Nice model, Ice giants vs. Saturn Options

[Rob Pinnegar]

I've been reading a bit about the Nice model recently, and was wondering about a few things.

In a nutshell, this model hypothesizes that Uranus and Neptune started out closer to the Sun than they are today, and then got propelled into their current, more distant orbits by Jupiter and Saturn. There seems to be some speculation that this "propelling" may have involved a couple of relatively close encounters between Saturn, and at least one of the ice giants.

Presumably, these encounters couldn't have been *too* close. A really close encounter would have severely disrupted Saturn's satellite system, and would likely have put the intruding ice giant into a Jupiter-crossing orbit. So a more distant encounter is probably what we are looking for -- something that could have "gently" propelled the ice giant outwards into the proto-Kuiper Belt.

It would also be nice to have some sort of evidence that an encounter of this type had actually happened. This got me thinking about Iapetus's large orbital inclination, which as far as I know has never really been explained. Could Uranus or Neptune have been responsible for that?

Suppose that the culprit was Uranus. The situation becomes more complicated because we have to consider Saturn's effect on the orbits of Uranus' regular satellites, particularly Titania and Oberon. Saturn's gravity would have caused their orbits to become more elliptical. After the encounter, this ellipticity would have been damped over time by tidal forces -- but only if the moons didn't crash into each other in the short term. That might put a lower limit on how close the encounter could have been. (Also, I suppose we also can't have an induced ellipticity large enough to lead to cryovolcanism on Oberon during the damping phase.)

If the perturbing body was Neptune, in some sense things are simpler because one could argue that Neptune lost most of its satellite system at that point, with Triton arriving later only to discover that there was nobody home. However, I don't much care for this, because it substitutes a more complicated explanation for the loss of Neptune's original satellites in place of a relatively simple explanation that already has a very visible prime suspect.

Any comments?

[alan]

Iapetus orbits far enough from Saturn that it is in the transition zone where the Laplace plane is between the planets equatorial plane and the plane of its solar orbit.

Near the planet the Laplace plane is close to the planets equatorial plane, farther away (where the irregular satellites such as Phoebe are located) the Laplace place is close to the plane of the solar orbit.

According to Wikipedia Iapetus's orbit is inclined 7.52° relative to the Laplace plane, 17.28° relative to the ecliptic and 15.47° relative to Saturn's equator.


On a somewhat related note the dust ring associated with Phoebe, which in illustrations is shown with a large inclination (27°) has a much smaller inclination relative to the ecliptic making it appear much flatter in the actual images.

[Rob Pinnegar]

Thanks for the link about the Laplace plane, Alan.

Surprised I hadn't run across that one before. As I said earlier, pretty much every reference I've seen to Iapetus' orbital plane has described its inclination to Saturn's equator as an unsolved problem. Guess I've been looking at the wrong references, eh?