[X] My Assistant

[volcanopele]

As Saturn approaches periapsis for Rev 04 this week, time to start talking observations. Obviously there is the Encelaudus close flyby on Wednesday. In addition, there is a 84,000 km encounter of Tethys which should provide a great view of Ithaca Chasma.

Also, I have a post up on my blog about the latest mutual event, this time between Mimas and Janus.

[tedstryk]

How will the coverage compare to T3. Also what ended up being the highest resolution at which actual images of Enceladus were obtained (in otherwords, that didn't miss). Now, if you Cassini folks had the sense to put an instrument on Cassini to remove Titan's upper atmosphere so it couldn't effect Cassini's trajectory, we wouldn't have such problems!

[Guest_Sunspot_*]

It doesn't look like JPL are going to release an event timeline for the Enceladus flyby

[alan]

It doesn't look like JPL are going to release an event timeline for the Enceladus flyby 

Well they finally released the coverage map

http://saturn.jpl.nasa.gov/multimedia/imag...fm?imageID=1419

I guess we'll see the images when we see them.

[Guest_Sunspot_*]

Lots of new images just arrived:

this one is great:

http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=34703

However alot of the links from the thumbnails to the larger images don't work, about half the pics I clicked on wouldn't bring up the larger image with caption.

[OWW]

However alot of the links from the thumbnails to the larger images don't work, about half the pics I clicked on wouldn't bring up the larger image with caption.

Yes, I noticed that during previous flyby's as well. Maybe their software is buggy or the person posting the images is so tired he/she can't do a thing right. It's really annoying.

[volcanopele]

However alot of the links from the thumbnails to the larger images don't work, about half the pics I clicked on wouldn't bring up the larger image with caption.

Yes, I noticed that during previous flyby's as well. Maybe their software is buggy or the person posting the images is so tired he/she can't do a thing right. It's really annoying.

I have a list of interesting images up on my blog now:

http://volcanopele.blogspot.com/2005/03/ra...-of-day_08.html

Some are there but a few are still missing in action. I have no idea when this will get fixed, though I hope it will be up and running tomorrow morning.

[volcanopele]

It doesn't look like JPL are going to release an event timeline for the Enceladus flyby

Oh ye of little faith:

http://saturn.jpl.nasa.gov/multimedia/prod...by20050307a.pdf

[Bjorn Jonsson]

The fun is starting:

[Guest_Sunspot_*]

Here too:

http://ciclops.lpl.arizona.edu/view_event.php?id=16

[DEChengst]

http://paranoid.dechengst.nl/saturn/Enceladus3.jpg

[Decepticon]

Reminds me of Lava flows on earth.

[volcanopele]

Reminds me of Lava flows on earth.

lava flows...you haven't seen nothing yet...

[Gsnorgathon]

volcanopele, you're a terrible tease... but that's one of the things I like about you.

[gpurcell]

Looks like a Europa that froze solid and stopped resurfacing....

[alan]

I want geysers! Where are the geysers?

[volcanopele]

The rest of the raw images have been posted. A few new comments on my blog. Too busy right now to comment further.

[Guest_BruceMoomaw_*]

You want geysers, eh? Take a look at all those little black specks arranged neatly along the fracture lines at http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=34984 -- which is the closest raw image released yet (range: 4177 km). We saw some of these during the February flyby; we're seeing far more of them now. And I find it hard to imagine what else they could be besides vents.

[Guest_Sunspot_*]

This is a great shot:

http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=34971

Interesting fracture patterns

[SFJCody]

This is a great shot:

http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=34971

Interesting fracture patterns

You mean the way they bunch up around the craters? That's got to have an obvious explanation. I just don't know what it is.

[volcanopele]

This is a great shot:

http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=34971

Interesting fracture patterns

You mean the way they bunch up around the craters? That's got to have an obvious explanation. I just don't know what it is.

Crater focusing. The stress field for those fractures will focus on areas where the ground is weakest, like at an impact site.

[volcanopele]

You want geysers, eh? Take a look at all those little black specks arranged neatly along the fracture lines at http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=34984 -- which is the closest raw image released yet (range: 4177 km). We saw some of these during the February flyby; we're seeing far more of them now. And I find it hard to imagine what else they could be besides vents.

Those may still be just shadows, we're not sure yet.

[Guest_BruceMoomaw_*]

Looking at the enlarged version of that image: if they are shadows, they're shadows cast by little individual pimple-like bumps or mounds arrayed in chains along the fissures -- meaning, again, that we are likely looking at individual vents.

In this connection, by the way, I also note an abstract that was submitted to the upcoming April meeting of the European Geophysical Union ( "Keck Near-IR Observations of Saturn's Main Rings, E and G Rings, Bracketing Earth's Aug. 1995 Ring Plane Crossing": www.cosis.net/abstracts/EGU05/01365/EGU05-J-01365.pdf ). It has since been withdrawn, for unstated reasons -- but the same team stated the same conclusions in a paper at last fall's meeting of the American Geophysical Union:

"Our E and G ring data confirm that the E Ring is very blue. Its radial and vertical structure are found to be remarkably similar to that apparent in the HST ringplane crossing data at visible wavelengths, reinforcing models of the ring's peculiar narrow or very steep particle size distribution... The G Ring is found to be distinctly red, similar in color to Jupiter's main ring, indicative of a more typical broad particle size distribution."

In other words, if they're right, the G Ring seems to be made up of pieces of material over a wide size distribution, just as one would expect of ordinary solid debris that was blasted off a moon by meteoroids (or lifted off its surface by tidal forces), or was left over from the formation of the Saturn system. But the E Ring's particles are consistently very tiny, suggesting that they do not contain any bigger chunks of material that are colliding to produce the small dust-sized particles in it (as Larry Esposito has previously suggested to explain the periodic very large surges in the amount of water vapor in the E Ring that have already been observed by Cassini's UV spectrometer). In short, they look more like tiny droplets of liquid spray that have been expelled from a geyser and then quickly frozen into solid flecks of ice.

One other interesting note from this abstract: "Our data show unambiguously that Tethys is a secondary source of material for the E Ring." Tethys has been low moon on the totem pole up to now among the original nine Saturnian moons when scientists rank the importance of close-up observations of them by Cassini -- but does this make it possible that Tethys also has a few tidally energized geysers on it?

[volcanopele]

Looking at the enlarged version of that image: if they are shadows, they're shadows cast by little individual pimple-like bumps or mounds arrayed in chains along the fissures -- meaning, again, that we are likely looking at individual vents.

In this connection, by the way, I also note an abstract that was submitted to the upcoming April meeting of the European Geophysical Union ( "Keck Near-IR Observations of Saturn's Main Rings, E and G Rings, Bracketing Earth's Aug. 1995 Ring Plane Crossing": www.cosis.net/abstracts/EGU05/01365/EGU05-J-01365.pdf ). It has since been withdrawn, for unstated reasons -- but the same team stated the same conclusions in a paper at last fall's meeting of the American Geophysical Union:

"Our E and G ring data confirm that the E Ring is very blue. Its radial and vertical structure are found to be remarkably similar to that apparent in the HST ringplane crossing data at visible wavelengths, reinforcing models of the ring's peculiar narrow or very steep particle size distribution... The G Ring is found to be distinctly red, similar in color to Jupiter's main ring, indicative of a more typical broad particle size distribution."

In other words, if they're right, the G Ring seems to be made up of pieces of material over a wide size distribution, just as one would expect of ordinary solid debris that was blasted off a moon by meteoroids (or lifted off its surface by tidal forces), or was left over from the formation of the Saturn system. But the E Ring's particles are consistently very tiny, suggesting that they do not contain any bigger chunks of material that are colliding to produce the small dust-sized particles in it (as Larry Esposito has previously suggested to explain the periodic very large surges in the amount of water vapor in the E Ring that have already been observed by Cassini's UV spectrometer). In short, they look more like tiny droplets of liquid spray that have been expelled from a geyser and then quickly frozen into solid flecks of ice.

One other interesting note from this abstract: "Our data show unambiguously that Tethys is a secondary source of material for the E Ring." Tethys has been low moon on the totem pole up to now among the original nine Saturnian moons when scientists rank the importance of close-up observations of them by Cassini -- but does this make it possible that Tethys also has a few tidally energized geysers on it?

well, we do get 500 m/px imaging of Tethys on this pass...

[GregM]

.

[volcanopele]

I'm no expert here, but I can only recognize this as a mountain and caldera.
It probably isn't, since ice would be too plastic over the long run to hold such a shape. Sure looks like it though.

It could just be a dome in the middle of the crater, like many of the craters in the region, but the thought has crossed our minds about that being a volcano

[Decepticon]

I'm just floored!

These images are nothing like I could ever imagine.


This image here caught my attention. That crack looks very deep.
http://saturn.jpl.nasa.gov/multimedia/imag...9/N00030070.jpg



And Enceladus in front of Saturn!
http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=34891

[dvandorn]

I'm just floored!

These images are nothing like I could ever imagine.


This image here caught my attention. That crack looks very deep.
http://saturn.jpl.nasa.gov/multimedia/imag...9/N00030070.jpg

I can't tell if that's a crack or a cliff. You can almost believe that it's an upthrust fault that's formed a sheer wall several hundred kilometers long. And almost absolutely straight for a good length of it.

Would such a feature likely have been a product of crust shrinkage? Or maybe gravitational crustal deformation?

-the other Doug

[Decepticon]

Ummmm WOW! Tethys!!!

http://saturn1.jpl.nasa.gov/multimedia/ima...heQ=0&storedQ=0


I hope we get more!

[OWW]

Ithaca Chasma close-up. Doesn't look as fresh and sharp as the cracks on Dione.

Attached thumbnail(s)

 

[volcanopele]

Ithaca Chasma close-up. Doesn't look as fresh and sharp as the cracks on Dione.

very pleased with those images...

BTW, the first and last clear filter Tethys close-up image can be made into a nice anaglyph or stereo pair, if you feel compelled.

[Guest_Sunspot_*]

http://ciclops.lpl.arizona.edu/view_event.php?id=16

Lots more pictures at the CICLOPS site. These are of higher quality than the JPL RAW jpegs.

[Bjorn Jonsson]

Impressive images, however, Tethys looks a lot less interesting than the other satellites.

I have found only four Tethys closeups, all of them showing Ithaca Chasma. The other Tethys images (a lot of images in fact) are from a distance of roughly 190,000 km. I'm very surprised if there are not more closeups.

[volcanopele]

Impressive images, however, Tethys looks a lot less interesting than the other satellites.

I have found only four Tethys closeups, all of them showing Ithaca Chasma. The other Tethys images (a lot of images in fact) are from a distance of roughly 190,000 km. I'm very surprised if there are not more closeups.

Blame VIMS and their 30 minute stare at C/A. For that we could only target the sub-spacecraft point, Ithaca Chasma.

In terms of visual beauty, either some one make an anaglyph or you will just have to wait for mine...

[Guest_Sunspot_*]

Heres my attempt at an anaglyph of Ithaca Chasma on Thethys

Attached thumbnail(s)

 

[remcook]

I was at the RAS discussion meeting yesterday and Michelle Dougherty (sp?) said that the magnetometer indicates that either Enceladus has a small induced magnetic field, or that it produces a plasma (i.e. an atmosphere). Interesting...

[dilo]

I made a couple of color (rgb filters) images of Teti and Enceladus: left with "real" and right "enhanced" version (color are smoothed and saturations increased). Enceladus colors are very interesting! (click for full res):

[Decepticon]

^Great Work!

[DEChengst]

Six frame mosaic:

http://paranoid.dechengst.nl/saturn/Enceladus4.jpg

It's not perfect but still okay I guess. Need to find a way to compensate for the difference in distance at which the images are taken for bigger mosaics though.

[Decepticon]

^ Another great look at this moon!

[scalbers]

Greetings all,

Thought I'd give an update on my Enceladus map. I now have added 3 "raw preview" images from this month's flyby. The URL is http://laps.fsl.noaa.gov/albers/sos/sos.html#ENCELADUS.


Later,

Steve Albers

[Decepticon]

Cassini page update. http://saturn.jpl.nasa.gov/news/events/enceladus1/index.cfm


Blue Clues!

[TheChemist]

Thanks for the alert, Decepticon !
What an amazingly wonderful bunch of images

[tedstryk]

http://photojournal.jpl.nasa.gov/catalog/PIA07370

Check this out! I wonder what implications this has for current activity. It doesn't seem tiny Enceladus could hold even a rarefied atmosphere for long.

[chris]

Then perhaps there really are volcanoes....

http://mer.rlproject.com/index.php?showtopic=706

Chris

[dilo]

...I'm still sure!
Meanwhile, take a look to these enhanced-color/details views:



Marco.

[Guest_BruceMoomaw_*]

It's just been announced that Cassini found a faint atmosphere around Enceladus: http://www.jpl.nasa.gov/news/news.cfm?release=2005-046 .

As far as I can see, that nails it -- Enceladus DOES have active geysers somewhere on its surface, and these are the source of the E Ring. (The press release, however, does not state the density of the atmosphere.)

[OWW]

That's Very interesting. Pity the press release doesn't mention the pressure, only that it is a 'significant' atmosphere.
Surely must be awfully thin though? Somewhere in between Mercury and Triton?

[Decepticon]

Was there any night side imaging?

[tedstryk]

They may have not yet figured out things like pressure. Are there going to be any occultations of Cassini by Enceladus? If not, they definitely need to do that during the extended mission...that would tell us a lot about this atmosphere.

[David]

This is surprising, given that Enceladus is such a small moon (its surface area is about equal to that of the country of Namibia, for instance), and its density is barely above that of water, so its gravity must be something like seven-tenths of one percent of Earth's. With that kind of gravity, I don't see how Enceladus could hold on to even a slight atmosphere if it were not continually replenished.

[Guest_BruceMoomaw_*]

I don't know about any planned Earth occultations -- but keep in mind that stellar occultations of Enceladus monitored by its UV spectrometer would be far more sensitive than radio occultations of it. However, UV measurements of solar occultations of Enceladus would be even better in this regard. I don't know whether any such solar occultations are currently on the agenda, but I wouldn't be surprised now to see the final targeted Enceladus flyby in march 2008 modified to incorporate one.

And, yes, the fact that such a tiny moon has any atmosphere whatsoever is the interesting part -- prety much confirming that Enceladus is indeed currently venting gas from its interior, as was suggested earlier by the existence of the E Ring and the extremely white, apparently fresh nature of Enceladus' surface.

[tedstryk]

Some nice Iapetus shots are starting to come in. Great view of that giant crater seen during the December flyby.

[volcanopele]

A few of my anaglyphs have been released: http://ciclops.lpl.arizona.edu/view_event.php?id=17

Be kind, I created these on the same day I learned how to do anaglyphs.

[remcook]

wow! how does this work? I can't see the blue/red green/red images like normal anaglyphs, but when I put the glasses on I see depth. or am I hallucinating? is tis a different method?

[volcanopele]

wow! how does this work? I can't see the blue/red green/red images like normal anaglyphs, but when I put the glasses on I see depth. or am I hallucinating? is tis a different method?

For these anaglyphs, I reprojected the left and right images to orthographic projection (single hemisphere view) with the same center lat and lon. This removed the stereo caused by the spheroidal nature of Enceladus. Therefore the only stereo you see is topography, not curvature.

[David]

yes, the fact that such a tiny moon has any atmosphere whatsoever is the interesting part -- prety much confirming that Enceladus is indeed currently venting gas from its interior, as was suggested earlier by the existence of the E Ring and the extremely white, apparently fresh nature of Enceladus' surface.

Is the implication, then, that the vented atmosphere is condensing and precipitating onto Enceladus' surface? Could there be Enceladan snowfalls?

[Guest_BruceMoomaw_*]

Yeah, that's precisely the long-standing theory -- that Enceladus' surface is so dazzlingly white because (unlike Saturn's other moons) it is regularly covering over the dark lag deposits produced on its surface by micrometeoroid impacts with fresh new ice flakes, which are also the source of the E Ring particles. The latter, according to the latest Earth-based observations, seem to be remarkably uniform in their small size, unlike the wide array of particle sizes one gets from chunks of orbiting solid stuff simply colliding with and pulverizing each other, as is the case with Saturn's other rings (including, according to those same observations, the G Ring).

Cassini's VIMS analysis of Enceladus' surface showed, surprisingly, no trace of frozen ammonia in its surface -- it seems to be almost pure water ice -- but I think that may actually reinforce the geyser theory: we may be looking at surface geysers of liquid water/ammonia mixture, with the ammonia permanently volatilizing while the water then resolidifies into flecks of pure ice that rain back down onto Enceladus (perhaps after a period in the E Ring). Cassini's UVS has also found that the nitrogen torus around Saturn seems to come entirely from the E Ring, rather than from Titan's atmosphere.

However, the very interesting new Planetary Society article on the presentations on Saturn's moons at the ongoing LPSC ( http://www.planetary.org/news/2005/cassini_moons_0322.html ) reveals that Cassini's UV spectrometer DID observe a stellar occultation by Enceladus, looking for an atmosphere -- and couldn't detect one, indicating that the apparent shroud of gas detected around Enceladus by its magnetometer is less than one-trillionth as dense as Earth's. It's possible that Enceladus' geysers function only intermittently, as the degree of eccentricity induced in its orbit by Dione, and its resultant tidal heating by Saturn, changes over periods of thousands of years -- and that at the moment it's in shutdown mode, with the E Ring material left over from its last active period. On the other hand, we do have that sudden massive outburst of additional water vapor sensed in the E Ring by Cassini as it approached Saturn in early 2004, which doubled the total mass of gas in the E Ring -- so it may simply be instead that Enceladus' gravity is so weak that virtually all the material expelled by its geysers immediately either falls back onto its local surface or escapes completely from the moon and ends up in the E Ring until it later recollides with Enceladus, leaving no significant pall of actual atmospheric gas retained by Enceladus itself at any one time.

(This Planetary Society article -- which, like Emily Lackdawalla's earlier one on the Titan papers, is extremely interesting -- also has a lot to say about Cassini's Iapetus observations, which remain puzzling in the extreme. Saturn's moons seem determined to remain stubbornly enigmatic for as long as possible.)

[Guest_Sunspot_*]

The giant ridge that defines the equator on Iapetus must be one of the strangest geological formations in the solar system....i'm sure it's allignment with the eqquator says something about it formation.

[alan]

I found the lastest article at the Planetary Society very interesting particularly these two parts

"Water ice on Iapetus' dark side would be very unstable, especially at low latitudes," Spencer said. "Ice would sublime [evaporate] near the equator at much higher rates on Iapetus" than on other icy satellites. In fact, he continued, "dark terrain daytime temperatures are warm enough that -- uniquely in the Saturn system -- water ice sublimation is likely to be important in controlling surface albedo [brightness]." In other words, water ice could be evaporating out from underneath the dark stuff, concentrating the dark stuff even more

and

So Hendrix turned to Iapetus. She compared the spectra of the light material and the dark material on Iapetus, and discovered that the reason that the dark material looks dark is not because there is more "Phoebean dark stuff" on it -- the previously popular theory -- but because there is "very little frost" in Iapetus' dark material. "The dark material on Iapetus does not look like Phoebe, because Phoebe is much more water rich. We need very little water to model Iapetus dark material. What we're seeing is a loss of volatiles," such as water -- which was exactly what John Spencer had talked about during his presentation on CIRS observations of Iapetus. In the audience at the conference, one could almost hear the puzzle pieces clicking into place in scientists' brains. After Hendrix finished her talk, Spencer commented that "We're all on the same wavelength, even though we're on different wavelengths." (UVIS studies the ultraviolet, and CIRS the infrared -- get it?)

Now if I understand this correctly its possible that the dark material which lands on the leading side of Iapetus is losing the brighter and more volatile components making it darker which may account for the difference between the color of Iapetus and Phoebe.
I see a problem with this. It appears that this would only happen after the surface of Iapetus is already darker than Phoebe, otherwise the process would also occur on Phoebe and they would have similar colors. Is the dust losing the brighter materials between Phoebe and Iapetus? Or does the impact of the dust on Iapetus generate enough heat to vaporize it? Of course there is dark material on the trialing side of Iapetus which is still left unaccounted for.

[volcanopele]

I found the lastest article at the Planetary Society very interesting particularly these two parts

"Water ice on Iapetus' dark side would be very unstable, especially at low latitudes," Spencer said. "Ice would sublime [evaporate] near the equator at much higher rates on Iapetus" than on other icy satellites. In fact, he continued, "dark terrain daytime temperatures are warm enough that -- uniquely in the Saturn system -- water ice sublimation is likely to be important in controlling surface albedo [brightness]." In other words, water ice could be evaporating out from underneath the dark stuff, concentrating the dark stuff even more

and

So Hendrix turned to Iapetus. She compared the spectra of the light material and the dark material on Iapetus, and discovered that the reason that the dark material looks dark is not because there is more "Phoebean dark stuff" on it -- the previously popular theory -- but because there is "very little frost" in Iapetus' dark material. "The dark material on Iapetus does not look like Phoebe, because Phoebe is much more water rich. We need very little water to model Iapetus dark material. What we're seeing is a loss of volatiles," such as water -- which was exactly what John Spencer had talked about during his presentation on CIRS observations of Iapetus. In the audience at the conference, one could almost hear the puzzle pieces clicking into place in scientists' brains. After Hendrix finished her talk, Spencer commented that "We're all on the same wavelength, even though we're on different wavelengths." (UVIS studies the ultraviolet, and CIRS the infrared -- get it?)

  Now if I understand this correctly its possible that the dark material which lands on the leading side of Iapetus is losing the brighter and more volatile components making it darker which may account for the difference between the color of Iapetus and Phoebe.
  I see a problem with this. It appears that this would only happen after the surface of Iapetus is already darker than Phoebe, otherwise the process would also occur on Phoebe and they would have similar colors. Is the dust losing the brighter materials between Phoebe and Iapetus? Or does the impact of the dust on Iapetus generate enough heat to vaporize it? Of course there is dark material on the trialing side of Iapetus which is still left unaccounted for.

the other possibility is that dark material ISN'T being emplaced but instead the dark terrain is losing bright material. so it isn't the addition of dark material, it is the absense of bright frost.

[alan]

the other possibility is that dark material ISN'T being emplaced but instead the dark terrain is losing bright material.  so it isn't the addition of dark material, it is the absense of bright frost.

For the frost to sublimate on one side only wouldn't there have to be an asymmetric energy source driving it, or at least a temporary one to get it started?

[Guest_BruceMoomaw_*]

One theory which has been around for a while -- and which the U. of Hawaii's Jeffrey Bell supports -- is that impacts from Phoebe dust onto Iapetus' leading side are darkening it INDIRECTLY. Specifically, the Phoebe micrometeoroids are hitting at high speed and sublimating the ice on Iapetus' leading surface away, leaving only the dark stuff that was originally mixed in with the ice (plus a relatively small admixture of the material from Phoebe). This would neatly explain Bonnie Buratti's results showing that the near-IR spectrum of Iapetus' dark material does not well match that of Phoebe -- but DOES match the dark surface of Hyperion, which (thanks to its chaotic rotation) has been evenly exposed to such incoming Phoebe material. (Titan intercepts virtually all the rest before it can reach any of the inner moons.)

Indeed, the heat from the particles' impact may well modify the dark material from both worlds chemically as well -- and a Fischer-Tropsch reaction set off by such heat in the organic compounds within the dark particles (and maybe within small amounts of methane clathrate ice on the moons' surfaces) would also nicely explain the carbon dioxide ice that Cassini has found on Iapetus. It will be interesting to see if Cassini also finds large amounts of CO2 ice on Hyperion in September -- as well as whether Cassini's dust detector senses a flux of Phoebe material spiralling in through the outer reaches of the saturn system.

[alan]

The Planetary Society artical also mentions Iapetus's od shape.

"In fact, Denk said Iapetus' topography is so extreme that it is quite difficult to say exactly what its diameter is. Small bodies are usually defined as "triaxial ellipsoids," 3-D shapes like slightly squashed spheres. The size of a triaxial ellipsoid is specified by three measurements, one from pole to pole, one in the plane of the equator from the sub-Saturnian point to the anti-Saturnian point, and one perpendicular to those two. But "it is very difficult to fit an ellipsoid to Iapetus because the topography is so uneven," Denk reported. He joked that they may have to just call it an "Iapetoid" and be done with it"

I have an idea that may explain why Iapetus's shape is so odd: Iapetus is a rubble pile. I don't beleive it was shattered and reacreted, it would probaly look like Miranda if this was the case. Instead I think a large object hit it leaving a crater so large that the remaining pieces were no longer gravitationally stable. The remaining pieces shifted positions until it was roughly spherical. This creates some interesting possibilities. First the reason that the "belly band " looks like a seam is that it is a seam. The other is that the dark material could have started as impact melt. The volatile and brighter materials boiled off rapidly, perhaps explosively spreading some of the dark material. The bright materials would form a brief atmosphere which frooze out near the poles and on the side opposite the crater burying the dark material which was spread onto the trailing side. Later the dark material on the trailing side would be uncovered by impacts.
Of course this may only work in my imagination, particularly my explanation for the dark material.

[john_s]

  Now if I understand this correctly its possible that the dark material which lands on the leading side of Iapetus is losing the brighter and more volatile components making it darker which may account for the difference between the color of Iapetus and Phoebe.
  I see a problem with this. It appears that this would only happen after the surface of Iapetus is already darker than Phoebe, otherwise the process would also occur on Phoebe and they would have similar colors. Is the dust losing the brighter materials between Phoebe and Iapetus? Or does the impact of the dust on Iapetus generate enough heat to vaporize it? Of course there is dark material on the trialing side of Iapetus which is still left unaccounted for.

This is the abovementioned John Spencer... The key difference between Phoebe and Iapetus is that Phoebe rotates much faster, and thus doesn't get as warm during the day- it's about 15 K cooler at noon. 15 K makes a big different in water volatility, so ice is (moderately) stable at Phoebe's equator but is quite unstable at Iapetus' equator, even for the same albedo. I'm not sure the color difference can be attributed to the lack of water on Iapetus, or at least that wasn't my original idea (though it's worth considering- Hmmm...).

[David]

  I have an idea that may explain why Iapetus's shape is so odd: Iapetus is a rubble pile. I don't beleive it was shattered and reacreted, it would probaly look like Miranda if this was the case. Instead I think a large object hit it leaving a crater so large that the remaining pieces were no longer gravitationally stable. The remaining pieces shifted positions until it was roughly spherical.  This creates some interesting possibilities. First the reason that the "belly band " looks like a seam is that it is a seam.

I guess you mean "a seam between two shattered chunks of Proto-Iapetus" -- but why would they end up aligning right on Iapetus' equator? Or are you suggesting that the reaccretion somehow altered Iapetus' axis of rotation?

[alan]

I think reaccreating left it in an odd shape. I'm assuming the collapse was not symmetric, rather than collapsing to fill the crater from all sides my model has it collaping from two sides which are now the poles. I think an oblique impact could have left a crater which was narrower in this dirction.
The tides should shift the resulting body until the longest radius, not the poles or the impacted side, faces Saturn. If I remember correctly the shortest radius should end up as the poles. The collapse must have had enough momentun for the diameter running through impact to end up larger the the polar diameter.

[alan]

Now this is an interesting idea
http://www.universetoday.com/am/publish/ia...ng.html?1252005

[Jeff7]

Now this is an interesting idea
http://www.universetoday.com/am/publish/ia...ng.html?1252005

Does seem to make sense - a dense ring, with a lot of dark material around it. Dense ring forms ridge, while the surrounding material coats Iapetus' leading hemisphere. Their theory of a "transient atmosphere" carrying particles away from the "Rindge" is also intriguing.

Iapetus does have a few huge craters too, which look fairly old. There would be a possible source of an orbital modification.

[dvandorn]

Now this is an interesting idea
http://www.universetoday.com/am/publish/ia...ng.html?1252005

Rings very true with me. Especially where Freire says "The dark streaks observed at the edge of Cassini Regio indicate that it was a wind blowing from the equator that deposited the 'dust'. We can be certain of this because Cassini imagery shows clearly that the dust is deposited downward from the crater rims."

The first close approach images of Iapetus from Cassini sure looked to me like they showed material literally piled into craters, especially along the edge of Cassini Regio. And the material appears to be piled against the *far* walls of the craters in relation to Cassini Regio -- in other words, the material was blown from the direction of the dark area, which means that it settled on the far inside rims, leaving the near inside rims uncovered (and therefore retaining their bright surfaces).

I don't recall if I mentioned here my perception of how the material collected against the far rims of the craters along the edge of the dark area, but I sure noticed it... and then, when several members of the forum suggested that no material was deposited but, instead, icy coverings were being stripped to create the dark albedo, I decided that my eyes must have lied to me.

It's nice to know somneone else saw the same things I did!

-the other Doug

[Guest_BruceMoomaw_*]

Virtually all the descriptions by the Cassini ISS team themselves noted the same thing (as did quite a lot of outside observers the moment they looked at the first good Cassini shots) -- the dark material tends to be piled up against the rear walls of the bright-side craters but tends not to cover the parts of their walls and floors adjacent to the dark side, confirming pretty firmly that a lot of the dark material got sprayed at a shallow horizontal angle from the dark side toward the bright side. The trouble is that this pattern isn't completely consistent, and this continues to puzzle them -- they're still thinking about two separate sources for the dark material, one from outside Cassini and one from inside it (perhaps expelled by geysers from the now-famous "belly band"):

http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2268.pdf
http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2262.pdf