492 Encyclopedia of the Solar System
Comets also contain a host of other exotic, presumably
interstellar, ices, some of which may have been impor-
tant in Triton’s geological history. For example, methanol
(CH 3 OH) pushes the minimum melting temperature of
ammonia-water ice down to∼152 K, and the resulting lava
is even more viscous, equivalent to certain types of silicic
lavas on Earth. The range of viscosities available to liquids
in the H 2 O–NH 3 –CH 3 OH system is compatible with the
appearance of the undulating smooth plains seen in Fig. 10.
[SeePhysics and theChemistry ofComets.]
The abundances of original ices may also have been al-
tered, and new ices created altogether, during Triton’s tidal
heating epoch (see Section 8). For example,copiousNH 3
and CO 2 may have been chemically produced within Tri-
ton provided there was a sufficient supply of nitrogen and
carbon. Despite these exciting possibilities, neither NH 3
nor any complex, exotic ices have yet been discovered by
ground-based spectroscopy (Section 5).
6.2 Walled and Terraced Plains
Shown in the northwest corner of Figure 10 is an∼175-km
wide, remarkably flat plain, Ruach Planitia, that is bounded
on all sides by a rougher plains unit that rises in one or more
topographic steps (scarps) from the plain floor. It is one of
four so-called walled plains identified on Triton; these are
generally quasicircular in outline, with typical relief across
the bounding steps or scarps of∼200 m. Ruach Planitia
and the other walled plains are the flattest places seen on
Triton, which implies infill by a very fluid lava or other liquid.
Clusters of irregular, coalesced pits towards the centers of
these plains have been likened to eruptive vents or drainage
pits.
The planitia themselves have been likened to calderas,
but they are generally much larger than the nearby paterae
and do not resemble them structurally. Specifically, there is
no evidence for collapse at the periphery of any of the walled
plains. Rather, the outline of the inward-facing scarps is in-
dented and crenulate, with islands of the bounding plains
occurring in the interior. If anything, the outlines of walled
plains resemble eroded shorelines. How erosion occurred
and under what environmental conditions on Triton is un-
clear. If the fluid that filled the planitia was responsible for
the erosion, it does not explain the similar outline of the
plains that overlap the eastern edge of Ruach Planitia (Fig.
10), which gives this area a terraced appearance and indi-
cates that the rougher plains were laid down in layers. A
distinct possibility is that the layers are composed at least in
part of a more friable or volatile material, and that over time
(or with higher heat flows) the layers disintegrated and the
scarps formed by retreat. Similar processes of mass wast-
ing, removal, and scarp retreat are believed responsible for
the etched plains of the martian south polar highlands and
similar terrains on Io.
6.3 Smooth Plains and Zoned Maculae
Other plains units can be seen in Figure 11, as well as the
transition to the bright polar materials. At the top left is
a hummocky terrain, composed of a maze of depressions
and bulbous mounds. Stratigraphically, it is older than the
volcanic plains to the north that overlap it, and appears
older (more degraded) as well. The hummocky terrain gives
way to a much smoother plains unit to the south. At the
available resolution it is unclear whether this smoothness is
due to volcanic flooding, volcanic or condensation mantling,
or some other form of degradation. These hummocky and
smoother units are the most heavily cratered regions on
Triton, but by solar system standards are not heavily
cratered at all.
Among Triton’s most perplexing geological features are
the large zoned maculae (spots) close to the eastern limb are
shown in Figure 11. Each such macula consists of a smooth,
relatively dark patch or patches surrounded by a brighter
annulus or aureole. The width of any given annulus tends
to be relatively constant (20 to 30 km for the three majorFIGURE 11 Southeastern limb of Triton, showing (from top)
hummocky terrain, smooth terrain, and bright polar terrain. A
prominent bulbous ridge zigzags across the top, and distinct
bright-ringed dark features of uncertain origin, termed maculae
(spots), are seen at the right, and more faintly, along the limb and
in the bright terrain. The largest crater on Triton, the 27-km
diameter, central-peaked Mozamba, is to the left of the largest
prominent macula, Zin. (Courtesy of NASA/Paul Schenk, Lunar
and Planetary Institute.)