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Nice model, Ice giants vs. Saturn
Rob Pinnegar
post Oct 23 2009, 11:48 PM
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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?
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nprev
post Oct 24 2009, 12:12 AM
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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. sad.gif


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Juramike
post Oct 24 2009, 12:55 AM
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QUOTE (nprev @ Oct 23 2009, 08:12 PM) *
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.


...And until really good models of how atmospheres and the holding bodies would have evolved over time given a set of initial conditions.

If most the solar primordial gas got swept up and the atmosphere stabilized, then the atmospheric constituents would have been locked in based on starting location.

But that's assuming that all the chaos that happened later didn't add stuff to the mix. How much atmospheric component got added by later comet impacts?
(Those gases would be from the "new location".)

Methinks sniffing isotope ratios will help tell the tale. (Would we dare call it a Uranal probe?)





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Greg Hullender
post Oct 24 2009, 04:25 AM
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Note also that all four gas-giant planets are sufficiently oblate to force their inner satellites into equatorial orbits on fairly short timescales. Without running the numbers, I'd guess Iapetus is far enough out that this effect is too small.
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Hungry4info
post Oct 24 2009, 06:42 AM
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Nitpick: Two gas giants and two ice giants.


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brellis
post Oct 24 2009, 07:09 AM
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A better understanding of the atmospheric makeup of the four outer planets would provide some clues, n'est-ce pas? We gotta send an Armada of Cassini-type crafts out there, with dozens of tiny probes to drop smile.gif
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alan
post Oct 24 2009, 03:02 PM
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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.
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Rob Pinnegar
post Oct 27 2009, 03:29 AM
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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?
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alan
post Oct 27 2009, 04:46 AM
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That still leaves the 7 degree inclination relative to the Laplace plane. I recall reading that Saturn's inclination is related to some resonance with Neptune. Iapetus's inclination could be caused by something similar.

Of course it may be related to a close pass by one of the ice giants, as you mentioned in your original post. The Nice model does offer an explanation for the irregular satellites

I've recently seen an abstract claiming encounters between Jupiter and the ice giants may be necessary to cause Jupiter's orbit to migrate fast enough to avoid excessively depleting the outer asteroid belt.
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Hungry4info
post Oct 27 2009, 02:55 PM
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I would offer that close planetary encounters wouldn't be needed. Perhaps as Jupiter migrated outward, it caught Saturn in a 2:1 resonance, so their orbits co-evolved. This ramped up against the orbits of Uranus and Neptune, and those orbits were pushed outward as well, sort of like how Ganymede is interacting with Callisto to raise it's orbit. Then I'm not sure how Uranus and Neptune would have been able to get out of resonance.

Sort of like what is described in this paper.
http://arxiv.org/abs/0910.1004


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Rob Pinnegar
post Oct 27 2009, 07:10 PM
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It's probably worth keeping in mind here that the proto-Kuiper Belt is thought to have had something like 35 Earth masses just before Uranus and Neptune's migrations started. That's roughly equal to their combined masses.

So Uranus and Neptune would have encountered, and interacted with, a very large number of planetesimals over a short time on their way out. Their orbits would have evolved rapidly, perhaps to the point where further encounters with Saturn were no longer possible.
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