[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.

[mike]

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.

[tasp]

{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 ?

This post has been edited by tasp: Nov 11 2005, 01:28 AM

[jmknapp]

The bellyband is just crazy steep. Has everyone seen this image given in Tillman Denk's 1/25/05 presentation?



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:

Then, another point was the endogenic origin.  I mentioned that the Cassini came up with this idea again, when we first saw the pictures then the idea came up that the big ridge might be the original source for the dark material.  This sounds very weird, and it's probably weird, but the elongated shape of the Cassini Regio is arguing for this, as are the streaks in the transition zone.  So this ballistic emplacement is not necessarily from outer space, but might also be something that was coming from the center of the region itself.  The big strength of this would be that we might combine two unique features to one origin.

However, there are several severe arguments against this.  The ridge is probably very old; it is highly cratered.  Then the ridge is not exactly symmetric.  The Cassini Regio ends somewhere near 50 degrees west, so the question is why there more of the dark terrain east of this?  Then Iapetus and the rich interior are probably bright, so the icy material is probably inside, and the dark is only a layer.  The question is why should material erupt and the bright component should disappear very quickly, while the dark component only would return back to the surface?

Then, we also have the bright mountains on the anti-Saturn side, which are a problem because we have the dark mountains at Cassini Regio, but then there are some mountains that are absolutely bright.  This is also not consistent with something of a ridge that is spewing out dark material.  The main obstacle for this of course, is the energy source, because Iapetus is believed to be very cold and very dead in its interior.  It's very unclear why material should come out of the surface.

link

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:

Into the lower left, you might also see some double or even triple ridges.  It's not very clear if these are really triple ridges, but it looks like that.

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:

When you go to the next picture, this is a close up of the bellyband from two different views from the first request of our eight mosaics, and from the second mosaic.  I think you can see very nicely how steep the mountains are, so it's up to 20% or even 30% in steepness in some cases.  You can also see that these are cratered very much.  This is indicative that the ridges are indeed a very old structure on the surface of Iapetus.  I wonder sometimes why it was not destroyed completely by any bigger impacts.

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