494 Encyclopedia of the Solar System
Ridges on Triton bear more than a passing resemblance to
those on Europa, and a similar mechanical origin has been
proposed.
6.5 Bright Polar Terrains
Most of Triton seen byVoyageris actually bright terrain
of one type or another, but the imagery is generally not
of sufficient quality for geological analysis. Interpretations
are further confused by the numerous dark streaks, plumes,
and clouds. Nevertheless, the bright terrains represent sub-
stantial, not superficial deposits. The view shown in Figure
12 looks across the edge of the cantaloupe terrain, into a
band of subdued or mantled cantaloupe-like topography,
and then into brighter materials beyond. Cantaloupe-like
topographic elements and sections of a linear ridge appear
engulfed by bright ice, probably up to a few hundred me-
ters in thickness. The important questions are whether the
bright ice thickness increases into the interior of the bright
materials in the distance, and does it become sufficiently
deep to qualify as a true polar cap.
Low-resolution imagery shows that quasicircular ele-
ments can be made out at many locations well within the
bright materials. Ridges also cross into the bright terrains,
and one bright lineament is seen close to the south pole.
The implication is that much of the polar topography is in-
completely buried. On the other hand, there are extensive
bright, featureless regions as well (up to several 100 km
across), which indicate either complete burial at these lo-
cations or obscuration by clouds. Overall thickness of the
bright polar ice is therefore probably less than 1 km, but
even if not organized as a uniform ice cap or sheet, a thick
deposit of a volatile ice such as N 2 could be warm and de-
formable enough at its base to flow laterally. Although not
literally a polar cap, much of the bright polar terrains may
behave as if glaciated.
6.6 Geological History
It is notable that the volcanic province shown in Figure 10
is one of two similar ones, with the second occurring to the
southeast and together stretching across 1000 km of Triton’s
surface. The alignments of volcanic vents in both provinces
suggest extension and rifting of Triton’s relatively strong icy
outer shell, or lithosphere. The volcanic plains shown in
Figure 10 are also very sparsely cratered (the largest crater
visible is 16 km across), much less cratered than, say, the
lunar maria. Estimates of the rate at which comets bombard
Triton suggest that these provinces are no more than 300
million years old, and possibly much less. A broad region of
Triton’s sublithospheric mantle was thus hot and partially
molten very late in solar system history, and probably re-
mains so. Such internal warmth is also consistent with a
deep subsurface ocean (Fig. 7).
The high volcanic plains postdate most of the other ter-
rains on Triton. They stratigraphically overlie the terraced
plains to the west and the hummocky plains to the east.
The terraced plains grade into and appear to superpose
the cantaloupe terrain. The relative age of the cantaloupe
terrain cannot be determined by traditional crater counting
methods, because the rugged topography there prevents re-
liable crater identification inVoyagerimages. Stratigraph-
ically, however, cantaloupe terrain appears to be the old-
est unit on Triton. The linear ridges obviously postdate
the cantaloupe terrain, yet some ridges fade into the ter-
raced plains to the east and another is discontinuous as it
crosses the hummocky and smooth plains near the equator
to the east (Figs. 1 and 11); no ridges cut the high volcanic
plains.
The eastern hummocky and smooth plains comprise the
most heavily cratered region on Triton, and when due ac-
count is taken of the concentration of cometary impacts
on Triton’s leading hemisphere, appears to be somewhat
older than the high volcanic plains to the north and north-
west. The cantaloupe terrain, then, must be even older. The
hummocky terrain may be a degraded version of cantaloupe
terrain. Indeed, cantaloupe terrain has been suggested to
underlie much of Triton’s surface. (For example,
cantaloupe-like topography extends well south into the
bright region of the trailing hemisphere.)
The youngest surfaces on Triton, naturally, involve the
mobile materials of the bright terrains. These probably in-
clude the zoned maculae of the eastern hemisphere. The
geological substrate upon which the bright materials reside
may of course be older. The walled plains themselves are
locally the youngest stratigraphic units. Ruach Planitia and
a larger planitia immediately to the west are less cratered
than the high volcanic plains, albeit with a large statistical
uncertainty. The filling of these walled plains may thus rep-
resent the most recent volcanic activity on the hemisphere
of Triton seen byVoyager.7. Atmosphere and Surface
7.1 Atmosphere
Triton is one of only seven solid bodies in the Solar System
with an appreciable atmosphere, and one of only four in
which the major component of the atmosphere also con-
denses onto the surface. Triton’s atmosphere is composed
primarily of nitrogen. The complicated oscillation of the
subsolar latitude with time drives an exchange of N 2 and
trace species between the atmosphere and surface frost de-
posits in the two hemispheres that is equally complicated
and as yet not fully understood. Internal heating (which is
comparatively important because of Triton’s extreme dis-
tance from the Sun and large proportion of rocky mate-
rials containing radioactive elements) and even glacier-like
creep of solid nitrogen caps may also play important roles in