[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.