Triton 489
as ammonia and methanol, which are among the minor ices
a body formed in solar orbit might have accreted. And if
Triton formed in solar orbit, it should have also accreted a
large carbonaceous component, upwards of 10% by mass if
comets such as Halley are a guide. But with or without these
additional components, the heat flow from Triton today is
sufficient to maintain an internal water layer or “ocean.”
Similar oceans have been discovered within the large jo-
vian moons Europa, Ganymede, and Callisto byGalileo,
so there is no fundamental reason why Triton would not
possess one as well.
Voyager 2confirmed the presence of nitrogen ice on
Triton’s surface. Specifically, a thin nitrogen atmosphere was
detected with a surface pressure and temperature consis-
tent with N 2 gas in vapor pressure equilibrium with N 2 ice
(see Section 7). All of Triton’s surface appears to be icy; even
the darker northern hemisphere shown in Figures 1 and 6
has a geometric albedo of∼0.55. Nitrogen is obviously very
volatile, and theoretical models show nitrogen ice grains
on Triton’s surface can rapidly (over many decades) anneal
and densify into a transparent glaze or sheet. It is thought
that such a nitrogen glaze covers much of Triton; the bright
equatorial fringe may be an unannealed frost deposit of
other ices.
Triton’s surface appearance also appears to be variable
on short time scales. Between 1977 and theVoyager 2flyby,
Triton become remarkably less red, particularly at shorter
wavelengths (Figure 8). Presumably, deposition of fresh ni-
trogen ice and frost have obscured more reddish surface ice
in this interval. On any other moon, this would be a major
event. On Triton, with its (presumably) active geology, ex-
treme seasons, and sublimation, transport, and condensa-
FIGURE 8 Historical visual spectral reflectance of Triton. The
differences between the data for 1977 and 1989 are evidence for
changes on the surface of Triton.VoyagerISS and PPS refer to
the imaging camera and the photopolarimeter, respectively.
[From R.H. Brownet al.(1995)In“Neptune and Triton” (D.P.
Cruikshank, ed.). University of Arizona Press, Tucson.]
tion of highly volatile ices in response to both, it seems an
almost forgone conclusion that the satellite’s global color, if
not its overall brightness (and thus its surface temperature
and atmospheric pressure), are not constant. Changes over
time in Triton’s methane spectral absorptions and ultravio-
let albedo have also been noted over the years.
As mentioned earlier, the overall redness of the ice
(Fig. 1) is thought to be due to UV and charged particle
processing of CH 4 (along with N 2 ), which can yield darker,
redder chromophores—heavier hydrocarbons, nitriles, and
other polymers. CH 4 exists as an atmospheric gas as well as
a surface ice.Voyager’sultraviolet spectrometer solar oc-
cultation experiment determined the CH 4 mole fraction at
the base of the atmosphere to be∼2to6× 10 –^4 , near
or at saturation for 38 K. Dark streaks and patches on the
polar cap and elsewhere may be methane-rich; if they are
depleted of N 2 ice, they should be warmer than the global
mean surface temperature, which is buffered by the latent
heat of nitrogen condensation/sublimation.
The nature and chemistry of Triton’s surface ices have
been determined by advanced ground-based spectroscopy
(Figure 9). In 1991 astronomers detected the spectral ab-
sorptions of CO and CO 2 ice, along with CH 4 and N 2 ice, on
Triton. Later work confirmed the presence of water ice, and
most recently, ethane ice has been detected. The shapes of
the absorption bands are so well determined that the abun-
dances, grain sizes, and degree of mixing of various compo-
nents can be modeled. It turns out that CH 4 and CO ice are
dissolved in solid solution with the far more abundant N 2
ice, which covers about 55% of Triton’s surface. The CH 4
abundance relative to N 2 is about 0.1% and CO abundance
of half that. CO is an important tracer of outer solar neb-
ula or cometary chemistry (as discussed in Section 3.3), butFIGURE 9 Modern, high-resolution, near-infrared telescopic
reflectance spectra of Triton. Absorptions due to individual
species are indicated. The spectral resolution (λ/λ)isa
remarkable 800. [From D.P. Cruikshank (2005).Space Sci. Rev.
116 , 421–439.]