Lost moons of Neptune, Still out there? Options

[Rob Pinnegar]

Here's something I've been wondering about for a while:

The general feeling regarding the current Neptunian system seems to be that Triton is a captured Kuiper Belt object. When Triton was captured, any previously existing Neptunian satellite system would have been severely disrupted, which is presumably why we don't see many other moons there now. Only Proteus seems to have survived the catastrophe.

If we assume that Proteus is the "Miranda" of the original Neptunian system, then (using the Uranian system as a model) Neptune is probably missing about four major moons right now. It is of course possible that some of them could have collided with Neptune, or with Triton. However, given Triton's large mass, it's also likely that some would have been ejected into heliocentric orbits. (In fact, this could have helped slow Triton down a bit during the capture process. Transfer of energy and all that.)

Any ejected moons would most likely have made close passes by Neptune within the first few million years after being tossed, which presumably would have caused their orbits to evolve pretty rapidly. I don't claim to understand the subject well enough to predict what would have happened then, but let's suppose for the sake of argument that some mechanism could have gotten them into the Kuiper Belt -- perhaps multiple approaches with KBOs, or something along those lines.

It seems very unlikely, but I wonder if any of those old Neptunian moons are still kicking around in the outer Solar System? At Uranus, Titania and Oberon are both over 1500 kilometres across, and Ariel and Umbriel are over 1100 as well. If Neptune's original satellites were of similar size, any survivors in the Kuiper Belt could rank among the largest objects there. Wouldn't it be a hoot if Ixion, Varuna or Quaoar turned out to be an ex-Neptunian moon?

Intuitively, I think this probably isn't very likely. Neptune's original companions probably ended up as super-sized Centaurs, and got chucked out of the Solar System by Jupiter and Saturn. And really, this hypothesis isn't particularly useful because it won't be falsifiable in any of our lifetimes. But it's neat to think about, anyways.

[Guest_BruceMoomaw_*]

I have extemely fuzzy memories of some paper done some time in the last few years on how likely these various fates for Neptune's original moons were after Triton arrived (crashing into Neptune, crashing into Triton, or escaping from Neptune altogether). Note that something must have slowed Triton down into orbit around Neptune; and gas drag has always been considered a less likely explanation for this than Triton either crashing into another moon or doing a close do-si-do around it that slowed Triton down into Neptune orbit at the same time that it accelerated the other moon enough to fling it completely away from Neptune. In either case, the other moon must have been pretty big -- but it's doubtful that it was Pluto, because it's hard to see how Pluto could have then gotten into its current 3:2 orbital resonance that keeps it from ever getting anywhere near Neptune.

[Rob Pinnegar]

[Edit: This one's a response to Bruce's message above; Ljk4-1 and I put up nearly simultaneous posts.]

There's also frictional dissipation, caused by distortion of Triton's shape during capture. That energy has to come from somewhere, and Triton's kinetic energy would be a good source.

However, intuitively, my guess is that this couldn't be the main energy sink, because Triton would probably have had to pass really close to Neptune -- possibly well within the Roche limit -- for enough energy to be expended to slow it into Neptunian orbit. It's tough to imagine that Proteus could have survived Triton's next few close passes by Neptune. (Proteus' continued existence also is likely a problem for the aerobraking hypothesis, I suppose.)

I guess that if Neptune once had several "Titanias", and if Triton came in roughly in Neptune's equatorial plane, the chances of one of them interacting with Triton would be pretty good.

Explaining Triton's origin is going to be a tough puzzle because, out of all the thousands of KBOs that Neptune encountered in the early days of the Solar System, Triton is the only one that survived -- and this means that it could very well have survived due to some unbelievably unlikely set of circumstances or coincidences that didn't save all the others.

Occam's Razor might not help us out very much here. We might have to go more with Conan Doyle-type reasoning -- eliminate the believable theories, then work on the unbelievable ones. This is not as impossible as might be thought. Look at the history of theories of the origin of Earth's Moon; same deal. Coaccretion, capture, fission -- they're all simple, but they don't work. Collisional ejection does.

Anyways, I'm veering out of the subject matter of my own thread here. ("But I digress...")