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?

[nprev]

No easy answers there, Rob. However, it's pretty hard not to strongly suspect that Uranus had a fairly wild history with that bizarre axial tilt. I've never really believed that its satellite system is OEM for that reason alone, really; perhaps they & the rings are leftovers from a few larger satellites that bashed together after the disruption?

Neptune's systemic anomalies are comparatively mild, and as you observed, the apparent capture of Triton looks like the logical explanation.

Iapetus...I don't know. It could be a disrupted artifact, but it sure acts a lot like a capture. Compositional comparisons with the other icy moons should prove most interesting someday; might be the only way to tell for sure.

Here are two extreme alternative thoughts:

1. Uranus & Neptune strongly interacted with each other back in the day, not with Saturn, and managed to get themselves ejected out to their present neighborhood through some favorable resonances with Jupiter & to a lesser degree Saturn.

2. One or both of them had a VERY close brush with Saturn, incidentally shattering its satellite system as well (with the possible exception of Titan) to form the rings & the icy moons. The interloper--Uranus, let's say, for argument's sake--continued its streak of bad luck by interacting with Neptune, and again resonances eventually pulled them out to the boondocks, where they settled down, raised new kids, and sinned no more.

Paleo-orbital dynamics is fascinating, but I'm afraid much of it is gonna be pure blue-sky until truly exhaustive comparative composition studies are done. It'll be awhile.