Geology Reference
In-Depth Information
Figure 8.28. A map of Ganymede showing prominent named features (from Spencer et al.,
2004
).
crater depth is considered to be a consequence of impacts
into ice, in which crater rims
“
relax
”
by viscous
ow.
Given the properties of the ice and estimates of temper-
ature as a function of depth below the surface, models
show that after the impact the crater
floor would
rst
flatten and then upwarp to a convex form as the crater
rim subsided. Under these conditions, long-wavelength
features would be more affected and larger craters would
be relatively more deformed than small craters, as seen in
their in depth-to-diameter ratios
(Fig. 8.30)
. However,
shallow craters could also re
ect conditions and proper-
ties of the target at the time of their formation in which the
ice-rich crust could have been warmer in the past and
more subject to deformation.
Craters smaller than ~20 km are typical bowl-shaped
depressions with depth-to-diameter ratios of 1:6 to 1:12
and are not greatly affected by viscous relaxation. Thus, as
models would predict, the ice-rich crust behaves essentially
as rocky material for small impacts. Craters ~20
-
40 km in
diameter generally have
flat or slightly convex
oors.
Central peaks occur in many craters of this size but are
generally absent among the larger craters; rather, they
appear to be replaced by central pits
(Fig. 8.31)
,which
could result from the impact punching through the ice
crust into warmer, softer ice or even liquid water.
Ejecta deposits are seen around many craters on
Ganymede and include
flow-like patterns
(Fig. 8.32)
sim-
ilar to the
craters on Mars (
Fig. 7.7)
. Ejecta
rays extend from fresh craters and include both bright and
dark forms. Some rays include mixtures of bright and dark
deposits
(Fig. 8.33)
, suggesting excavation into materials
of different ice
“
ejecta ow
”
rock mixtures. The
floors of some craters
are very dark, possibly resulting from flooding by darker
subsurface materials
(Fig. 8.34)
.
Among the many remarkable discoveries from the
Voyager mission were the crater structures on Ganymede
-
