462 Encyclopedia of the Solar System
FIGURE 9 Views of smooth bright terrain on Ganymede. (a)Voyager 1image of smooth bright
terrain in Harpagia Sulcus; (b) Galileo high-resolution image of smooth terrain from the center of
(a), showing ridges and hills not visible in regional-scale views; (c) Galileo image of Arbela Sulcus,
a narrow lane of smooth terrain cutting through the dark terrain of Nicholson Regio.
terrain, and other areas with more subdued topography,
termed smooth terrain.
Smooth terrain may occur either as patches bounded by
grooved terrain on all sides or as lanes of smooth mate-
rial tens of kilometers wide cutting across bright and dark
terrain. In either case, the terrain appears to be smooth
in kilometer-resolution regional images (Fig. 9a), leading
to the hypothesis that it formed by low-viscosity cryovol-
canic flows flooding the underlying terrain. At higher reso-
lution, it becomes apparent that the smooth terrain is not
so smooth after all. In some areas, it appears to be a flat
plain crossed by ridges or sets of aligned hills (Fig. 9b).
In other areas, especially where the smooth terrain occurs
as narrow lanes, it appears to be a flat or gently undulat-
ing surface crossed by parallel dark lineations, which may
be narrow valleys formed by tensile fracturing of the ice
(Fig. 9c). The presence of parallel sets of ridges and val-
leys in smooth terrain suggests that tectonism plays an im-
portant role in shaping this terrain, in addition to possible
cryovolcanism.
Though cryovolcanism is an attractive explanation for
the smooth, flat areas found within smooth terrain, its role
has not been conclusively demonstrated. No obvious vol-
canic constructs or flows have been observed, though it is
unclear if we know what an ice volcano is really supposed
to look like. A few features that may possibly be volcanic
calderas have been observed (Fig. 10), but most areas of
smooth terrain exhibit no such features. While the smooth
regions shown in Fig. 10 are topographic lows, as one would
expect if they were troughs filled by low viscosity volcanic
flows, the smooth regions shown in Fig. 9 have been found
to lie locally higher than parts of their immediate surround-
ings. Another possible interpretation for the linear bands of
smooth terrain is that they formed through separation and
spreading of the crust in a manner analogous to Europa’s
gray bands. In either case, the formation of smooth terrain
appears to involve the extrusion of liquid water or warm ice
from Ganymede’s subsurface.
Tectonism plays a more obvious role in the formation
of grooved terrain. In kilometer-resolution images, grooved
terrain is characterized by parallel valleys and ridges spaced
about 5–10 km apart (Fig. 11a). At higher resolution, each
ridge and valley is itself composed of many smaller ridges
and valleys (Fig 11b). Each of these smaller ridges is thought
to be a fault block, a piece of the icy crust that has been
separated from its surroundings by faults and then moved
and tilted as it slid along those faults. The shapes and in-
tersections of the faults are suggestive of a style of faulting
known as tilt-block normal faulting, in which many paral-
lel faults slice the upper portion of the crust into roughly
rectangular blocks that then tilt over and slide against each
other as the crust extends, much like books sliding over
on a bookshelf when a bookend is removed. This style of
faulting creates parallel ridges with a sawtooth topographic
profile, matching the triangular ridges on Ganymede with
their sharp crests, frosty upper slopes, and dark V-shaped
valleys between them.
In a few places on Ganymede, large impact craters have
been cut by these networks of faults (Fig. 12). Since almost
all craters are formed in a roughly circular shape, these
cut craters offer an opportunity to measure directly how
the crust has deformed as their shape becomes progres-
sively distorted by motion along the faults. Measurements
of these craters confirm that the development of grooved
terrain on Ganymede is dominated by extensional tecton-
ics. At the extreme end of the spectrum, some small parts
of the crust appear to have been pulled apart to more than
twice their original width, but in most cases the extension