Triton 485
theory. The modern view of Triton’s origin, and that of Pluto
and other bodies in the deep outer solar system, is discussed
later in this chapter. [SeePluto.]
3. Pre-VoyagerAstronomy
3.1 Radius, Mass, and Spectra
Through the telescope, Triton is a faint, 14th magnitude ob-
ject, never more than 17 seconds of arc from Neptune. Con-
sequently, physical studies of the satellite from the ground
have historically been very difficult. Showing no visible disk,
only crude limits could be put on its size, or mass, for many
years. But mid-20thcentury estimates implied Triton was
one of the largest moons in the solar system and massive,
possiblythemost massive moon in the solar system. Triton
was clearly a moon of mystery.
The first real breakthrough occurred in 1978, when
infrared detector technology had improved to the point
that a methane (CH 4 ) band was detected in Triton’s in-
frared spectrum. Soon more bands were found. The rel-
ative depths of the new bands, plus their variability as
Triton orbited Neptune, indicated that much (if not all)
of the methane detected was in solid form, that is, an ice
on the surface of Triton. Ices on the surface implied that
Triton might be a relatively bright, smaller world, rather
than a darker, larger body of the same visual magnitude.
[SeeTitan.]
Methane ice on the surface also offered a potential
explanation for Triton’s reddish visual color. Experiments
had shown that when solid methane is irradiated by so-
lar ultraviolet rays, or bombarded by charged particles, it
turns pink or red as hydrogen is driven off and the remain-
ing carbon and hydrogen form various carbonaceous com-
pounds. Continued radiation or charged particle bombard-
ment ultimately turns methane into a blackish carbon-rich
residue, however, so Triton’s persistent redness also implied
the satellite’s methane ice is refreshed on a relatively short
time scale.
3.2 Seas of Liquid Nitrogen?
An even more amazing discovery was made in the early
1980s. A single infrared spectral feature was found at
≈2.15μm (see Fig. 3), a feature that could not be at-
tributed to any of the usual spectral suspects (CH 4 ,H 2 O,
silicates, etc.). Nitrogen (N 2 ) does have an absorption at
this wavelength, and becauseVoyager 1had recently deter-
mined the dominant atmospheric gas on Titan to be N 2 (not
CH 4 ), finding nitrogen on Triton was not far fetched. The
amount of nitrogen gas required to account for the absorp-
tion was quite large, however, as nitrogen, a homonuclear
diatomic molecule, is a very poor absorber of infrared light.
The astronomers concluded that in order to get the neces-
FIGURE 3 A pre-Voyagerprediction for the state of Triton’s
surface. A subliming N 2 ice cap is centered on the illuminated
south pole (dot). [From J.I. Lunine and D.J. Stevenson (1985).
Nature 317 , 238–240.]sary pathlength for the absorption, the nitrogen had to be in
condensed form, either solid or liquid. Liquid nitrogen was
the favored interpretation, and a fantastic vista emerged—a
satellite covered with a global or near-global sea of liquid
nitrogen, along with methane-ice-coated islands or even
floating methane “icebergs”!
The “problem” with liquid nitrogen is that it freezes at
zero pressure at about 63 K. For Triton to have a global
ocean at that temperature requires (1) Triton absorb most
of the sunlight striking it (have a low albedo) and (2) Triton’s
surface radiate infrared heat very inefficiently (have a low
emissivity). For this and other reasons, planetary chemists
offered a competing concept for Triton’s surface, one in
which both the nitrogen and methane were solid and dis-
tributed nonuniformly (Fig. 3). Because of nitrogen’s great
volatility, it was argued that crystals of up to cm size could
grow on Triton’s surface over a season and so provide the
pathlength for the 2.15-μm absorption. Methane, as in all
the spectroscopic models, would be only a minor compo-
nent; it dominates Triton’s near-infrared spectrum by virtue
of the relative strength of its absorptions.
The difference between the two models for Triton’s sur-
face had important implications for the atmosphere and
for Triton’s seasons. Triton’s seasonal cycle is complicated.
Because of the precession of its orbit, its seasons vary in
intensity and length, and in the decades before theVoyager
encounter Triton was moving towards the peak of maximal
southern summer (Fig. 4). Correspondingly, Triton’s north-
ern hemisphere was (and is) enduring prolonged darkness.