In a post further down the page Bruce Moomaw made the following comment:

>It's considered just about certain at this point that Triton is indeed a
>captured Kuiper Belt object -- but the trouble is that its capture has very
>dramatically modifed it. It almost certainly started out in a very elliptical
>orbit around Neptune, which was gradually circularized by tidal forces --
>and the tidal heating of Triton during this period was more than adequate
>to completely melt all its ices, turning it into a sort of giant teardrop
>with a rocky core, and totally changing its initial physical and chemical
>makeup. It is certainly different in major ways from Pluto, both in surface
>appearance and in surface and atmospheric chemical makeup -- and so, if we
>want to get a look at a big, relatively unmodified Kuiper object, we still
>have to go to Pluto.

Bruce's comments brings up some very interesting points. Some physical data
concerning Neptune's two major moons:

Triton: radius 1900 km, orbital eccentricity <0.01, [retrograde orbit]
Nereid: radius 150 km, orbital eccentricity 0.7483, [posigrade orbit]

If an orbit has an eccentricity of zero then it is a circle. If the
"orbit" is 1.0 then it is a parabola and on an escape trajectory. The
Galilean moons of Jupiter are typical large moons with posigrade orbits
having eccentricities around 0.005 [almost perfect circles]. Our Moon
has an average eccentricity of 0.0549 . However the Earth-Moon system is
a bit odd because it is actually a "double planet" (primary world and
moon have similar masses) like the Pluto-Chiron system. The Moon's radius
is 1738 km which makes it a bit smaller than Triton. Triton is unique in
the Solar System as the only large moon in a retrograde orbit. There
are several tiny moons (big rocks really) orbiting Jupiter in retrograde
orbits. Triton with its retrograde orbit along with Nereid with an
eccentricity of 0.7483 is clear evidence that something cataclysmic happened
in the Neptune system. We know from Voyager data and direct observation
that Triton has a tenuous nitrogen atmosphere with frozen volatiles on its
surface. These facts have led many people (including me) to believe
that Triton was a Kuiper Belt Object that was captured into a Neptune orbit.
It is not possible for an object to travel in from infinity and get
captured into orbit without collision (the object would simply leave
the Neptune system along a hyperbolic trajectory). To achieve capture, the
object must either collide with something already in orbit about the primary
planet (another moon or ring) or have a glancing impact with the primary.
Our Moon was captured by the Earth when a Mars sized object broke up near
the Earth due to a glancing impact. Neptune is a very large gas giant.
A glancing impact with Neptune by a Kuiper Belt Object would probably
result in the object either being injested by Neptune's atmosphere or
simply breaking up into smaller objects that then exited the system.
However if the object struck (or had a close encounter with) an existing
moon, then the object and moon would both breakup into fragments. Some
of the fragments might form moons in retrograde or highly eccentric
orbits while other fragments might leave the Neptune system altogether.
The Pluto-Chiron double planet has a very eccentric heliocentric orbit
of 0.25 (Earth's eccentricity is 0.0167). Pluto's orbit is well out of
the plane of the ecliptic with an inclination of 17.1 deg. (the other
planets except Mercury are less than 3.4 deg. inclination). Pluto's
orbit actually passes within the orbit of Neptune. Of course the notion
that Pluto was ejected from the Neptune system is an old one. Many experts
dispute the theory that Pluto came from the Neptune system because the
orbits of Neptune and Pluto are resonant (the two worlds never occupy the
same location). However it is my opinion this counter argument fails
to take into account that the initial orbit of Pluto would be susceptible
to gravity perturbations from the gas giants. These perturbations could
over billions of years reshape the initial orbit of Pluto into its current
stable resonant orbit with Neptune. Therefore by this counter argument
I assert that Pluto, Chiron, Triton and possibly Nereid all resulted from
a single Kuiper Belt Object that collided with one of Neptune's original
moons (which is now incorporated within one of these resultant bodies). I
believe the elemental abundances and isotope ratios of Pluto, Chiron and
Triton will prove to be very similar. I also believe the atmospheres of
Pluto, Chiron and Triton will prove to be similar when allowances are made
for their different gravitational fields and average distances from the Sun.
Bruce commented that tidal heating will have significantly changed Triton
from its inital state as a Kuiper Belt Object. I suspect that the effects
of tidal heating were insignificant when compared to the heating due
to the initial impact event that formed Triton. Since all three objects
(Triton, Pluto and Chiron) would have been created by the same event, I
believe they will prove to be very similar.

Gary