to Iapetus, it's not particularly close to anything else, so there's not much in the way of competition.

Based on what we've seen this time around, there is some interesting stuff going on in the south polar region. One would think that Cassini could improve on Voyager here, even if Iapetus is only going to be a crescent.

Even if Iapetus is imaged, one of the unfortunate things about the April encounter is that, from the looks of things, we wouldn't get any Saturnshine images of the non-lit part of Iapetus. This is because we'll be seeing the side of Iapetus that faces away from Saturn -- hence, no Saturnshine.

[Rob Pinnegar]

One more thing...

Take a look at the image of Iapetus at:

www.nineplanets.org/iapetus.html

and compare it with the map shown earlier in this thread.

The Nine Planets Cassini image shows a big impact structure near the south pole (on the terminator at the extreme right side of the image) whose location can be referenced to the even larger Roncevaux Terra basin, here located on the terminator to the right of centre.

Looking at the "red circle" map: The Roncevaux Terra impact basin can also be seen here, and when the two images are compared, it looks very much as if that south-pole impact structure should be located quite close to the red circle.

Thus the irregularity in the "white mountain" region of the south pole could be due to the topography of this impact structure. Perhaps its outline is not as easy to discern when seen from the Saturn-facing side of Iapetus, as compared with the Saturn-backing hemisphere (as in the Nine Planets image).

[TritonAntares]

The Nine Planets Cassini image shows a big impact structure near the south pole (on the terminator at the extreme right side of the image) whose location can be referenced to the even larger Roncevaux Terra basin, here located on the terminator to the right of centre.

Looking at the "red circle" map: The Roncevaux Terra impact basin can also be seen here, and when the two images are compared, it looks very much as if that south-pole impact structure should be located quite close to the red circle.

Thus the irregularity in the "white mountain" region of the south pole could be due to the topography of this impact structure. Perhaps its outline is not as easy to discern when seen from the Saturn-facing side of Iapetus, as compared with the Saturn-backing hemisphere (as in the Nine Planets image).

I guess you mean this encircled south-pole impact structure:

This crater alone should be to small to cause such a great irregularity.
The large Roncevaux Terra bassin seems more likely to have disturbed the whole region,
or there probably exists another hidden bassin.
---------------------------------------------------------------------------------------------------------------------------------------------------------------
Concerning the upcoming April far-encounter, I do not believe an opportunity to observe
the south-polar hemisphere under optimal contrast conditions as cresent will be thrown away.

[TritonAntares]

Is that flattened area on the sunlit side at about 10 o'clock Odysseus in profile? If so, wow!

No, as you can see in my post above.

Odysseus is actually the largest impact on Tethys:


Bye.

[nprev]

No, as you can see in my post above.

Odysseus is actually the largest impact on Tethys:


Bye.

Whups! My bad; new names, new features, all jumbled together in my limited and very random access memory. Thanks for setting me straight!

[TritonAntares]

Whups! My bad; new names, new features, all jumbled together in my limited and very random access memory. Thanks for setting me straight!

Doesn't matter...

Here a Voyager image and map with a few names:


Ups...
The 'Roncevaux Terra impact bassin' doesn't seem to be in Roncevaux Terra...
Where are the borders here???

The IAU should better hurry up giving names to the new Iapetus surface structures found and imaged by CASSINI...
We're still talking about features like 'moat', 'snowman', 'belly band', 'white mountains', '450 km bassin' and so on,
as a lot of new found structures on Titan were named weeks or months ago.

Bye.

[TritonAntares]

Another 13 Iapetus cresents released...

Two takeouts:

Date: 11/28/06
Distance: 1.115.139 km
Filters: CL1 and CL2


Date: 11/28/06
Distance: 1.116.538 km
Filters: CL1 and CL2

Should have been the last sequence for january...

Btw:
We didn't get any real saturnshine images, maybe its possible to enhance & manipulate some of those bright cresents...
This should be possible as longitudes between 270° and 90° are facing Saturn.
Poorly I've got no sufficent software on my PC at work - only pain(t)...

Bye.

[Phil Stooke]

Steve Albers suggested a long time ago that I post some polar versions of some of his maps, but I never got around to it. But here are a couple at last...

Iapetus. I took Steve's excellent map, and brightened Cassini Regio a bit to show surface features better - so this new map is not photometricallyaccurate, but you can see more in the dark areas. Then a polar map from each hemisphere. The outer edge is the equator in each case.

North:



South:



Phil

[Rob Pinnegar]

The 'Roncevaux Terra impact bassin' doesn't seem to be in Roncevaux Terra...
Where are the borders here???

The basin is in the blacked-out region in the lower-right corner of the Voyager map you have shown. The reason that area is blacked out is that it was not imaged by Voyager. It isn't part of Cassini Regio.

The IAU should better hurry up giving names to the new Iapetus surface structures found and imaged by CASSINI...
We're still talking about features like 'moat', 'snowman', 'belly band', 'white mountains', '450 km bassin' and so on,
as a lot of new found structures on Titan were named weeks or months ago. 

I wholeheartedly agree with this.

Features on Iapetus are named after characters in the old Song of Roland. A lot of the names of the major characters were used up after the Voyager images, which means that the IAU may now be in the position of having to name these huge basins after minor characters that don't really deserve the distinction. This could be part of the reason for the delay.

If they can't find enough characters in the Song of Roland, maybe we ought to look for another literary source. How about Watership Down? We could name the four biggest basins Hazel, Fiver, and Bigwig (in Cassini Regio) and Woundwort (in Roncevaux Terra). And the giant, nearly-invisible 850-km basin could be El-ahrairah.

Of course I'm kidding -- but, heck, we've gotta call 'em _something_.

By the way -- those are some nice polar projections, Phil.

[TritonAntares]

The basin is in the blacked-out region in the lower-right corner of the Voyager map you have shown. The reason that area is blacked out is that it was not imaged by Voyager. It isn't part of Cassini Regio.
I wholeheartedly agree with this.

That was clear to me...
I was surprised to read 'Roncevaux Terra' only in the northern hemisphere of this old map.
I always thought 'Roncevaux Terra' is identical with the bright Iapetus surface,
as 'Cassini Regio' is with the dark parts.
If not - as this old map implies - there is still the possibility to give a new name to this unknown southern region.

------------------------------------------------------------------------------------------------------------------------------
Phil, wow excellent polar projections...

Something I was long waiting for...

Actually, from this viewpoint you can see more dark regions than bright ones...

Bye.

[Toma B]

Here is final version of image covering Iapetus non-targeted flyby from 20th January 2006 to 28th January 2006.




All images NASA/JPL montage by me...
Toma B.

[tasp]

Here is final version of image covering Iapetus non-targeted flyby from 20th January 2006 to 28th January 2006.


All images NASA/JPL montage by me...
Toma B.

Thanx, nice to see them all together.

Interesting seeing how slowly the terminator moves. (think I worked it out once at less than 3 mph)

Also, can appreciate how slowly Cassini moves in the outer part of its' path around Saturn in these pictures.

[tasp]

Here is final version of image covering Iapetus non-targeted flyby from 20th January 2006 to 28th January 2006.




All images NASA/JPL montage by me...
Toma B.

The 879,000 and the 890,000 images can be fused (poorly) for 3D.

(caution, I got a headache in less than a minute)

The 'scratch' area seems very dramatic. They are really deep.

Is it possible the poor matching will exagerate the actual depth? Amazing feature either way.

[Rob Pinnegar]

Is it possible the poor matching will exagerate the actual depth? Amazing feature either way.

If you want the apparent depth to correlate with the actual depth, you will have to make sure that you are sitting at the right distance from the computer screen, so that this equation is roughly satisfied (I've just worked it out now so if it's wrong somebody yell at me):

(E+I)/S = M/D

where E is the distance between your left and right eyes, S is the distance from your face to the computer screen, and I is the distance between the two Iapetus images on the computer screen. The quantity M is the distance Cassini moved "sideways" between taking one image and the next, and D the distance from Cassini to Iapetus (here about 900,000 kilometres). You might be able to estimate M/D by looking at the change in Iapetus' phase angle between one image and the next.

(By the way, I do realize that I've left out Cassini's radial motion relative to Iapetus here. Tilt your head sideways!)

[scalbers]

Here's an Iapetus map update, including the 956000km image, as well as a reworking of the Saturn-shine ones...

http://laps.noaa.gov/albers/sos/sos.html#IAPETUS

Phil, I appreciate your polar projections even if I'm a bit of a moving target at times

[TritonAntares]

...
In April there's an opportunity to get a bit of the south polar region,
but as far as I can tell from the info in the SPICE kernels, they do not plan to image Iapetus then.
Here's a predicted view:

News concerning the April 11 far-encounter:
Tilmann Denk told me there are 35 sequences planned between March 25 and April 18.

Additionally a Solar System Simulator v4.0 animation with data for this event:

Unfortunately this simulator is only using this poorly painted Iapetus-map...
It should really be updated...

Bye.

[TritonAntares]

Greuti - a member of a german forum - kindly simulated this with Celestia:

japetus_april.avi
Thanks Greuti.

Bye.

[Tman]

Greuti - a member of a german forum - kindly simulated this with Celestia:

Hi Ralf, you're welcome!

Cheers, Greuti

p.s. I see you don't read regularly the Mars Rover threads in this German forum.

[TritonAntares]

...
In April there's an opportunity to get a bit of the south polar region, but as far as I can tell from the info in the SPICE kernels, they do not plan to image Iapetus then. Here's a predicted view:

Good news...

...there will definitly be Iapetus observations between March 25 and April 18 (24 days!).
I was lately informed by Tilmann Denk.

The first image (resolution 14.3 km/pixel) on March 25 will show the saturn facing part of the leading side, 'Snowman' will be again at the right edge.

Apart from March 31 there will be Clear-, UV3-, GRN-, IR1-, IR3- and 3 GRN-pics every day until April 9.
Two or more photosessions are planned from April 10 to 12.
Minimum distance will be on April 11; 603.000 km;phase 125°; resolution 3.6 km/pixel.
Parts of the south polar region will be visible in sunlight.
Also the first saturnshine images will be made beginning with this date.

After April 13 there is the possibility to recognize the large south polar bassin discovered in August 2004 in saturnshine.
Hope we'll see anything at all...

Session will end on April 18 (phase 149°;resolution ~8 km/pixel) showing 'Snowman' on the left edge.

More later...

Btw1: I'm still looking for an animation for this far-encounter using an updated iapetus map.
Would be very nice if somebody could post such a toy...

Btw2: There should be a new extra thread for this event opened...

Bye.

[Decepticon]

I got my fingers crossed for some saturnshine images on this pass. Filling those polar areas will be great!