[X] My Assistant

[Guest_BruceMoomaw_*]

WhileTitan and Enceladus have been stealing all the recent attention,
scientists have begun devising possible answers to the two really big
puzzles about Iapetus:

(1) Regarding the startling "belly band" -- that 20-km tall ridge
discovered by Cassini running precisely along 1300 km or more of the moon's
equator -- Papers # 39.03, 39.04,and 47.08 of this week'sDPS conference (
http://www.aas.org/publications/baas/v37n3...s2005block.html )
suggest that the answers lies in the fact that when Iapetus was initially
forming it was spinning very rapidly (a 17-hour period), thus generating
centrifugal force that both caused it to bulge at the equator, and formed
the equatorial ridge by making soft crust and mantle material from the north
and south shift out toward the equator and collide to thrust the belly band
upwards.

However, this leaves us with the next set of puzzles: why was it spinning so
fast, and how did it cool and harden from its intial soft and plastic state
fast enough to freeze and preserve its equatorial bulges? Castillo et al
(paper 39.04) suggest that the latter is due to the fact that Iapetus formed
with an unusually high concentration of Al-26 in it -- the isotope that
produces lots of heat, but by the same token decays much faster than
standard U-Th-K radioisotopes. But how did so much Al-26 get into it? And
was it spinning so fast either because it's actually a captured moon (like
Phoebe), or because it started out as an inner moon that happened to make a
close flyby of Titan and got flung into a highly elliptical orbit? Both
suggestions have been made. But in either case, how did its initially
elliptical orbit get so well circularized (although it's still at a decided
tilt to Saturn's equator)? Alternatively, did it just initially form at
that great distance from Saturn, and form in a way that caused it to
initially spin fast?

(2) The other big puzzle about Iapetus remains its dark/light dichotomy.
Cassini's photos make it very hard to see how the dark patch's origin could
not be exogenic -- that is, material detached from Saturn's little outer
captured irregular moons by meteoroid impacts, and then spiralling gradually
in toward Saturn to be hit from behind by the leading face of the
faster-moving Iapetus. For one thing,the dark region is perfectly centered
on Iapetus' leading face. For another, Cassini confirmed that Iapetus'
craters near the edges of its trailing light side have dark patterns on
their floors of exactly the sort you'd expect from dark material hurtling
toward Iapetus' surface from its leading side, rather than oozing up from
the moon's interior.

But the patterns don't entirely match that model,either -- such dark
material sprayed onto Iapetus' leading face should simply cover that leading
face evenly. Instead, it doesn't extend all the way to the poles -- but it
DOES stretch partway around Iapetus onto its trailing side in the
lower,equatorial latitudes. It looks, in fact, like a saddle.

Well, J.R. Spencer (DPS paper 39.08) proposes an entirely new solution: the
initial patch of dark exogenically-deposited material, being dark, absorbed
enough sunlight and thus got warm enough for the remaining water ice on the
gradually darkening leading side to sublimate into vapor from the
lower-latitude regions of the dark patch and refreeze at Iapetus' poles,
relightening them. And the dark leading-face material -- where it bordered
the light-colored trailing-face surface in Iatpetus' equatorial regions --
also warmed the ice in those bordering light-colored surface regions enough
to make it slowly sublimate away, thus widening the dark region to stretch
further around Iapetus in its equatorial regions,but not at higher and
cooler latitudes.

T. Denk (paper 39.07) mentions a fact that would seem to back this up: the
fuzzy nature of the light-dark boundary. He also notes a difference in
color between the parts of the dark region that are on Iapetus' leading side
and those that stretch back onto the trailing side--and one with a "rather
sharp color boundary". Denk himself says this can't be explained yet. But
could it be due to the fact that the dark surface on Iapetus' leading side
is actually mostly not the material itself that was deposited there from the
irregular moons, but was instead created by the very high-speed impact of
those particles from the irregular moons heating the native material on
Iapetus' leading side enough to not only boil away the ice in those regions,
but chemically change and "redden" Iapetus' own native dark chondritic grit
left behind there? By contrast, the native Iapetan dark grit left behind in
those parts of its trailing side where the ice has been boiled away
indirectly by the warmth from the neighboring leading-side dark surface has
not been chemically modified and "reddened" by that much gentler warmth.
Viewed this way, the color difference would constitute still more evidence
of the truth of Spencer's model. So the centuries-old puzzle about the
strange appearance of this moon may at long last have been answered.

[Guest_paulanderson_*]

As Iapetus is of much interest to me... if this was noted already somewhere before and I missed it, sorry, but the latest Cassini Update from JPL (August 26) also includes this:

Cassini Significant Events for 08/18/05 - 08/24/05

Friday, August 19 (DOY 231):

A talk was given at noon today in Von Karman Auditorium entitled "26Al in Iapetus - Consequences for the Formation and Evolution of the Saturnian System." This seminar was about the dynamics and shape of Iapetus, a distant satellite of Saturn, and how it turns out to yield crucial clues for unveiling the history of the Saturnian nebulae and the Solar System. With its short half-life, 26Al has been used as a fine-scale chronometer to date events occurring in the early history of the Solar System. Iapetus is the first case among planetary satellites where other models cannot suffice and heat from Calcium-Aluminum Inclusions (CAI) is absolutely required. This allows us to date the age of Iapetus as 4.565 8 ? 0.000 6 Gy. This sets a lower bound on the age of Saturn, the upper bound being the age of the CAIs. This result has important consequences for our understanding of the Saturnian system and provides new constraints for models for the formation of the outer Solar System. Implications for the geology of Iapetus and the other Saturnian satellites was also discussed.

Paul

[tasp]

The symmetrical attendent ridges, angling away from the main ridge structure very tightly constrain possible formation scenarios for this structure. Additionally, the large crater basin off the east end of the main ridge structure contains no trace of the ridge structure anywhere in its area clearly photographed to date.

That Iapetus swept up ring material seems an attactive idea, but how can the details all fit together?

*Perhaps a ring system around Iapetus itself? Any 'lost' fragments of Saturns main ring system would not stay aligned to such high precision in its 3+million kilometer journey from Saturn to distant Iapetus. By the time it arrived near Iapetus it would be disspiated such as meteor streams are in the vicinity of earth, this material could never accumulate in a straight structure 20 km wide. Additionally, having Iapetus traverse the rings of Saturn in their current location and then arrive somehow at its current orbit without disrupting or being disrupted by all the objects in between is profoundly unlikely.

* How can a ring form around Iapetus? Primordial debris leftover from its formation? Not suspect this anywhere else, why here? Perhaps a grazing impact of a large body lofted material in a manner similar to that believed to have created earth's moon? That seems plausible, perhaps. Any oddly elongated craters on Iapetus? Yes, the long distance color shots from summer 2004 show a highly elongated possible crater on the terminator.

*How would a ring around Iapetus, created by a randomly oriented impact wind up almost perfectly aligned with the equator? A large number of individual mutually colliding materials in orbit around virtually any object will collapse into the LaPlacian plane as a result of the collisions. Objects inclined to the equator will pass through this plane twice per orbit, collisions preferentially alter materials patths to over the equator. This process continues to maintain the near perfect 2 -dimensional collimation of Saturns' rings in its equatorial plane.

*How does the ring material get down? If the density of material in the ring system is high enough, gentle collisions between ring particles in adjacent orbits happens virtually continuously. An object in a slightly lower orbit, when it contacts an object in a slightly higher orbit will transfer momentum during the 'bump'. The effect is to enlarge the orbit of the higher particle, and to contract the orbit of the lower object. This process, aggregated across the ring system causes the top side to rise, and the low side to lower. The is a limit to how low you can go around Iapetus. The highest spot along the equator will 'scrape' the low side of the ring as it 'whizzes' by. Material in its final orbits around Iapetus take roughly 3 hours to go around Iapetus at just over 900 mph. Material 'smacking' the highspot won't vaporize to any great degree at this speed if made of water ice at -250 to -350 F. What does happen is, you start to accumulate material in a pile at the surface of Iapetus on the equator under the ring system.

*How does that make a ridge? The pile can only spread out so much as the incomming materials dissipate their 900+ mph speed. Once the 'pile' is large enough, the accretion point will start to move upstream into the oncoming flow of additional ring material. The ridge forms 'upstream' into the flow, if you will. As I said, once the pile is large enough the incoming velocity cannot over come the growing piles inertia, it can only grow upstream, that is the only place where there is room.

* How do you get symmetrical attendent ridges out of this? It is possible the outer 1/3 of the descending ring system is inclined slightly to the equator or that a large impact during emplacement of the ring materials 'upset' the system slightly. It is also possible, the material accreting onto the 'pile' or ridge splatters somewhat, and interacts with portions of the ring system above the contact point. Materials in a low orbit about Iapetus encountering abruptly deceled materially will themselves be deceled and will acumulate down range.

Time constraints right now keep me from giving more detail, but I will happily entertain comments, con and pro on this.

[Guest_Richard Trigaux_*]

What you say tasp is interesting.

The idea of an equatorial bulge does not hold, it would have produced an eliptical globe. The idea of Japetus passing through a ring, although appealing, does not hold, as the material deposit would have be much less accurate. On the other hand, the idea of Japetus passing through a ring of Phoebe's material explains well the darkening of the leadind side, with some adaptation such as a slight rotations of Japetus, for instance under the effect of added mass.

On the countrary the idea of a Japetus ring falling on its ground is much more interesting, it explains well the narrow mountain range and its symmetry in latitude and assymmetry in longitude (once a spot starts to gather material, it grows up so that it catches more material)

The question is how such a ring could have formed. You suppose a meteorite impact, but such an impact is likely to have formed a ring in a random inclination, not just at the equator. And anyway we should observe other such mountain ranges on other bodies, associated with large impacts.

My idea is that Japetus had a moon in former times. Hey, the moon of a moon, how should we name this, in the as raging as useless debate about naming objects. Impossible? we see that many Kuyper belt objects have moons, and even more than one, as it was found recently with Pluto (which has the large Charon and two small 100kms moonlets). So Japetus formed with such a small moon, perhaps 100kms wide. This is consistent with the idea of a fast rotation for the former Japetus.

We can imagine many ways to make this small moon lower its orbit untill it breaks with tide effect. Perhaps it even never melt, so that it broke in a kind of snow. (Similar sized moons such as Hyperion obviously never melt, unless it is a fragment of a former larger body). There are certainly many disturbances in the Saturn system.

And then the ring slowly spiraled in (I think rings spiral in, not both in and out like you think. But only simulations would discriminate us).

Eventually the same process happened several times, to produce the attendant ridges, or there was some precession effect due to disturbances, which changed the relative inclination between the ring and Iapetus. Where can we see the attendant ridges?

[tasp]

____________________________________________________________________

The question is how such a ring could have formed. You suppose a meteorite impact, but such an impact is likely to have formed a ring in a random inclination, not just at the equator. And anyway we should observe other such mountain ranges on other bodies, associated with large impacts.
____________________________________________________________________

In The New Solar System book, the Planetary Rings chapter has a diagram (that with my crappy computer skills I am unable to post) that really illuminates how large quantities of material in orbit in any inclination (exactly 90 degrees might be a special case for another topic) will, if there is enough of it, always wind up in the equatorial plane. Assymetries (even tiny ones caused by oblateness) in the gravitational field cause the inclined material to spread around the host object. Collisions of the orbiting material result and the effect over time puts all the material in the equatorial plane. It is an amazing effect, and it is entirely 'automatic'. I was convinced of the ring around Iapetus causing the ridge as soon as I comprehended the diagram. It really is a stunner.

That subsequently, there appears to be a suitable 'donor' crater, some thoughts I've had about solar wind drag on a possible Iapetan ring, and the symmetrical attendent ridges, really push me towards this scenario.

The 'bump' process (officially called dynamical ring spreading, IIRC) that raises and lowers the top and bottom of the ring is the main mechanism for lowering the material to the surface of Iapetus. Drag forces on the ring system, solar wind, photon pressure, Saturn's magnetospheric effects, all 'keep a lid' on the high end of the ring system. Iapetus won't form little moons at the Roche limit from material 'bumped up' there, all the material winds up on the surface, eventually.

More later.

[Guest_Richard Trigaux_*]

tasp,

If matter gathers around a perfectly symmetrical globe, there is no reason that a ring forms in the aquatorial plane, that depends only of the average velocities of the particules. The particules will average their movements from collisions, but eventually they can gather in a high inclination ring, even 90°.

All the rings we observe are in the equatorial plane of the parent body, because there is a relation: the rings and the main body formed simultaneously (or, if the ring was formed by the breaking of a moon, the moon and the parent body formed simultaneously.

Also Large ringed planets are not symmetrical, they are elliptic, perhaps this is the cause of the rings being brought afterward into the equatorial plane. But Japetus is not elliptic, it rotates slowly, and always have its heavier side toward Saturn. Can really a ring be stable around such a body? If not, it is likely to fall on the ground.

When I suggested a moon around Iapetus, I was speaking of a body which formed simultaneously with Japetus, and not of a moon which formed afterwards from a ring. This is, with my opinion, the only way to have something symmetrical to the equator plane.

[tasp]

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Also Large ringed planets are not symmetrical, they are elliptic, perhaps this is the cause of the rings being brought afterward into the equatorial plane. But Japetus is not elliptic, it rotates slowly, and always have its heavier side toward Saturn. Can really a ring be stable around such a body? If not, it is likely to fall on the ground.
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Good point.

{and you can see the 'however' rushing towards you}

-however-

Upon its formation, Iapetus was surely not rotating every ~80 days. Probably more like 10 hours, more or less. Saturn not yet having had time to tidally arrest its rotation.

And,

Of all Saturn's tide locked satellites, which one 'locked up' last?

Iapetus.

Our little buddy is so remote from Saturn the tidal braking of its rotation must have taken a very long time indeed, the tidal braking effect depending upon an inverse square law.

Additionally, as we look at an Iapetus rotating on its axis faster, the difference in velocity between the (hypothetical) emplacing ring material and the surface decreases. The 'touch down' velocity will be less than the 1540 km/hr of a stationary Iapetus.

(Assuming Iapetus and the ring both are spinning in the same direction.)

Shame we never had a probe go by while it was happening, fascinating to watch all of this.

[Guest_Richard Trigaux_*]

Upon its formation, Iapetus was surely not rotating every ~80 days.  Probably more like 10 hours, more or less.  Saturn not yet having had time to tidally arrest its rotation.

Nearby all bodies, I think, formed from a primary nebula. The process is today not exactly known, and there are still large discutions. Perhaps even several processes can lead to the formation of a hard body from a nebula. But I think that we can have some confidence in this statement: there is alway, in a nebula, a mechanically linked subset of this nebula which will form a given body, and its overal movement (translation and rotation) will give the translation and rotation of this body. Think at an accretion disk formed of ringlets, each forming a planet (although things may be more complicated and less visually appealing). If we consider that a primary nebula formed Iapetus, perhaps this nebula was rotating too fast, so that some of it was left in orbit, forming a moon or a ring (depending if it was near or close to Japetus). This process is commonly invoked to explain planet systems, double stars, etc.

But also some process may brake the overal rotation of the nebula, otherwise it would never condense in a small body. This process is not yet understood, but it seems linked to the polar jets often seen in planetary nebulaes (and also in quasars).

So that it is perfectly plausible that Iapetus formed with a little moon like Pluto's. In this case there would be no need of any further mechanism to align the ring plane with the equator plane, as they formed from the same mechanics. The only extraordinary thing is that Iapetus would be the only one in this case. Or perhaps there was others in the saturnian system, which were unstable and formed the huge impacts on the inner moons like Tethys?