[X] My Assistant

[TritonAntares]

First of all, again the two existing encounter animations:
Animation 1
Animation 2

Nearest approach:

Resolution about 3,6 km/pxl.

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

Days around Sep 10 belong to the targeted fly-by in about 1000 km distance,
others will only be some OPNAVs.

Bye.

[Bob Shaw]

The flaw in the 'bowling ball' theory is that there's no chance of the plane of the orbit of the ring remaining fixed with relation to the surface of the moon, so the decaying ring particles wouldn't hit in a line but rather in a belly-band centered under the average orbital path (which migt well be sinusoidal). Saturn's system would have altogether too many and varied perturbations for the simplistic model to work!

Bob Shaw

[tasp]

Laplace worked out the scenario for getting all the materials in randomly inclined orbits over the equator long ago.

If you have a copy of The New Solar System, check out the chapter on planetary rings for a more complete description of the collapse of materials to the Laplacian plane.

When I saw that, I knew the equatorial ridge structure had to be the emplaced residue of ring system around Iapetus long ago.

Of all the major satellites in the solar system, Iapetus has the most isolated orbit from perturbing effects.

To anticipate another question, vaporization, the impacting debris on Iapetus would be traveling less than 1550 km/hr. Rather less if one feels Iapetus was rotating more rapidly in the past. (the weak tidal effects of distant Saturn braking Iapetus' primordial rotation would have been uniquely slow)

[Bob Shaw]

Of all the major satellites in the solar system, Iapetus has the most isolated orbit from perturbing effects.

To anticipate another question, vaporization, the impacting debris on Iapetus would be traveling less than 1550 km/hr. Rather less if one feels Iapetus was rotating more rapidly in the past. (the weak tidal effects of distant Saturn braking Iapetus' primordial rotation would have been uniquely slow)

It's the perturbations - the *many* perturbations - which are the fly in the ointment. Yes, rings do tend to become planar, and astonishingly self-organised, but in practice we just don't see them wherever there's anything other than one big(ish) body for the effect to work around.

If Iapetus were anything other than unique I might agree with you, but not this time!

Bob Shaw

[TritonAntares]

So we have exogenous and endogenous theories for the equatorial ridge as well.

What kind of observations will it take to sort this out? I guess if the ridge is eruptive, there should certainly be evidence of that in the close-ups in 2007. The white mountains in Western CR might look like cryovolcanoes, forex, or piles of welded cannon balls.

One question to answer should be:
What is older, the equatorial ridge or its surrunding/overlaying area, speak the craters?

Therefore I compared these two shots of the 'belly band':

Visible left is a large bassin in the western part of CR cut by the part of the ridge with the 'white peaks'.
Thus the crater must be older than them, even if its central peak doesn't coincide with the 'white peak' NW of it
and it is also lower. So you could guess the ridge there is fairly young, maybe eruptive....

In the right image another part of the ridge in central CR is shown. It seems to be quite old.
Lots of craters (red circles) crashed into it and disturbed its line.
The craters in the blue circles look somehow tilted, probably raised up by the ridge.
But due to bad resolution this is difficult to discern...

And don't forget there is no evidence for the ridge east of CR, only some uncertain albedo features.

Is the equatorial ridge powered by some longitudinal subsurfaced source or is there a gravitational cause from one direction?
The belts different heights and ages then could be explained by a longitudinal shift over a longterm period.

Bye.

[tasp]

Thanx for the picture, nice to see the ridge right side up.

I am having trouble imagining an internal process that can create a feature dead straight, aligned almost perfectly to the equator, that can operate for over 90 degrees of longitude . . .

Additionally, at the east end of this structure, is a conveniently huge circular impact depression (the one with the big landslide) that is miles deep. There seems to be no sign of any defect or discontinuity in the exposed crustal materials at that location.

Cratering of the adjacent plains seems similar to the cratering on the ridge to my untrained eye. Ridge is ancient.

Craters on such an elevated structure might have 'strange' morphologies compared to their analogs on the plains.

I also note, the steepness of the sides of the ridge. Plausibly the angle of repose for solid materials placed from above, rather than the expected slopes of a presumably runny iceyCO2/ammonia slurry?

The symmetry of the diverging attendant ramps is also striking. An internal process doing that . . . .?

Perhaps the 'attendants' reflect precession of the axis of rotation of Iapetus during the final stages of emplacement. Or similarly, slightly inclined structures in the outer portion of the Iapetan ring system.

I claim no special training in gravitional perturbation theory, but about the only thing I can imagine external perturbations doing to a potential ring system on lonely Iapetus, is to slightly speed up the momentum transfer process that places the materials onto the surface.

[Guest_BruceMoomaw_*]

I am having trouble imagining an internal process that can create a feature dead straight, aligned almost perfectly to the equator, that can operate for over 90 degrees of longitude . . .

The idea of Matson et al is that the centrifugal force of Iapetus' initial fairly fast spin, while its interior was still warm and plastic, created the major oblate bulge one sees in it, and in the process created convection loops of warm ice on either side of the equator that "folded" the cooler surface ice into a nice neat ridge at the equator (which is what they claim their calculations show would happen in such a situation). Then -- at about the time Iapetus' spin had been tidally slowed to once every 17 hours -- the Al-26-generated heat died out and Iapetus "froze" with the degree of oblate shape it had at that point; and during the eons since then, Saturn's tides have finished braking it into completely synchronous rotation and dragged its now-solidified oblate bulge into line with the planet. More on this shortly.

[Guest_BruceMoomaw_*]

The best summaries of the views of Matson, Jonathan Lunine and Julie Castillo are from the two 2005 AGU meetings:
http://www.agu.org/cgi-bin/SFgate/SFgate?&...t;P14A-03"

http://www.agu.org/cgi-bin/SFgate/SFgate?&...t;P21F-02"

As for WHY they believe this, see the second paper: "We revisited Iapetus' dynamical evolution models in order to understand under which conditions the observed non-hydrostatic shape of this satellite (corresponding to a past 17-hr geoid) became frozen and was preserved until the present. This study highlights the need of extra sources of energy not considered in previous thermal evolution models for the Saturnian satellites. Thermal evolution models driven by the decay of long-lived radiogenic species do not produce a temperature high enough for a body such as Iapetus, composed only of 20% silicate by mass and 3.561 10e6 km distant from its primary, to despin over the age of the Solar System. Different approaches were considered to increase the internal temperature: alteration of the surface properties to limit heat loss (greenhousing, low emissivity, insulating layer), impact heating, and enrichment in long-lived radiogenic species. None of these approaches can account for the despinning, except enrichment in long-lived radiogenic species. However that option cannot produce models that preserve the 17-h geoid. We demonstrate that short-lived radiogenic species, especially 26Al, as found in the Calcium-Aluminum-Inclusions (CAIs) provide conditions suitable for models that match the observational constraints. The relatively short half-life of 26Al allows the constraint of Iapetus' formation time with high accuracy: 1.0+/-0.2 to 1.6+/-0.4 My after the production of CAIs. This new result provides a strong constraint on the formation of Saturn. Based on current values for the absolute formation time of the CAIs, Saturn's age is estimate as 4.566 5 By."

That is: Iapetus -- given its great distance from Saturn -- had to be quite warm for a while in order to be plastic enough for the friction from Saturn's tides to have slowed it down to synchronous rotation by now. But if it was warm enough for that because it was unusually rich in the long-lived radioisotopes (U, Th, K-40), it would have stayed soft long enough to lose its current shape, which seems to be the shape it would have had from centrifugal force as a sphere spinning as fast as once every 17 hours. So it had to be very warm and soft for a very short period very early on, to tidally decelerate very quickly down to a rotation rate of once every 17 hours -- and then it had to suddenly lose its internal heat and freeze at that point, after which very slow continuing tidal dissipation despite the fact that it was now solidified would be adequate, over almost the entire lifetime of the Solar System, to slow it down further to its current synchronous 79-day rotation by now. And the only heat source that could do that sort of thing would be an initial abundance of intense but short-lived radioisotopes in it, with the only plausible candidate being Al-26.

[nprev]

Hmm. Almost makes you wonder if Enceladus was similarly enriched in radioactives, doesn't it? Dissapating fossil heat might be a bit harder to do deep in the gravity well of Saturn with Dione & Tethys adding their own increments of tidal heating.

Odd things happening out there...

[Guest_BruceMoomaw_*]

Actually, Castillo et al are thinking of precisely that -- especially since Enceladus' rock-ice ratio is considerably higher than that for any of the other icy moons:

http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2200.pdf

http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2219.pdf

Their belief is that this is exactly what "jump-started" Enceladus' heating -- which later, after the radioisotopes had decayed, was able to sustain itself due to the amount of tidal friction acting on Enceladus' internal magma (although such tidal-frictional heating could never have started up in the first place in an Enceladus that was initially cold and thus rigid).

[TritonAntares]

The idea of Matson et al is that the centrifugal force of Iapetus' initial fairly fast spin, while its interior was still warm and plastic, created the major oblate bulge one sees in it, and in the process created convection loops of warm ice on either side of the equator that "folded" the cooler surface ice into a nice neat ridge at the equator (which is what they claim their calculations show would happen in such a situation). Then -- at about the time Iapetus' spin had been tidally slowed to once every 17 hours -- the Al-26-generated heat died out and Iapetus "froze" with the degree of oblate shape it had at that point; and during the eons since then, Saturn's tides have finished braking it into completely synchronous rotation and dragged its now-solidified oblate bulge into line with the planet. More on this shortly.

Interesting point,
raises the question why Iapetus' spin degraded from say once a few hours to now about 80 days?
Couldn't Iapetus been capatured by Saturn?

It has a 15° tilted orbit far out of the other saturnian satellites, the most distant out of all the larger solar systems moons.
As a lonely object (centaur, KBO, left planetesimal from solar systems early ages...) it would have had a rotation period of say a few hours to a few days, for sure vastly shorter than today.
Once being captured, Iapetus was forced in its locked orbit, first its cool crust and than its warmer pseudoliquid interior had been tidally slowed.
For some time this inner part could even have been heated up by shear & relaxation effects - and perhaps the Al-26 decay.
In this period the ridge was created. The longitudinal distribution of the belt could be caused by some tidal effect,
when Iapetus was in a transition period between faster spinning and locked rotation
and the interior was still warm enough to transport material in this part of the crust.

Paradoxically the ridge is now positioned on the anti-saturn hemisphere, a position not to be expected.
Maybe Iapetus cooled down to todays conditions before synchronous rotation was achieved.

Not to forget is the strange triaxial ellipsoid figure (radii 732x726x722 km) of Iapetus...

Bye.

[TritonAntares]

So,
one lonely Iapetus images was released lately:

Date: 2006/04/06
Distance: 1.140.455 km
Filters: CL1 and CL2

Bye.

[tasp]

My thoughts on 'the good old days' on Iapetus seem to be at variance to the mainstream now.

Iapetus, while in a somewhat tilted orbit, has a comfortably low eccentricity, I never felt Iapetus formed anywhere else.

Considering the ~80 day period around Saturn, it seemed that Iapetus' accretion phase would have been quite leisurely, it just taking so much longer to 'sweep up all the fixins'.

In view of a long accretion process, we might expect Iapetus to have had time to radiate the impact derived heating and avoiding, or greatly minimizing a globally molten phase. This might explain the 'lumpy limb' we see in various areas in the Cassini pictures (not talking about the equatorial ridge here). Iapetus having had a rigid crust earlier in it's history than most (all?) other moons and able to retain greater deviations from sphericity from impact damage.

Having a more rigid crust earlier in the game than other moons allows preservation of features perhaps not seen on other objects, like the equatorial ridge structure.

Iapetus shares a trait with several other moons of Saturn (although I am not claiming any special significance for this) in that it is close (but not in) to a strong resonance of another satellite. Weird (to me) that Io, Europa, and Ganymede are in a mutual resonance, and other than scale, the highly similar Uranian satellites are not. And then we have Saturn, resonance features all across the rings, and moons (except for Hyperion, probably a special case) only close to resonances.


The synchronous rotation findings are certainly a surprise.

Iapetus remains a 'weird wee beastie'.

[angel1801]

I checked the raw images just a few minutes ago. Images ranging from about 922,000 km to about 1,050,000 km are available now.

[Steve G]

My thoughts on 'the good old days' on Iapetus seem to be at variance to the mainstream now.

Iapetus, while in a somewhat tilted orbit, has a comfortably low eccentricity, I never felt Iapetus formed anywhere else.

Considering the ~80 day period around Saturn, it seemed that Iapetus' accretion phase would have been quite leisurely, it just taking so much longer to 'sweep up all the fixins'.

In view of a long accretion process, we might expect Iapetus to have had time to radiate the impact derived heating and avoiding, or greatly minimizing a globally molten phase. This might explain the 'lumpy limb' we see in various areas in the Cassini pictures (not talking about the equatorial ridge here). Iapetus having had a rigid crust earlier in it's history than most (all?) other moons and able to retain greater deviations from sphericity from impact damage.

Having a more rigid crust earlier in the game than other moons allows preservation of features perhaps not seen on other objects, like the equatorial ridge structure.

Iapetus shares a trait with several other moons of Saturn (although I am not claiming any special significance for this) in that it is close (but not in) to a strong resonance of another satellite. Weird (to me) that Io, Europa, and Ganymede are in a mutual resonance, and other than scale, the highly similar Uranian satellites are not. And then we have Saturn, resonance features all across the rings, and moons (except for Hyperion, probably a special case) only close to resonances.
The synchronous rotation findings are certainly a surprise.

Iapetus remains a 'weird wee beastie'.

What effect would have Saturn's protoplanet heat had on the inner moons compared to Iapetus? If Saturn was radiating heat to the inner system, the icy moons would have had a warmer crust than Iapetus. Perhaps Ithica Chasma may have resulted from a warmer environment until Saturn itself cooled down?

[TritonAntares]

Hi,
this time some more Iapetus images (altogether 29);
Here three takeouts:

Date: 2006/04/05
Distance: 1.207.215 km
Filters: CL1 and CL2


Date: 2006/04/06
Distance: 1.062.356 km
Filters: CL1 and CL2


Date: 2006/04/07
Distance: 923.304 km
Filters: CL1 and CL2

Some details of the latest image (2,75 x enlarged):

3 certainly identified craters are marked.
The mountain (quadrangle) is casting a large shadow,
it must be quite high...
As mentioned before, the curvilinear ridge seems more and more connected with 3 sunlighted crater rims...

Bye.