486 Encyclopedia of the Solar System
FIGURE 4 Seasonal excursion of the subsolar latitude on Triton.
Dot shows the subsolar latitude at the time of theVoyager
encounter. [From R.L. Kirket al.(1995).In“Neptune and
Triton” (D.P. Cruikshank, ed.). University of Arizona Press,
Tucson.]
The possibility of long-term cold traps at both poles, with
strong seasonal atmospheric flows from pole to pole, was
recognized. The illustration in Figure 4 was in fact based
in part on an analogy with Mars, with N 2 replacing CO 2 as
the dominant, and condensable, atmospheric constituent,
and CH 4 replacing H 2 O as the secondary, less volatile com-
ponent (an analogy that is strengthening, as will be dis-
cussed later). Specifically, a large cap of solid nitrogen was
predicted for the south pole, sublimating slowly in the fee-
ble summer sun.
3.3 Similarities with Pluto
As Triton was coming into clearer astronomical focus in the
1980s, parallel developments were occurring for other outer
solar system bodies, especially Pluto. Methane ice had been
discovered on Pluto prior to Triton, and overall, Pluto’s vis-
ible and near-infrared spectrum bore a strong resemblance
to that of Triton, although Pluto’s methane absorptions were
deeper. Their common bond was reinforced by their simi-
lar visual magnitudes (Pluto and its moon together are only
∼0.3 magnitudes fainter than Triton when referenced to a
common distance and solar phase angle).
Pluto’s fundamental properties (mass and radius) were
by the time of theVoyager 2encounter with Neptune (and
Triton) relatively well constrained. Pluto’s relatively large
satellite, Charon, had been discovered in 1978, which al-
lowed determination of the mass of the Pluto–Charon sys-
tem by means of Kepler’s Third Law. Careful monitoring of
the Pluto-Charon system’s lightcurve, plus observations of
the occultation of a star by Pluto in 1988, established that
Pluto’s radius lay between 1150 and 1200 km. Pluto turned
out to be a smallish, bright, more-or-less ice-covered world,
and a relatively dense one as ice-rock bodies go, close to
2gmcm–^3 .[SeePluto.]
Pluto’s density corresponds to a rock/ice ratio of about
70/30, and is, curiously, close to what is predicted for a
body accreted in the deep cold reaches of the outer solar
system. According to current thinking, the solar nebula at
that distance from the Sun, when the Sun and planets were
forming, was relatively cold and unprocessed. The outer
nebula thus retained many of the chemical signatures of
the interstellar gas and dust (molecular cloud) that was the
ultimate source of the nebula. Specifically, carbon would be
in the form of organic matter and carbon monoxide (CO)
gas. CO is very volatile, and the solar nebula was unlikely
to have ever been cold enough for it to condense in bulk
(though small amounts could be adsorbed on or trapped in
water ice). The key point is that volatile CO ties up oxygen
that would otherwise be available to form water ice. There-
fore, bodies formed in the outer solar system, but not near
a giant planet, should have relativelyhighrock/water-ice
ratios. In contrast, in the high-pressure environment near
a giant planet, CO combines with H 2 to make H 2 O and
CH 4 , which can both condense. The resulting satellites are
predicted to be much icier, with rock/ice ratios of 50/50 or
less. [SeeTheOrigin of theSolarSystem.]
That Pluto was so rock-rich was one line of reasoning that
pointed to Pluto being an original solar-orbiting body and
not an escaped satellite of Neptune. Dynamical evidence
against Pluto being an escaped satellite also accumulated.
By the 1980s it was being argued that Triton and Pluto
should be considered as two independent solar system bod-
ies, with independent histories. The link between the two,
in terms of brightness (and presumably size) and composi-
tion, was that they formed in the same region—the outer
solar nebula near or beyond Neptune. Essentially, they are
surviving examples of large outer solar system protoplanets.
Pluto became locked in a dynamical resonance with Nep-
tune, which preserved its peculiar orbital geometry, while
Triton was later captured by Neptune’s gravity. [SeePluto.]
If the analogy with Pluto is correct, then Triton should
also be rock-rich. If Triton had a relatively bright, icy surface
like Pluto, Triton’s visible magnitude implied it would prob-
ably be somewhat larger, but its density would be similar
to that of Pluto–Charon. Of course, the surface state and
thus the size of Triton could not be pinned down before
theVoyagerencounter, but the consequences for Triton of
being captured (as has been alluded to) were potentially
spectacular. These include intense tidal heating and whole-
sale melting of the satellite. These ideas were appreciated
by the planetary community on the eve of theVoyager 2
encounter. So with the observational and theoretical back-
drop just described, and with the promise of resolution of