490 Encyclopedia of the Solar System
is not expected to survive in giant planet satellite-forming
nebulae. The detection of CO thus directly supports a cap-
ture origin for Triton. Some discrete CH 4 patches probably
also exist, and the ethane ice is one of the “heavier hydrocar-
bons” predicted to form from methane. CO 2 and H 2 O are
distributed as discrete units covering the complementary
45% of the rest of the surface. Within these units, CO 2 ice
particles represent about 10–20% of the material present.
Water ice and CO 2 ice thus represent the composition of
Triton’s involatile “bedrock.”
The geology revealed by theVoyagerencounter is as
remarkable as it was unprecedented. The surface is almost
whollyendogenicin nature. Intrusive and extrusive volcan-
ism (calderas, flows, diapirs, etc.) dominates the landscape
outside the polar terrain, with tectonic structures (mainly
ridges) being decidedly subsidiary. Impact cratering is an
even more minor process. Triton’s surface is geologically
young and has apparently been active up until recent times.
Triton’s topography can be rugged, but does not exceed a
kilometer or so in vertical scale (and usually no more than
a few 100 m), due to the inherent mechanical weakness of
most of the ices that comprise its surface. Polar ices appear
to bury much of this topography, and so may in this sense
constitute a true polar cap. It is usually assumed that this
cap is mostly nitrogen, similar to the surface ice. Details of
Triton’s geology are pursued in the following section.
Triton’s atmosphere is unique as well. It is too thin and
cold for radiative processes to play a dominant role. Heat
is transported by conduction throughout most of its ver-
tical extent, which is by definition athermosphere,up
to an exobase of∼950 km, where the mean free path of
N 2 molecules equals the pressure/density scale height. The
thermospheric temperature is a nearly constant 102±3K
above∼300 km altitude, and is set by a balance between
absorption of solar and magnetospheric energy in a well-
developed ionosphere between∼250 and 450 km altitude
and both radiation to space by a trace of CO and pho-
tochemically produced HCN below∼100 km and down-
ward conduction to the cold, 38 K surface. The lowermost
atmosphere is characterized by an interhemispheric, sea-
sonal condensation flow. Turbulence near the ground forces
the temperature profile to follow a convective, nitrogen-
saturated lapse rate of∼–0.1 K km–^1 up to an altitude of
∼8 km (as determined by observations of clouds, hazes,
and plume heights; Section 7), forming atroposphereor
“weather layer.” Unlike in the atmosphere of the Earth and
other planets, there is no intervening radiatively controlled
stratospherebetween Triton’s troposphere and thermo-
sphere.
6. Geology
Triton’s surface, at least the 40% seen byVoyagerat resolu-
tions useful for geological analysis, can be roughly separated
into three distinct regions or terrains: smooth, walled, and
terraced plains; cantaloupe terrain; and bright polar mate-
rials. Each terrain is characterized by unique landforms and
geological structures. Substantial variations within each ter-
rain do occur, and the boundaries between each are in many
locations gradational, but in general the classification of Tri-
ton’s surface at any point is unambiguous. Certain geological
structures are common to nearly all terrains, specifically, the
tectonic ridges and fissures, and impact craters, naturally,
can form anywhere.
Although Triton’s surface is composed almost entirely
of ices, many of the individual geological structures can
be readily interpreted as variations of structures terrestrial
planet geologists would find familiar, such as volcanic vents,
lava flows, and fissures. The volcanic features in particu-
lar have inspired a designation “cryovolcanic” in order to
distinguish them from those formed by traditional silicate
magmatic processes. The physics and physical chemistry
are fundamentally the same, however, whether one deals
with silicate or icy volcanism. There are in addition geolog-
ical structures and features on Triton that are unusual and
notreadily interpretable in terms of terrestrial analogues.
Some defy explanation altogether.6.1 Undulating, High Plains
Plains units are found on Triton’s eastern or leading hemi-
sphere (referring to the sense of orbital motion, to the right
in Figure 1) and to the north of the polar terrain boundary.
Figure 10 shows a regional close-up of various plains near
the terminator in the center of Figure 1. To the bottom and
right of the image are flat-to-undulating smooth plains cen-
tered around circular depressions or linear arrangements of
rimless pits. These plains are relatively high-standing and
bury preexisting topography, with edges that may be well
defined or diffuse. There is little doubt that these high plains
are the result of icy volcanism and that the various pits and
circular structures are the vents from which this material
emanated. In general, eruptions along deep-seated fissures
or rifts often manifest as a series of vents, and the irregu-
lar,∼85-km wide circular depression toward the lower left
resembles a terrestrial volcanic caldera complex. This fea-
ture, Leviathan Patera (all the features on Triton have been
given names drawn from the world’s aquatic mythologies),
sits at the vertex of two linear eruption trends. Towards the
terminator (northeast), one of these trends is anchored by
another caldera-like depression of similar scale.
Volcanic activity on the Earth often occurs in cycles,
whereby magma formed by partial melting in the mantle
rises due to buoyancy, accumulates at intermediate, crustal
levels to form a magma chamber, and subsequently erupts;
things are then quiescent until the magma is replenished
and the cycle begins anew. The loss of magma volume often
leads to collapse of the vent region over the magma cham-
ber, forming a caldera. Cycles of eruption and collapse can