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_*]

It's a very real possibility -- Triton is supposed to have scrambled the hell out of Neptune's original population of moons when it arrived -- but, as you say, it's probably unverifiable.

[Guest_Richard Trigaux_*]

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. (snip)

An alternative is indicated by a recent discovery of a star with TWO accretion disk turning in opposite site. If such a discovery is confirmed, accretions disks would be highly homogenous, and depend on the clouds which formed them. So it is not impossible that Triton formed around Neptune in its position (which may have evolved since). But, argument for you, there is also the position of Pluto, which intersects Neptune's orbit. It was often said that Pluto could be an ejected Neptune Moon, one of the disappeared moon you seek. But if so, it may not have satellites...

[JRehling]

Possibly isotopic analysis could reveal a match between a KBO and the existing Neptunian satellites. Don't see an in situ analysis happening soon, though!

Besides ejection, there are two other possible fates for the lost moons, though: Dropping into Neptune or smashing into Triton itself. I think the former is more likely, and would truly leave no fingerprints behind.

[Bob Shaw]

I think the former is more likely, and would truly leave no fingerprints behind.

Hmmm... ...depending on whether the impacting objects formed in the same area of the Solar nebula as Neptune, there might be an unexpected variation in the makeup of Neptune. I wonder if there are any 'blips' in the composition of Neptune which have been identified so far?

Bob Shaw

[Rob Pinnegar]

I wonder if there are any 'blips' in the composition of Neptune which have been identified so far?

Neptune probably gobbled up thousands of "Varunas" back in the old days... it's tough to imagine that any particular one could affect its composition in an identifiable way.

I guess that one signature sign of an ex-Neptunian moon would be evidence of unusually strong internal activity for a body that size, due of course to tidal heating, both long-term (satellite-satellite resonances) and short-term catastrophic (Tritonian trebuchet). Maybe someday.

[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.

[ljk4-1]

Funny you should mention Neptunian moons...


Paper: astro-ph/0604552

Date: Wed, 26 Apr 2006 20:27:17 GMT (73kb)

Title: A Survey for "Normal" Irregular Satellites Around Neptune: Limits to
Completeness

Authors: Scott S. Sheppard (Carnegie Inst. of Wash.), David C. Jewitt and Jan
Kleyna

Comments: 10 pages, 9 figures (6 color), 3 tables; Accepted for the July 2006
Astronomical Journal
\\
We surveyed 1.75 square degrees of sky near Neptune to an R-band 50%
detection efficiency of 25.8 mags (corresponding to radii of about 17 km for an
assumed albedo of 0.04). We discovered one new outer satellite, Psamathe
(S/2003 N1), about 20 km in radius and having a distant retrograde orbit and
moderate eccentricity. Until 2003 Neptune was only known to have two satellites
which exhibited orbital signatures indicative of capture. Both of these, Triton
and Nereid, are unusual when compared to the irregular satellites of other
giant planets. With recent discoveries of four additional satellites by Holman
et al. (2004) it is now apparent that Neptune has a distant ``normal''
irregular satellite system in which the satellites have radii and orbital
properties similar to those of the satellites of other giant planets. We find
that the satellite size distribution at Neptune is not well determined given
the few objects known to date, being especially sensitive to the inclusion of
Triton and Nereid in the sample. Finally, we note that Psamathe and S/2002 N4
have similar semi-major axes, inclinations and eccentricities. They may be
fragments of a once larger satellite.

\\ ( http://arXiv.org/abs/astro-ph/0604552 , 73kb)

[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...")