460 Encyclopedia of the Solar System
diameter increases beyond 35 km, instead of the transition
to larger central peaks or peak rings seen on the inner plan-
ets, large craters on Ganymede and Callisto exhibit central
pits (Fig. 7b). Young craters undergo another transition at
about 60-km diameter, where the central pits begin to ex-
hibit round domes of material in their centers (Fig. 7c).
These central domes have fractured surfaces reminiscent
of lava domes, and they may be formed by rapid extru-
sion of warm, viscous ice into the center of the crater just
after its formation. Most large craters on Ganymede and
Callisto are very shallow, especially the older craters, indi-
cating that warm subsurface ice has flowed in toward the
crater depressions and bowed their floors back up to the
topographic level of their surroundings (a process known as
viscous relaxation). Some central-dome craters have been
so flattened that they do not exhibit any obvious rim struc-
ture; the central dome and surrounding pit wall are the
only obvious structures remaining (Fig. 7d). A few large
craters, known as penepalimpsests, exhibit only subdued
topographic rings, with a smooth patch in the middle (Fig.
7e). Where these occur on Callisto and the dark terrain
of Ganymede, they show up as a distinct circular patch
of bright material against the dark background. Still other
bright circular patches, which are almost completely flat
(except sometimes an outward facing scarp can be seen
around the outside and a depressed smooth patch found
in the middle), are found within Ganymede’s dark terrain
(and a few exist on Callisto) (Fig. 7f). These features are
called palimpsests, a word for an ancient piece of parch-
ment where the writing has been erased. In a similar way
these large ancient craters have almost been erased by the
process of viscous relaxation.
On the Moon, the largest craters form multiring basins
[seeTheMoon]. On Ganymede, one large basin called
Gilgamesh shares similar characteristics with the lunar
basins: a smooth central region surrounded by large
irregular massifs, which is in turn surrounded by a few
large concentric mountain ranges. However, most large
impact basins on Ganymede and Callisto exhibit a distinctly
different morphology, with a large palimpsest in the middle
surrounded by many evenly spaced concentric rings. The
best example of such an impact basin is Valhalla on Callisto,
which is about 1000 km across and exhibits about 20 con-
centric rings around its central bright palimpsest (Fig. 8).
Most basin rings on Callisto are troughs that appear to
have formed by extension of the surface material. These
multiring structures are thought to form as subsurface
material rapidly flows in from the sides to fill the center
of the impact basin, pulling a thin brittle veneer of surface
material inward.FIGURE 8 View from theVoyager 1spacecraft of the Valhalla multiring basin on Callisto. The
extensive system of concentric troughs surrounding the impact site is over 3000 km across.