500 Encyclopedia of the Solar System
thicker and slightly warmer. Strong winds aloft are also indi-
cated. Surface-atmosphere interactions on the polar caps of
Mars are also being studied in great detail, and the lessons
learned can be applied to Triton. And of course, continued
monitoring of Pluto provides a valuable second case against
which to test theoretical models.
8. Origin and Evolution
Triton and Pluto turn out to be remarkably similar in size,
density, and in surface and atmospheric compositions as
well. There is little doubt that they share a common her-
itage. Moreover, they are not isolated in the outer solar sys-
tem. An entirely new reservoir of minor planets has been
found orbiting near and beyond Neptune—the Kuiper Belt.
The first Kuiper Belt object was found in 1992, and as of
this writing over 1200 have been discovered. A number are
as large as Pluto or Triton. The largest, Eris, has a density
similar to that of Triton, and methane and nitrogen ice on
its surface. [SeeKuiper Belt Objects: Physical Studies.]
The link between Triton, Pluto, and the Kuiper Belt is
strengthened by what is known of the orbital dynamics of
this region. For example, a number of Kuiper Belt objects
share the same dynamical resonance with Neptune that
Pluto occupies (this orbital resonance prevents encounters
between Neptune and Pluto, and is one of the strong argu-
ments against the Pluto-as-escaped-satellite hypothesis). In
this sense, Pluto and its companion “Plutinos” are more like
the Trojan or Hilda groups of asteroids (which are locked
in orbital resonances with Jupiter), only that Pluto-Charon
is the clearly dominant member of its group.
Dynamical calculations show that Pluto and its compan-
ions were probably swept into this orbital resonance as Nep-
tune’s orbit expanded early in solar system history. During
this time the flux close to Neptune of bodies orbiting near
and beyond Neptune would have been quite high, and even
today Neptune continues to deplete the inner Kuiper Belt
population, the short period comets being one result. It is
perhaps not surprising then that Neptune should have had
a catastrophic encounter with at least one escapee from the
Kuiper Belt: Triton. [SeeCometaryDynamics.]
Satellite capture does not occur easily. Generally, objects
passing near a planet leave with the same speed that they
came in with. Even complicated trajectories called tempo-
rary gravitational captures (enjoyed by Comet Shoemaker-
Levy 9) are just that, temporary. To be permanently cap-
tured, a cosmic body must lose energy (velocity) by running
into or through something. In Triton’s case, it could have
collided with another stray body just passing by Neptune,
but the probability of this having happened is quite low. Be-
cause Triton orbits close to Neptune, in the region usually
occupied by regular satellites, it is much more likely that
it ran into a regular satellite or its precursor protosatellite
disk.
FIGURE 15 A possible capture mechanism for Triton. In this
example “exchange capture” calculation, an equal mass Triton
binary approaches from the upper left, and is disrupted by tides
from Neptune. One member of the binary is captured into an
elliptical orbit with a semimajor axis of≈ 70 RN, while the other
escapes (RNis Neptune’s radius). [From C.B. Agnor and D.P.
Hamilton (2006)Nature 441 , 192–194.]A recent, alternative model proposes that Triton was
once part of a binary, and when it passed too close to Nep-
tune, strong tides from the planet split the binary in two.
One member of the binary escaped back into solar orbit,
while the other stayed behind in Neptune orbit (Fig. 15).
In this case, the captured member of the binary loses or-
bital energy to the escaping member. While this may at first
glance seem far-fetched, we now know that a good fraction
of Kuiper belt objects are binaries, and tidal stripping close
to a much more massive planet such as Neptune simply re-
quires a close passage. Although permanent capture of one
of the original binary members is not assured, the proba-
bility is much greater than, say, being captured by colliding
with an original Neptune satellite.
The inclination of Triton’s postcapture orbit depends on
the initial encounter geometry, and is essentially random.
Triton could have ended up either prograde or retrograde.
After capture, Triton’s orbital evolution would be strongly
influenced by tides. Every time Triton reapproached Nep-
tune, Neptune’s gravity would raise a tidal bulge on Triton.
The periodic rise and fall of the bulge would dissipate en-
ergy as heat, which would be extracted from the energy
of Triton’s orbit. Because the tidal couple between Triton’s
bulge and Neptune would be (on average) radial, no change
in Triton’s orbital angular momentum would occur. Based
on these constraints, and ignoring for the moment any en-
counters with original satellites, Triton’s orbital configura-
tion after capture would evolve as depicted in Figure 16.
Triton may have begun with a semimajor axis of 1000RN