Triton 493
maculae shown in Fig. 11). The maculae betray almost no
topographic expression, and so must vary in height across
their extents by no more than a few tens of meters. The dark-
ness and redness (Fig. 1) of the central patches implies the
presence of carbonaceous material, which probably means
some methane ice is present. The brightness of the annuli
is similar to that of the bright terrain, so they may consist
of similar ices (predominantly N 2 ).
The extreme eastern limb shown in Figure 11 is com-
posed of a mosaic of maculae, and much of the bright terrain
in the rest of the image contains similar, though generally
less distinct, features (see also Fig. 1). Perhaps the mac-
ulae are outliers of the southern polar cap, which should
have been retreating at the season observed (late south-
ern spring). Furthermore, another walled plain can be seen
along the middle left edge of the frame. Its eastern rim is
incomplete, and breaks down into a region of small mesas.
If this planitia were filled with bright ice, it would passably
resemble, in plan and in albedo, the bright terrains to the
south, especially those near the boundary with the smoother
plains. The resemblance would be further improved if the
planitia are bowed upwards, for which there is independent
topographic evidence (see Fig. 10). Perhaps the maculae are
planitia underneath, and the mysterious erosive process that
cut back the planitia scarps has operated more extensively
on Triton.
6.4 Cantaloupe Terrain, Ridges, and Fissures
The entire western half of Triton’s non-polar surface shown
in Figure 1 is termed cantaloupe terrain, as it appears cov-
ered by large dimples and criss-crossed by prominent quasi-
linear ridges. Much of the terrain displays a well-ordered
structural pattern: at high resolution the dimples become a
network of interfering, closely spaced, elliptical and kidney-
shaped depressions, termed cavi (Fig. 12). Unlike impact
craters, the cavi are of roughly uniform size,∼25-to-35 km
in diameter, and do not overlap or crosscut. They are clearly
internal in origin, but the leading explanation is not volcan-
ism, butdiapirism.
Diapirism is triggered by a gravitational instability in-
volving a less dense material rising through overlying denser
material. The required buoyancy may be thermal or com-
positional. Probably the best known terrestrial examples of
diapirsare salt domes, in which a layer of salt rises as a se-
ries of individual blobs, or diapirs, through overlying denser
sedimentary strata. In one region of extreme dryness, the
Great Kavir in central Iran, the salt diapirs breach the sur-
face, rotating and pushing the overlying strata to the side.
The shapes, close spacing, and interference relations of the
diapirs of the great Kavir in fact bear a significant resem-
blance to the cavi.
The implications of a diapiric origin for cantaloupe ter-
rain are that Triton possesses distinct crustal layering, and
based on the spacing of the cavi, that the overlying denser
FIGURE 12 Cantaloupe terrain at the bottom and polar terrain
at the top, in this high-resolutionVoyagerimage taken from a
distance of 40,000 kilometers. Each cantaloupe “dimple” is about
25–35 kilometers across. A tectonic ridge and fissure set runs
through the cantaloupe terrain, probably formed by the extension
of Triton’s icy crust. Towards the south (upper right), smooth
materials, and beyond them, brighter ice, appear to mostly bury
cantaloupe and fissure topography. (Courtesy of NASA/JPL.)layer or layers is∼20 km thick. This crustal layer could sim-
ply be a weaker ice (possibly ammonia rich) that responded
to heating from below, or it may be an ice denser than the
ammonia-water ices presumably below (such as CO 2 ice).
Triton’s surface is crosscut by a system of ridges and fis-
sures, which are best expressed in the cantaloupe terrain
(Fig. 1). The ridges occur in a variety of forms: pairs of
low, parallel ridges bounding a central trough,∼6–8 km
across crest-to-crest and a few hundred meters high; similar
but wider ridge-bounded troughs with one or more medial
ridges (one, Slidr Sulcus, can be seen in Fig. 12); and sin-
gle, broad, bulbous ridges (e.g., Fig. 11). The fissures, which
are less numerous, appear to be simple, long, narrow val-
leys only 2–3 km wide. All of these fundamentally tectonic
features appear to result from extension and/or strike-slip
faulting of Triton’s surface. The medial ridges may be due
to dike-like intrusions of icy material, and the bulbous ap-
pearance of some may be due to overflow of such injected
ice, which could also be a source for smooth plains deposits.