First time post from a long time lurker!
I am interested in what (if any) consensus there is among planetary scientists on the origin of Iapetus's current orbital inclination, or if there is no consensus, what plausible theories are out there. Although there has been much speculation on the origin of the various strange surface geologic features of Iapetus, there seem to be very few detailed theories on how it arrived in the ORBIT that it currently follows. (If this has been hashed out before on UMSF, you can just post a link to the relavent thread)
What needs to be explained:
The 15.47 degree inclination to the plane of the rest of the Saturnian system, which is by far the highest inclination of any regular satellite in the Solar System. It seems impossible that Iapetus could have accreted in an orbit with this inclination.
The unusually large distance from Saturn compared to the rest of the major satellites. The orbit of Iapetus has a semimajor axis nearly three times that of Titan's, and nearly 2.5 times that of Hyperion, and there are no known intermediate bodies (of any size) in the vast space between it and the next nearest neighbor. Again, this is not what we would expect for any normal accretion model. (this kind of gap is also not seen in the Jovian or Uranian systems, which feature more regularly spaced satellite orbits)
An orbit that is both prograde and has a low eccentricity, neither of which seem plausible in any likely capture scenario that could explain the orbital inclination. (such as Phoebe, or Triton & Nereid at Neptune)
The lack of any known current or past gravitational resonance with any extant major satellite, which might be used to explain its current inclination. (For example, Miranda at Uranus has a notable 4.2 degree inclination, but this is very likely due to a past period of 3:1 resonant forcing with Umbriel)
In short, Iapetus seems stranded in an orbit that it has no right to be in. How did it arrive in this orbit??
It has been proposed (via a link in an old post here at UMSF which I can't find at the moment) that faint long range gravitational interaction with Jupiter can explain Iapetus's inclination, probably helped by Saturn's relatively weak gravitional control at Iapetan distances. Has this been modeled in any detail?? At what distance from Saturn do Jovian gravitational effects become strong enough to influence sat. orbital inclination?
Another proposal by Wing Ip
(which was linked to in this thread
and seeks mainly to explain the equatorial ridge) suggests that a giant impact with Iapetus may have knocked it into its current inclination, and possibly formed a ring which later gave rise to said ridge. It seems to me that any impact energetic enough to change the orbital inclination by 15.5 degrees should totally disrupt the satellite and disperse the debris too widely to re-accrete.
I am asking this because I have been thinking over a little theory on how Iapetus ended up where it is today.........and it's one which I've never seen any one else propose. Just wanted to see what alternative theories are out there before making a fool of myself by posting mine.
-David Van Avery