[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.

[Bill Harris]

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

[silylene]

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?C...ear&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?

[ljk4-1]

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

[David]

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

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

[nprev]

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

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...

[JRehling]

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?

[jmknapp]

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?

[ugordan]

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?