464 Encyclopedia of the Solar System
FIGURE 12 This crater in the dark terrain of Marius Regio on
Ganymede has been cut by faults extending from a nearby region
of grooved terrain. Measurement of the postdeformation shape
of the crater demonstrates that the faults have extended the
terrain and also horizontally translated the eastern part
northward relative to the western part.
Alternatively, we may have missed seeing the contractional
features on Ganymede either because the crust shortened
mostly in a ductile fashion, leaving few obvious surface fea-
tures, or because we simply don’t recognize contractional
features formed in ice.
In places where narrow lanes of grooved terrain cut
across dark terrain, it is clear that bright grooved terrain
can form simply by extension of the dark terrain, without
the cryovolcanism that may have taken place in smooth ter-
rain. Faulting can serve to erase the impact craters on the
dark terrain by slicing them up and making their rims unrec-
ognizable. Brightening of the terrain can occur by breaking
through the dark regolith layer and exposing bright subsur-
face ice along the fault scarps.
5.2 Dark Terrain
Aside from the swaths of bright grooved terrain that cut
across the dark terrain and small peripheral fractures ad-
jacent to the grooved terrain, dark terrain on Ganymede
is primarily dominated by systems of arcuate to linear fea-
tures known as furrows. Furrows are usually composed of
two bright ridges spaced 10–20 km apart, with a dark trough
in between them. Most furrows are arranged in concentric
sets of arcs (Fig. 13), indicating that they are probably an-
cient multiring basins that originally resembled Valhalla on
Callisto but are now sliced up into fragments by the forma-
tion of bright terrain. This interpretation is aided by some
small furrow systems that appear to have an impact basin in
the center. Some sparse systems of linear furrows appear to
radiate out from a point rather than being concentric arcs.
The origin of these radial systems is unclear.
There was speculation based onVoyagerimages that
some areas of dark terrain had a splotchy appearance due
to patches of dark cryovolcanic material oozing onto the
surface. At higher resolution, however, these splotchy areas
were revealed to be dark plains interrupted by networks of
bright fractures.5.3 Callisto
The story of tectonism and volcanism on Callisto is remi-
niscent of Ganymede’s dark terrain. All of the obvious tec-
tonic features are arranged in concentric rings and sur-
round large impact basins. The rings are high scarps or
deep troughs with sharp boundaries (Fig. 14). The scarps
and troughs are formed by faults that have extended the
crust by a small amount during the formation of the im-
pact basin (see Section 4). Multiring basins on Callisto are
examples of the ancient impact basins that formed the fur-
row systems on Ganymede before they were broken apart.
There is also a strange system of troughs near Callisto’s
North Pole that seems to radiate out from a point. Un-
fortunately, the center of this system on Callisto was never
imaged at high resolution, so the origin of these features re-
mains mysterious, as does the origin of similar radial furrow
systems on Ganymede. Early speculation that smooth dark
patches on Callisto might be cryovolcanic in origin has been
largely dispelled by evidence that Callisto has a loose re-
golith that smoothes over the underlying terrain like a thick
dark blanket.6. Unanswered Questions and
Future Exploration
Several interesting unanswered questions remain about
Ganymede and Callisto. Since the general properties of
these satellites appear to be so similar, understanding the
processes and events that have driven their interior evolu-
tion and geological records to different states is an impor-
tant problem in comparative planetology. In the interiors of