Printable Version of Topic

Posted by: BruceMoomaw Sep 4 2005, 01:05 PM

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/dps2005/dps2005block.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.

Posted by: Rob Pinnegar Sep 4 2005, 06:59 PM

Admittedly, I haven't read the above cited abstracts yet, but, for what my opinion might be worth, several of the hypotheses described above only serve to highlight how little we know about what is going on at Iapetus. This is particularly true for the ideas concerning the equatorial ridge. Are _any_ of them falsifiable from the data collected so far? In particular, that business about Al-26 sounds totally conjectural. I can only assume that the authors must have some reason for thinking that this could have happened.

I hasten to add that this partially-informed criticism is aimed at the ideas themselves, not at their originators. When you don't have the huge amounts of evidence that are needed to solve a complicated problem, you've got to start somewhere. For example, look at how many times we've had to change the "currently accepted theory" of the formation of the Earth's Moon. Journey of a thousand miles, eh?

Posted by: ilbasso Sep 4 2005, 09:19 PM

IMHO, that equatorial ridge is just too narrow to be considered a "bulge". It's much more a "pucker". (note the use of highly technical geophysical terminology) Of all the theories I have heard so far, I like the one of Iapetus sweeping up a ring - but that still wouldn't account for the ridge going all the way around Iapetus. All the other processes explained in the papers (as Bruce graciously summarized for us) just don't seem to pass the Occam's Razor or "smell test" for me.

Posted by: Bob Shaw Sep 4 2005, 09:25 PM

Whatever the reason for the ridge, it's global in effect. Similarly, the albedo difference. So perhaps there *is* something special about Iapetus' origin or current location! Beyond that, it's difficult to say much that isn't speculation, but perhaps with a few more images...

Posted by: Myran Sep 4 2005, 09:42 PM

I agree ilbasso, the ridge are not quite the kind you would expect as the result of just rapid rotation. For that it would be more of a wide bulge and flattening at the poles, and we have Saturn itself as one example on that.
Yet rotation could have been part in creating this odd formation, as for example one impact that happened when Iapetus interior was partially liquid. We might see one frozen shockwave in the icy surface layer.
But again, this feature could originally have had one different orientation, and it have subsequently toppled over bringing it to the equatorial region due to the excess mass.

Posted by: Richard Trigaux Sep 5 2005, 07:35 AM

How could a fast rotation produce a "puck" rather than an oval-shaped moon? If Japetus was to spin fast, and then slow, the change of shape would have produced a system of cracks with a cylindrical symmetry.

I think that icy moons first formed liquid, from the heat of the accretion. But they quickly freeze, having nearby no radioactive heating, in perhaps some thousand of years. Freezing certainly produced cracks, but these primary cracks were erased by the heavy meteorite bombing during the first billion years, lefting icy evenly cratered globes. This explains why these moons are white: the ice is in the surface, while the darker carbonated or rocky materials fell in the core. Eventually some of these moons, like Miranda and Enceladus, experienced re-heating, for reasons likely linked to orbit changes.

As ilbasso said, it looks obvious that Japetus crossed a ring, and that the ring material mostly fell on the equator, while some of it swept on nearby all the planet.
This ring material could come from Phoebe, which is fairly dark and exhibits extensive bombing by large craters. (Eventualy Phoebe could be a former icy moon stipped from its ice layer by impacts, lefting only the concentrated black material).

This hypothesis is very temptative and eye-sriking, but it raises two difficulties.

The first, as says Ilbasso, is that the equatorial ridge extends to nearby all the way around Iapetus. Like most large planets moons, Iapetus is always looking to Saturn with the same hemisphere. This is because, not being completelly symmetrical, it has a heavier side, which turns to Saturn. And if the leading hemisphere gathers mass, it will fall toward Saturn in its turn. This process could produce several rotations of Japetus. The only difficulty here is that it should be completelly black.

The most serious difficulty anyway is that Japetus is not in the ring plane, and likely not in Phoebe's plane. If a ring formed from Phoebe material, then Iapetus could sweep through it first with its south pole, then lower latitude, equator, upper lattidude, northern pole, and back. To produce a ring centered right in Japetus's plane, with an accuracy of tens of kilometres all around the orbit, would require to form this ring from Japetus material itself, or a complex system where Iapetus would shepherd a ring formed from Phoebe. But it is difficult to change an orbit plane...

So the Japetus-Phoebe system is more complicated than it looks like, and a subtle issue arises: why Phoebe is much more severily bombed by meteorites than, say, Hyperion? Why all Saturn moon exhibit an uniform bombing with 10-30kms impacts, while featuring some very huge ones? This is not statistically even.

Posted by: Rob Pinnegar Sep 5 2005, 03:27 PM

The notion of Iapetus sweeping up a ring seems totally unworkable to me. I could probably be convinced that Mimas might have swept up a ring at some point in its history --- but as Richard has already pointed out, the orbital inclinations of _both_ Iapetus and Phoebe, and their distances from Saturn, make it difficult to envision how it could have happened. Not without invoking some Velikovskian weirdness in the time between then and now, at least. If Iapetus "swept up" any material from Phoebe, it would have hit all over Iapetus (or at least all over the leading face), not just the equatorial region.

Besides that, if the ridge is to be thought of as the result of thousands of micro-craters piled on top of each other, shouldn't we be able to see some of the larger craters? The only other possibility is that all the impacts were small (differential accretion) and it's tough to imagine that small impacts, even a very large number of them, could have built up such a big ridge. No, it's _got_ to be internal, even if Cassini Regio is the result of external processes.

The other thing about thinking of the ridge as having an external origin is that it necessitates the assumption that the ridge is completely unassociated with Iapetus' strange non-ellipsoidal shape. And that's a tough one to swallow.

Posted by: paulanderson Sep 6 2005, 01:20 AM

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

Posted by: tasp Nov 3 2005, 06:37 PM

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.

Posted by: Richard Trigaux Nov 3 2005, 07:46 PM

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?

Posted by: tasp Nov 3 2005, 10:56 PM

____________________________________________________________________

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.

Posted by: Richard Trigaux Nov 4 2005, 03:05 PM

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.

Posted by: ljk4-1 Nov 4 2005, 04:35 PM

Has anyone seriously considered that Iapetus has been reshaped artificially? And I don't mean by the Hoagland cult.

Or has the Intelligent Design flap made everyone so afraid of ascribing anything that looks like it was "made" on purpose skittish?

If we are the only ones in the Universe (yikes), then I guess Nature does have a good time playing with its parts. But if we aren't....

I'm sure Iapetus has some natural (albeit relatively bizarre) explanation for its features. But it also does not hurt to think outside our rather small box, too.

Posted by: JRehling Nov 4 2005, 05:00 PM

Circa the 17th century, continuing to some diminishing extent into the 20th century, the notion of complex life (>= multicellular plants) native to other planets in the solar system -- or the possibility of spacefaring civilizations coming into our solar system bringing high technology along with them -- has been given credence in various circles -- at times even being taken for granted.

The exploration of the planets (remotely and otherwise) has fed that hypothesis a steady diet of crow, and whenever we have been able to put it to the test, the no-life (again, not including possible microbiotic life) results have been just as consistent as the Sun rising in the morning. Taking the evidence (or lack thereof), the enormous distance between the stars, the great probability that an explorer would also be a pillager, and Occam's Razor into account, one must come away, I think, allowing a probability perhaps on the order of trillionths (eg, not zero, but very near zero) that spacefaring aliens have entered our solar system.

When Europeans arrived in Iceland, and when proto-Amerinds arrived in Alaska, and when the first peoples arrived in Australia and Hawaii and Madagascar, they did not sniff the air and then resolve to leave those places pristine so that over billions of years they could observe from a distance and see if a new intelligent species would evolve in those lands. They moved in, displacing no intelligent species, and became the intelligent species.

Given the very long length of planetary lifespans compared to the quick rise of civilization, it is enormously likely that any spacefaring civilization wandering into another one would find no other civilization, but would colonize it bloodlessly for their own use. Without that urge, the civilization would not have become the dominant species of its own planet -- wouldn't have settled their own Alaska, Iceland, etc.

So, our existence says a great deal about whether or not a spacefaring civilization has been able to reach Earth in the past billion years. Almost certainly not. And not much chance in the next ten million. By which time, if spacefaring is economically viable, we would have the opportunity to take the whole galaxy for our own, because we can see from our having had the opportunity to arise, no one else is likely here to do it instead.

All told, the idea of two civilizations evolving in the same galaxy is about as likely as two technology-wielding genera evolving on the same planet. It won't happen because whoever gets there first will preclude the other.

Posted by: dvandorn Nov 4 2005, 05:51 PM

Sounds like a justification for Isaac Asimov's all-human galactic civilization, to me...

-the other Doug

Posted by: mike Nov 4 2005, 06:10 PM

Why exactly is 'Occam's Razor' taken as such gospel? Who is this 'Occam' guy? Aren't razors sharp enough to slice you from stem to stern?

Posted by: JRehling Nov 4 2005, 06:48 PM

It is a shortcut for people who don't want to speak in the language of second-order probability, but if you'd like a mathematical model, those exist, and the degree to which it merits being "gospel" can be understood quantitatively.

Another everyday language way of saying the same thing is, if you see hoofprints, don't think zebra (of course, the example is not chosen for Africa).

On a Pennsylvania farm, a hoofprint could mean:

[A] a horse stepped on the ground
OR
[B] a zebra was anomalously set loose in Pennsylvania, and then it stepped on the ground

While the probabilities are not 1 and 0, respectively, the probability of [A] is, in the absence of other information, vastly greater than [B].

If no reasoning need be applied, there's no harm to being whimsical and considering [B].

But when one reasons over uncertain information, and has to reason deeply, with finite time available, alternative possibilities mean a combinatorial explosion -- anything you add in breadth costs you depth of reasoning. Then pursuing the [B]s of the world ends up lowering the expected value of your reasoning. Sometimes this leads to a rare breakthrough, but more often to lots of low signal-to-noise blather. The Internet is rich in the typings of people who find appeal in the [B]s. Some people prefer the extra depth a discussion gets to by following a chain of reasoning through the more likely probabilities without the almost-always-irrelevant [B]s. There are lots of [B]-heavy astronomy chat boards on the net; this one seems to be [A] heavy, which contributers frequently compliment. Birds of a feather flock together. Given that at least five people who work on space exploration projects visit this site and avoid the anomalist sites, I don't see a lot of upside in making this site yet another anomalist site and driving off the people who are the best sources of information.

Posted by: David Nov 4 2005, 06:48 PM

The point of William of Ockham's "razor" is merely that it is possible to infinitely multiply the number of hypotheses that account a given data set by adding a large number of "entities". There is no maximally complex explanation, as any theory can always have an additional bell or whistle (or equant or epicycle) added on. However, there is usually a severely constrained set of simpler explanations. As most of the more complex hypotheses are going to be variations of simpler hypotheses with some additional non-functional machinery invoked, Occam's razor is invoked to prune back the set of possible hypotheses to a more manageable level.

When creating achronic generalizations about the universe, it is impossible to test the relative truth of two equally adequate solutions to the same problem, and the choice of the simplest is a concession both to the limits of human memory and to human aesthetic preferences for simplicity and elegance. In diachronic sciences (i.e., those examining the development of entities over time), where it is sometimes possible to check the factual accuracy of generalizations by reference to various sorts of artificial or natural records, or (for shorter term developments) actually observing changes, it sometimes transpires that the simplest explanation, from an achronic point of view, actually results from the confluence of a number of processes that have the same or similar outcomes. In such cases, simple explanations may not actually be historically accurate. That does not stop them from being useful shorthands for making falsifiable predictions; if you know that A -> B, you do not necessarily need to consider any number of intermediate effects that have no influence on the outcome. That is one limitation on Ockham's razor.

William of Occam/Ockham (or Okeham, i.e. "Oak Village"; a town in Surrey, England) was a 14th-century Franciscan friar, philosopher, and theologian who advanced a destructive and devastating critique of contemporary scholastic theology (as exemplified, for instance, by Thomas Aquinas).

Posted by: tasp Nov 4 2005, 08:09 PM

----------------------------------------------------------------------------------------------
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.
-----------------------------------------------------------------------------------------------

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.

Posted by: mike Nov 4 2005, 08:19 PM

You certainly have a point there.. I personally find, though, that always applying the Razor of Occam leads to nothing in particular. However, never applying it would indeed prove ruinous to our current modes of thought - whether this is good or bad I still haven't been able to determine, and maybe I never will.. Moderation in everything, said the Greeks, until they were destroyed by the Romans, but then the Romans went away, except of course for everything they left behind, which is to say they are part of everyone, like everything in the entire universe. Time just seems slow.

I say it would be much more interesting if there really WERE aliens on Mars/Iapetus/anywhere. Their actual existence would eliminate the endless postulation by 'non-average' people expressing themselves via various forums everywhere. I imagine, though, that their existence will be so obvious that Occam's Razor will seem rather pointless.

William of Occam sounds like an interesting guy.. thanks for the information.

And I've found the theories about Iapetus' current state interesting, too.

Posted by: Rob Pinnegar Nov 4 2005, 08:25 PM

Aren't tidal effects inverse-cube?

Posted by: Richard Trigaux Nov 4 2005, 09:18 PM

The Occam razor is not an epistemological principle, but an heuristic principle. It does no say what IS true or false, but just what hypothesis is the most relevant to test. We start first to consider the simplest explanation (I once bough a new car, and a red light switched on. Why to panic? I checked the simplest explanation: I forgot the parking brake, and truly it was released but not completelly). But sometimes reality is complex or really unexpected, and we must look for complex or extraordinary explanations. There is no point today to say that Japetus was artificially engineered, but if in the future any other explanation is checked false, we shall have to envision this.

Posted by: JRehling Nov 4 2005, 09:18 PM

It would just up the ante. Then the anomalists would postulate that the aliens were really ghosts and claim that anyone who denied it was part of a coverup. If it were proved that the aliens were ghosts, they would claim that the aliens were ghosts from the future and claim that anyone who denied it was part of a coverup. If that were proved... etc.

Posted by: Richard Trigaux Nov 4 2005, 09:34 PM

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?

Posted by: Richard Trigaux Nov 4 2005, 09:50 PM

I do not agree with Mike and ljk4-1 about the odd formations on Japetus being artificial. But untill now these formations are unexplained, and I do not recognize to Hoaglandites the right to forbid us to envision such extraordinary explanations (understand to forbit them by ridiculizing them). Of course, each time it is the same, we find something extraordinary, oh, it may be artificial, and then we find a natural explanation so that we are getting more cautious than Shiaparelli. But it may happen one day that we find something REALLY artificial. In this case it would have to be accepted without hesitation.


Planet colonisation may be possible, even with thousand of years of travel between stars, it is nothing in comparizon witht he age of the galaxy.

But that the colonist will mandatorily destroy his world and pillage the others is not sure. It is what 19th century humans did. Only one century later we create reserves and have environment lobbies. And what about humans in 10 000 000 years? Think at what was on Earh 10 000 000 years ago, and what there is now, and extrapolate. What about the Moore law for brains? We cannot extrapolate in fact, as evolution of life is by breakthroughs not a linear process. So it is really impossible to guess what could be a 10 billion years old civilization. (which appeared on the first stars).

Posted by: tasp Nov 4 2005, 10:22 PM

Even better that way, but upon reflection, I gotta say, I dunno. One or the other.

Also, assuming Iapetus firmed up from moltenidity prior to tide lock, oblateness will persist to the present time. Sorry I didn't put that in before.

Posted by: tasp Nov 4 2005, 10:36 PM

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?

[/quote]

I feel strongly that many moons at one time had satellites of their own.

Hyperion could be the battered remnant of a Titanian moonlet. (moonoid? terminology of all this stuff is annoying, ain't it?)

Polydeuces, Calypso, Telesto, and Helene may be fromer moons of their host Lagrangian parent bodies.

That we don't currently see 'sub'moons, may be attributable to an efficient process that makes them 'go away'.

I'm thinking tidal evolution.

As our earth's moon is currently being advanced in its orbit by tidal interaction with the earth, and that Triton's orbit is decaying by the same mechanism, I am not hesitant to ascribe this effects to proto-moonlets.

Below the equivalent of geo-stationary altitude, moonlets will rapidly have their orbits decayed by tidal effects and wind up splattered on the surface of the host body, or they will be rapidly 'repelled' above that 'magic altitude' by tidal effects and wind up in an L4 or L5 relationship, or might just be 'lost'.

Due to Titan's large mass. I suspect Hyperion was 'repelled' vigorously, and wound up in a superior orbit with a resonant relationship to the former host.

I also wonder if the 'low density fluff' objects we seem to be finding might be good candidates for tidal interactions. Like earth's oceans the effect may be more pronounced in a nonconsolidated object.

Posted by: mike Nov 4 2005, 10:42 PM

Recent scientific research has demonstrated that the human brain can in fact grow long after adolescence has passed. With the advent of the Caesarian Section the heads of humans can now be larger than before, and with continued research into genetic engineering it seems to me that we are forever finding new ways to improve ourselves in every conceivable way.

As far as future colonists pillaging whatever they may find, first I will note that one man's pillaging is another man's improving. Second I will note that it is generally more advantageous to everyone to examine that which already exists rather than simply destroy it outright because it is 'weird' (scary). Therefore I conclude that while some future explorers will indeed wipe out less advanced forms of matter (more advanced matter being that which can consciously think, of course ), most will not. I shall generalize further and say that therefore the universe will steadily improve into perpetuity, as it has been since the beginning of time.

Oh, and I never said that I thought Iapetus was artificially constructed, but it certainly would be interesting if it was, wouldn't it?

Posted by: David Nov 5 2005, 01:57 AM

As a hypothesis, it has the defect of not explaining very much. What's the point of building a giant ridge on the equator of a battered moon? To separate the warring factions of North Iapetans and South Iapetans and keep them from arguing over which end of the egg to break first?

I admit that when I first saw the images of Iapetus showing the ridge (or pucker), I thought "How can that be natural?" I still don't know, of course; but speculation on its natural causes is at least productive, while speculation on artificial causes is a dead end that explains one mystery with another mystery. The course of knowledge is a progression from what is known to what is unknown, and one cannot leap into the unknown, miles from the shores of the known, and expect to do anything but sink. One has to build causeways into the unknown, keeping a link with the mainland of what we understand.

And that means that we cannot really speculate about what might or might not have been built by non-human intelligences until we already have some concrete evidence in hand that they exist; without that, we simply have no good way of distinguishing "weird artificial constructs" from "weird natural formations". There are natural geological formations on earth that, at first glance, one would swear had to have been built -- like the Yonaguni terraces under the sea off Japan. Iapetus' ridge, while bizarre, doesn't even come close to that degree of apparent structure.

Posted by: mike Nov 5 2005, 02:34 AM

The thing is, any speculation as to the possible causes for such a huge feature will be interesting. The scale is just too massive to be ignored. You don't NEED aliens.

I suspect something massive slammed into it, cracking the crust and turning roughly half the moon dark (or light, I suppose).. but I won't rule out that some passing ship's crew long long ago needed some mineral from deep beneath the crust, so they bombarded the moon and sucked out the juices deep within. Call me a dreamer.

Posted by: ElkGroveDan Nov 5 2005, 03:52 AM

I like the way you think. (this post and Occam's above) Working all day light-years from clear concise reasoning, it's refreshing to get a dose of it now and then.

Posted by: Richard Trigaux Nov 5 2005, 07:34 AM

Even on Earth, not all colonists were just destroying everything. The Mongols simply took control over the colonized countries, destroying them only if they resisted. And they eagerly learned and fostered the local science and traditions, as they had little by themselves. Alexander the Great is another example where the colonist and colonized traditions merged in a beautiful blend, allowing a very original culture to emerge. On the middle, the Romans did not destroyed the countries they conqueered, but they killed their soul (Greeks, Gaelics). On the other extreme the Catholics were perhaps the most merciless destroyers of local spirit and traditions. Modern colonists justified their attitude by bringing progress. In most part it was hypocrisy, as bringing progress can be done without war or colonization, example the Bhutan, a country which remained uncolonized and completelly locked in a Middle Age technology level until 1968, but which is now, from its own direction, at the leading edge of modernization.





Sorry!

Posted by: Richard Trigaux Nov 5 2005, 07:49 AM

There are some criterion however. For instance finding square forms. But, as you very relevantly note with the Yonaguni formation, even this can be natural. The Yonaguni terrasses are really impressive and artificial looking (evoking some very old civilization in the glacial age, which remnants are now under the sea level). But it is easy to check that the surroundings of the Yonaguni "pyramid" all exhibit such square forms, which are a consequence of the natural diaclase network in this rock.

On the other hand the absence of apparent purpose is not an evidence of being natural. Many artificial features on Earth do not have obvious apparent purposes, and they can be understood only with an insight into the culture or people who built them.

One thing sure about the Japetus formation is that it is very old, looking as much baterred with craters than the surrounding surface, telling an age of 4 billion years or more. This does not help to identify what formed it. Even traces or artificial earth work would be erased since, so that only the overal geometry gives some clues, as in my discution with tasp.

Posted by: Richard Trigaux Nov 5 2005, 08:03 AM

All this is interesting. In another discution, I wondered why it is the SMALLEST moons which exhibit geological activity (Enceladus, Miranda). It was replied that it is because, being lighter, they are more likely to be pulled away and thus experience tidal heating. So very small bodies which never melt and keep some fluffy or sandy constitution, may experience a strong tidal braking, even if the heat of this braking is dissipated before producing any melting).

And if you look at Hyperion, it looks very much as if it never melt, and shows a mixture of dark materials and icy materials. (Phoebe exhibits only dark material, perhaps because it is a former larger body, experiencing melting, which formed a dark core surrounded by white icy lawers. But it was later stripped off this icy layer by impacts, perhaps because of its reverse orbit which made impacts much more violent and destructive).

If the japetus ridge was produced by a former moon or ring falling on the ground, the accretion velocity was small, even subsonic, and thus the repeated impacts did not vaporized the falling matter (there was no explosions) so that the falls accumulated matter and formed a ridge rather that an elongated trench of craters (what would happen on Earth in similar conditions).

Posted by: tasp Nov 11 2005, 01:25 AM

{this quote is from Mike, sorry I messed up the post in responding}

I suspect something massive slammed into it, cracking the crust and turning roughly half the moon dark (or light, I suppose).. but I won't rule out that some passing ship's crew long long ago needed some mineral from deep beneath the crust, so they bombarded the moon and sucked out the juices deep within. Call me a dreamer.

[/quote]

Speaking of cracking the crust, the last Iapetus pictures up on the Cassini JPL site (if you turn your monitor brightness way up) show BIG cracks that seem radial to the largest crater in Cassini Regio.

Hard to be sure, the cracks curve around into the opposite hemisphere, there seems to be at least 4 of them and 2 seem dead on radial, and the other two , are close.

Does our little friend have another surprise brewing ?

Posted by: jmknapp Dec 22 2005, 01:12 PM

The bellyband is just crazy steep. Has everyone seen this image given in Tillman Denk's http://saturn.jpl.nasa.gov/multimedia/products/product-presentations.cfm?



Given the recent discoveries at Enceladus, this ridge seems to recall the tiger stripe features there, & raises the possibility that the bellyband may house (or have housed) cryovolcanic vents.

Talking about the mystery of the dark depsosit, Denk said in the January presentation:

http://saturn.jpl.nasa.gov/multimedia/products/docs/CHARM_050125_transcript.doc

So there are some problems with that idea, but of course now that ice volcanoes on Enceladus are confirmed, some of those problems--namely the "main problem" of the energy source--may need to be rethought. If Enceladus can do it, why not Iapetus?

I developed a sim for predicting the fallout from ice plumes on Enceladus & decided to try it on Iapetus, just to see how it comes out. First of all, not that there is any specific reason to do so, just for comparison I tried plume sources coming from the region of Iapetus' south pole (75S-90S), particle trajectory elevations from 75-90 degrees, and particle velocities from 200-600 m/sec (the latter being the approximate escape velocity):



That map is centered on 90W in order to show the Cassini Regio contiguously.

In contrast to the case with Enceladus, the distribution is very symmetric in longitude, with the impact locations trailing off in density northward. A similar Enceladus simulation shows strong asymmetries in longitude. I guess the differences are that Enceladus rotates much more rapidly (something like 60x more rapidly) and also the ratio of gravity of Saturn to that of the parent moon is much greater at Enceladus.

Anyway, of course the distribution of particles given sources along the bellyband would be of main interest. Here's the result of a sim using three locations along the bellyband:



So at the equator too the particle impact distribution from a given vent is symmetric, which would be consistent I guess with a series of vents along the bellyband causing the dark deposits. There aren't many unique features of either the distribution of the particles or of the dark material though that jump out to indicate a correspondence.

Denk also said of the bellyband image above:



Going back to the Enceladus tiger stripe region, we also see very steep ridges:



Note the long shadows cast by some of the ridges (sunlight coming in from the left).

Also triple-ridges of sorts on the stripes:



Knowing that sunlight is coming in from the left above helps to sort out the convex/concave ambiguties.

More from Denk:



So the cryovolcanic activity if any would have stopped long ago.

Posted by: Bill Harris Dec 22 2005, 01:45 PM

Joe--

Since we started discussing plumes and vents along fractures on Enceladus I've given thought to the idea that the BellyBand in Iapetus may have a similar origin. Keep working with it.

--Bill

Posted by: dvandorn Dec 24 2005, 03:55 AM

There are stretches of the Belly Band where the "plateau" at the top of the ridge is crenulated with chains of identically-sized, shoulder-to-shoulder craters. That are exactly the same width as the crest of the ridge.

That *strongly* suggests vents to me.

-the other Doug

Posted by: silylene Dec 26 2005, 06:04 PM

I am reposting this unusual hypothesis I formed (a concept which I consider highly speculative, but I am interested in feedback), and posted 6-12 months ago on the SDC forum. It involves extremely slow motion collisions between large masses...and whether this could cause the Iapetus equitorial ridge. And there is exactly one mechanism I can think of that would cause an extremely slow motion collision between large bodies.

I first suggested the (quite facetious) "tennis ball hypothesis" of Iapetus back when the pictures first got transmitted (in the SDC fora). I noted that the equatorial ridge looked like a seam formed by the merger of two half-spheres. I made the analogy that it looks like the seam on a tennis ball, if you remove the felt covering.

Well, the idea never left me, and I was thinking more about this highly speculative idea and then posted this concept in May 2005 on the SDC fora:

Let's imagine, long ago, there were two equal sized proto-ice-moons, each with a mass one half of Iapetus, sharing a nearly common orbit around Saturn (not each other!). These two proto-ice-moons perhaps had an orbital arrangement much like Janus and Epimetheus, and as the two moons approach each other they exchange momentum and trade orbits with each other. Because of gravitational drag from the other moons, and induced tides from Saturn, this is not a stable system.

So slowly, these ice-moons move closer and closer to each other. As they get real close the two moons start warming each other up from mutual tidal interactions (I wonder what's their Roche Limit in this situation?). The moons finally get warm enough from mutually-induced tidal interactions that they become softer-slushier ice, instead of -200C rock-hard ice.

I now note that the relative velocities of these two moons is very minimal, almost zero.......

Then suppose the two soft-ice-balls slowly close the gap between themselves, and have a slow-motion collision and merge? Imagine two snowballs slowly pushing into each other. And then the moons re-freeze? Could the ridge be the trace of the merger?

And maybe the dark deposits are bits of dark icey debris tossed out by this slow-motion collision, which are then gradually swept back up afterwards....

I am assuming that the two proto-moons were already tidally warmed by their interactions resulting from increasingly close-passage dances around each other. Thus the two moons, at the time they slowly collided were rather soft ice-balls, perhaps as soft as ice is at -20C. If the proto-moons were soft, then their collision would be completely inelastic and absorptive.

I would love to see my hypothesis modelled!! I wonder how slow a pair of co-orbital proto-moons would collide? How much tidal energy would they deposit into each other as they approach? What would a finite-element model for a slow-motion collision predict? Would the merged moon gain some angular momentum and rotate before becoming tidally locked with Saturn? Would the merged moon generate additional tidal warming as it becomes tidally locked, which would help it deform into a spherical shape?

I also notice that the older craters on iapetus are unusually "soft"-edged, compared to craters on other (cold) ice-moons of the outer planets. I think the soft-edged craters are a bit unusual too, and their existance is consistent my hypothesis that the proto-moons had been tidally warmed at some time in their existance. Ice at -200C is as hard as steel and will not become soft. But ice at -20C is rather soft and slowly will deform (such as Terran glaciers exhibit).

++
Back to the collision...

Consider two roughly equal sized proto-Iapetus satellites co-orbiting Saturn (not each other!)....in nearly the same orbital radius from Saturn. In my hypothesis, the two proto-iapetus satellites have an orbital geometries much like Empimetheus and Janus currently have (but at a radius correspnding to about where iapetus is nowdays.)

Tidal interactions between the two proto-iapetus satellites gradually bring them closer and closer each time they both revolve around Saturn. They have nearly the identical orbital velocities. So their delta-V is nearly zero. The tidal interactions just preceeding the merge warm and soften the ice. They get closer and closer until they make a slow-motion inelastic merge. The merge, and heat from the slow-motion collision is enough that the soft iceball slowly collapses to a mostly-spherical satellite, with the ridge remaining as the remnant of this slow-motion collision. (Actually iapetus is an irregular ellipsoid shape)

I think that it's possible for co-orbital moons to have a very low speed collision. For examples of co-orbital moons consider the following: Janus and Epimetheus, S1 and S3, Tethys has 4 co-orbital moonlets, and Dione has 2 co-orbital moonlets.

In these cases, both moons are moving in nearly circular orbits. They are not moving at exactly the same speed. The faster moon slowly gains on the slower moon. The faster moon is faster because it is in a slightly lower orbit. They approach each other very slowly, and are close to each other for a long period of time. The gravity of the trailing moon pulls on the leading moon, and the gravity of the leading moon pulls on the trailing moon. This adds energy to the trailing moon and takes energy from the leading moon. This does not speed up the trailing moon, but instead tugs it into a higher orbit. In this higher orbit, the trailing moon has more energy, and a slower speed. Similarly, the leading moon loses energy and is pulled into a lower, faster orbit.

I do think it is possible that co-orbital moons could conceivably collide someday due to gravtitational interactions with other bodies which bring them slowly closer to each other millenia after millenia. Someday, the passing distance of the two moons becomes closer than the intersections of their surfaces. At this time, The two moons would have an extremely low speed collision, perhaps with a closing velocity of only a few km/hr difference (or less!).

In such a case, the two moons would merge to make a lumpy single body. There would still be substantial heat generated from even an extremely slow speed collision. If the combined lumpy single body is massive enough (and warm), it will gravitationally deform slowly assume a spherical shape.

I propose this mechanism could be the source of the equatorial ridge on Iapetus (the equatorial ridge is the remnant of of an extremely low speed collision of two putative co-orbital moons, which formed the current Iapetus).

Several posts about this hypothesis begin in this thread, around here:
http://uplink.space.com/showthreaded.php?Cat=&Board=sciastro&Number=218501&Search=true&Forum=sciastro&Words=iapetus&Match=Entire%20Phrase&Searchpage=0&Limit=150&Old=1year&Main=118640

I know this hypothesis is "way out there", and I normally don't like tossing out such speculative ideas. The reason I do again is that I still have not seen a good explanation anywhere of the equatorial ridge on Iapetus. The conventional explanations for the ridge seem rather 'forced'; perhaps it becomes time to consider an unconventional explanation?

Posted by: ljk4-1 Dec 26 2005, 06:13 PM

My theory: The two halves are getting ready to open.

Posted by: jmknapp Dec 26 2005, 07:05 PM

Seem like something as momentous as a collision between two bodies of equal size would leave either much more of a trace (not just a little ridge around a mostly spherical moon) or else no trace if the whole shebang just melted and refroze.

How about: there is for whatever reason a tendency for the ice to flow, glacier-like, from the poles towards the equator? This would cause a compression zone at the equator with resulting uplift.

Now, what would cause flow to happen this way? The inclination of Iapetus' orbit is quite high, around 15 degrees, taking Iapetus in its 79-day revolution rather far above and below Saturn's equatorial plane. Iapetus is tidally locked to Saturn, eliminating most tidal effects, but due to Saturn's own equatorial bulge there will be tidal force in the north-south direction as the moon goes through its inclination extremes. So at the extreme high and low excursions from the plane a component of the tidal force will cause a compression north-south and as Iapetus crosses Saturn's equatorial plane that compression would be relaxed. This might cause some heating in addition to a tendency for the ice to flow from the poles to the equator?

Posted by: tasp Dec 26 2005, 07:32 PM

I very much appreciate the 'right side up' view of our fascinating ridge structure. (not to complain, but the flyby geometry of the Cassini pass has driven me crazy)

A few comments on the picture:

The ridge is old, subsequent cratering events have battered it pretty good.

The bilateral symmetry of the central ridge and the 2 'attendant' ridges ( the north and south diverging 'on ramps') is absolutely amazing. Any plausible explanation of this feature must account for these two ancillary structures.

The segments of the diverging ridge peak that remain unblemished by craters, are identical to high degree.

Internal geological processes of earth do not generate structures like that. We do not see volcanoes with symetrically placed secondary cones. Faulting processes do not generate patterns like that, either. We don't see geysers or mud pots generating geometrical patterns in this type of arrangement. Tesserated tundral patterns do not look like that, and would not scale to 90 degrees of longitude on an object ~1450 km in diameter.

We conveniently have (not pictured here) an enormous basin excavated off the east end of this stucture (kilometers deep!) and there is not a trace of the 'roots' of this structure visible in the photos taken to date at that location.

The sides of the ridge would seem to be approximately at the angle of repose for icy materials in the local gravitational field present. (help me out here, does anyone have experimental data on this subject?)

The local elevation of the ridge structure seems to smoothly and regularly decline from the 'over the horizon' segment down across your oriented photo. This is another complication for internal processes to generate.

(the Hawaiian volconic structure on earth is 'tapered' northwest to southeast due to erosional effects of the Pacific ocean, not sure of an analogy of this process that could operate on Iapetus)

To me, the picture clearly shows a primary stucture, dead straight and so closely equatorial aligned as to have suggested ET involvement to some folks (not me).

Additionally, perfectly symmetrical (less crater damage) secondary diverging stuctures trace away from the primary ridge north and south. An explanation of this detail of the ridge structure would seem to be insurmountable for any plausible internal geo process creating it. How would an internal geo process 'know' to create a symmetrical feature on this scale? The diverging ridges 'peter' out symmetrically (and short of) the 'petering' out of the central ridge, yet the 'petering' out of all 3 ridges look alike (less subsequent crater damage). Also, the diverging angle of the north ridge to the central ridge as compared to the diverging angle of the south ridge to the central ridge is absolutely identical, and also quite small, like 4 degrees or so.

How does an internal geo process do that on a structure so large?

Posted by: David Dec 26 2005, 08:18 PM

Is there maybe a hinge on the opposite side of Iapetus?

Posted by: nprev Dec 26 2005, 08:50 PM

Oy, be careful: you might have accidentally written the headline for next week's National Enquirer!!

Seriously, though, I think that the Bellyband almost has to be an uplift feature that presumably had some eruptive activity a long, long time ago. I almost wonder if Iapetus is so homogeneous internally that when the moon began its final cool-down the crust fractured right at the equator (which may have been the thinnest point due to the effects of rotation on internal convection) and exuded material as the mantle froze & all that ice expanded...

Posted by: Bill Harris Dec 26 2005, 09:33 PM

The Iapetus bellyband is an enigma.

Not trying to promote tunnel-vision, but this was my first impression of it.

--Bill

Posted by: JRehling Dec 26 2005, 09:51 PM

Thoughtful posts there.

The Iapetus mystery comprises two "big" items: the dark patch comprising Cassini Regio and the equatorial ridge. But smaller clues have to be satisfied too. The white mountains on the western end of Cassini Regio are collinear with the ridge, but are individual peaks instead of a ridge, and white, not dark. The Snowman craters are on the eastern end of Cassini Regio and seem to show dark stuff that was *not* emplaced from inside CR (eg, the ridge) outwards -- but those saturnshine images may be hiding the truth on that.

Additionally, it would be truly puzzling if distant Iapetus had a hot history -- could we reject both radiogenic and tidal sources and suppose that some giant impacts were at work?

Dione and Tethys also have dark patches -- are those endogenous stains of the same kind?

Posted by: jmknapp Dec 28 2005, 12:54 PM

Questions about Iapetus' orbit:

The inclination is about 15 degrees. Wouldn't tidal forces tend to bring Iapetus into Saturn's equatorial plane eventually? If so, may we postulate that the inclination was higher in the past and is presently decreasing?

Posted by: ugordan Dec 28 2005, 01:37 PM

Alternatively, could it be possible that Iapetus' orbit remained more or less as inclined as it is today, only it kept precessing around Saturn due to Sun's influence combined with the distance from Saturn?

Posted by: jmknapp Dec 28 2005, 07:05 PM

Oh yeah... good point. Being so far from Saturn maybe the sun's effect can keep things off kilter.

Posted by: JRehling Dec 29 2005, 04:49 PM

Did the ridge grow from the bottom pushing up, or by growing on the top?!? (FWIW: Grass grows the first way; palm trees grow the second way.)

If this was a fault that spewed accumulating layers of dark stuff extending out roughly a worldwide radius, then you would expect the most dark stuff to accumulate AT the fault. Perhaps there was never any uplift at all, just a continuous outflow until a global hemisphere had been dirtied, with the layer caked very thickly at the line of origin. You could imagine something like this if a garden hose had leaks evenly spaced along its length and it were left to run as the temperatures dropped. Eventually, the hose would be caked thickly in ice, whereas a light glazing would extend a meter or so in all directions.

The problems with this include:

1) The eastern end of CR is very round, whereas the western end is sort of pointy. If spewing from a linear fault created CR, there was some gross un-uniformity that led to the right end being almost a compass-trace out of some point in eastern CR, but a very different kind of termination in the west. That's not impossible or even implausible, but it means we need an ad hoc argument just to explain the raw shape of CR.

2) Dark stuff! Even if an icy moon did have volcanic eruptions from within, and even if they were tainted with ocean floor gunk, darker than the pure surface ice (certainly true on Europa)... can anyone explain why it would be *black*? That means that somewhere down there was stuff with just about no ice. Or somehow ice + gunk erupted, and the ice dissipated from the plume straight to deep space while the black stuff snowed down. The composition question has to be answered for any endogenous explanation. Perhaps a wiser soul than I can answer this straight off, but it seems odd to me.

3) White mountains in the west. Did this volcanic fault "finish" spewing black stuff and end up erupting out some modest-sized white icy volcanoes?

4) The Snowman craters. One way or another, dark stuff ended up dirtying crater rims outside CR, and it's to say the least entirely implausible that it would happen to snow down just onto crater rims. A huge question here is whether we are seeing the heights of Snowman 1 darkened from the west (thus, outside the western rim; inside the eastern rim) or from the inside (inside both rims); or from the east (inside the western rim; outside the eastern rim). Or was this a place where dark stuff was so thick that it ended up being deposited around by an impact that postdated an endogenous deposition?

5) Why would Iapetus have such a lively past? Particularly, why would the "life" be localized to one fault? The lumpy shape of this world -- *unprecedented* for a world of its size -- is suggestive of a world with no inner heat source. The distant orbit makes it the *least* likely suspect for tidal heating, and the size would not suggest radiogenic fireballs within.

I'm still backing an exogenous origin, but the ridge is a puzzler. Isn't this the kind of thing they figure out on CSI?

Posted by: Rob Pinnegar Dec 30 2005, 05:35 PM

<Grin> Yeah, except CSI is usually a lot more gory.

This may not be a very helpful observation, but all the speculation about Iapetus (including my own) sort of reminds me of the intense over-analysis of the Moon prior to the Apollo missions: way too many ideas, and just not enough evidence to hit the nail on the head in most cases. All it took was one piece of basalt from Tranquillity Base to burn down everything the cold-mooners had built up, for example.

I sure hope that we're not going to need a Cassini Regio sample return to clear the air concerning Iapetus. That could be a very long wait. Till then, though, here's hoping that the 2007 flyby does the trick.

Posted by: JRehling Dec 31 2005, 01:27 AM

I think high resolution imagery of the entire perimeter of Cassini Regio would do the trick. Every place where we see the relationship between the CR boundary and local topography is an "arrow" pointing to the source of the dark material. Given all of the arrows (which will presumably point to a geometrical point, a line, or an area), I think the mystery will be essentially solved. Right now, if CR were an (oblong) clock, we only have that information in high resolution for the 10 o'clock to 2 o'clock positions, and all we see is that the dark stuff on the northern border came from the south. We have seen the eastern boundary (3 o'clock) in saturnshine, which shows us the boundary, but leaves us guessing the topography. We need about 4x the coverage we have had, and unfortunately, the next couple of flybys simply repeat the same old ground.

A sample might not tell us as much as the boundary definition. That's either a good thing or sour grapes, because we're not going to GET a sample! Although an Icepick-style sample return would be possible, and would be at relatively slow velocity with the craft at aphelion out at 9.5 AU, overtaking Iapetus's farside.

Posted by: Steve G Dec 31 2005, 02:19 AM

Keep in mind that Iapetus is the most distant (large) icy world we've studied so far. All the others would have been influenced by Saturn's protoplanet heat. This is far outside so the ingredients making up Iapetus wouldn't be influenced by any heat sourse that may have been present while Saturn was forming. So it's interior may contain some neat goop that the others had stripped off.

Posted by: Rob Pinnegar Jan 3 2006, 04:35 AM

This is true... and it don't matter if it's exogenous or internal, do it?

Hey -- let's suppose, for the moment, that we can tell whether it's exogenous or internal. I wonder if the shape, topography, and albedo variations, of the perimeter and near-perimeter parts of Cassini Regio, could be used to construct an inverse solution for the source distribution? (I'm assuming here that the central part of CR is too dark to be useful as far as albedo is concerned.)

It'd probably be a pretty rough solution that would necessarily require a *lot* of assumptions, but it'd be worth a shot.

Posted by: ljk4-1 Jan 3 2006, 02:25 PM

My two WAG theories:

1. The moon shrunk when it cooled down and compressed to form the globe-encircling ridge.

2. A really big impact (or two) just as the moon was cooling left a shockwave that circled the moon and met from opposite sides that then froze in place - the ridge.

As a semi-example, Callisto has an impact crater named Valhalla that seems to have had its impact waves frozen in place. Not globe-circling, mind you, but somehow similar. And Callisto is the furthest of all the Galilean moons, located outside the Jovian radiation belts.

http://www.solarviews.com/eng/callisto.htm

Posted by: ljk4-1 Jan 3 2006, 05:04 PM

Dark Terrain on Saturn's Iapetus

Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

Explanation: Why are vast sections of Iapetus as dark as coal? No one knows for sure. Iapetus, the third largest moon of Saturn, was inspected again as the Saturn-orbiting robot Cassini spacecraft swooped past the enigmatic world again late last year. The dark material covers most of the surface visible in the above image, while the small portion near the top that appears almost white is of a color and reflectance more typical of Saturn's other moons. The unknown material covers about half of the 1,500 kilometer wide moon. The material is so dark that it reflects less than five percent of incident sunlight, yet overlays craters indicating that it was spread after the craters were formed. Iapetus has other unexplained features. The bright part of Iapetus is covered with unexplained long thin streaks.

The orbit of Iapetus is also unusual, being tilted to the plane of Saturn's orbit by an unusually high fifteen degrees. A strange ridge about 13 kilometers high crosses much of Iapetus near the equator and is visible near the bottom. Oddly, this ridge is almost exactly parallel with Iapetus' equator. The exact shape of Iapetus remains undetermined, but images indicate that it is quite strange -- something like a walnut. Research into the formation and history of mysterious Iapetus is active and ongoing.

http://antwrp.gsfc.nasa.gov/apod/ap060103.html

Posted by: alan Jan 4 2006, 05:09 AM

Large craters often have a central peak. Would a large oblique impact leave an elliptical crater with a central ridge?

Posted by: Steve G Jan 4 2006, 07:20 AM

Hopefully the extended mission will focus on the icy satellites and radar mapping of Titan. Assume we are near the end of the road for Cassini, rather than plunging it into Saturn’s atmosphere like Galileo, what would it take in terms of Titan gravity assists and fuel to put it in orbit around Iapetus?

Posted by: ermar Jan 4 2006, 07:34 AM

It would take a bit too much. See http://www.unmannedspaceflight.com/index.php?showtopic=1855&st=60&p=33747&#entry33747 on this.

Posted by: alan Jan 5 2006, 07:54 PM

How about this for an explanation of the ridge. A recent abstract claimed Iapetus' shape implies that it "frooze" when its rotation period was 17 hours. What if it formed with a much slower rotation than this and was spun up by a collision. This would change its shape from nearly spherical to what we have now. As it was compressed from the poles the crust could buckle along the equator.

Posted by: BruceMoomaw Jan 5 2006, 11:53 PM

I'll have to review this; but as I understand it the favored theory is that it did finish accreting while it was still spinning rapidly, and as a result was relatively "soft" and got stretched into its current shape by centrifugal force (including that ridge, which was produced because the outer crust solidified earlier and then got dragged along by the underlying still-plastic mantle material, so that its northern and southern sectors collided and buckled upwards to produce the ridge).

Posted by: Decepticon Jan 6 2006, 01:16 AM

That would only make sense if the ridge extend evenly around the whole planet right?

Posted by: tasp Jan 6 2006, 02:12 AM

Not explaining the symetrical (less subsequent cratering damage) diverging attendant ridges will be the downfall of many hypothesis.

Posted by: jmknapp Jan 6 2006, 03:01 PM

Wouldn't this image argue for an endogenous origin for the dark dust?



The big impact basin presumably predates the deposition of dark material. If dust was coming in externally, how would it "know" to deposit on one side of the ejecta blanket boundary and not the other?

Here's a detail of the part of the boundary I'm referring to:



The sun is coming in from the left, so evidently the ejecta blanket is a raised surface at that point.

Posted by: TritonAntares Jan 8 2006, 09:01 PM

http://www.unmannedspaceflight.com/index.php?showtopic=1978&st=0&p=35217&#