Geology Reference
In-Depth Information
in diameter from about 7 km down to ~25 m, the limit of
identi
cation. The
floors of small craters vary from
smooth to pitted or hummocky. Central-peak craters
range in size from about 5 to 40 km in diameter, and the
peaks are as large as 10 km across and 0.5 km high. Many
central-peak craters have scalloped rims, terraced inner
walls, and hummocky
floors, which is typical of similar
craters seen on the terrestrial planets and the Moon.
Craters > 25 km in diameter transition from central-peak
to central-pit morphologies, with central-pit diameters
being 15%
20% of the crater diameter. Most central-pit
craters have flat floors and, in some cases, interior terraces
on their walls. For example, Tindr is a 105 km in diameter
crater with a central-pit complex about 15 km across
(Fig. 8.46)
. Although the mechanism for the formation of
pits remains poorly understood, a primary factor appears to
be the nature of the target material (i.e., ice) rather than
properties of the object that caused the impact.
Some craters > 60 km in diameter show central domes,
such as Doh
(Fig. 8.46)
and Har. Doh crater is superposed
on the central zone of the Asgard multi-ring structure, and
has a rim composed of mountains and knobs that form a
ring about 65 km across. Individual mountains are as large
as 8 km across and 730 m high. Har
'
is dome is in the center
of the crater
floor and is ~25 km across, or more than
one-third of the crater diameter. The dome displays irreg-
ular troughs that form a crude radial pattern, giving the
impression of tensional fractures that formed in response
to uplift, similarly to the cracking of a bread crust.
Although there are no obvious ejecta rays, a higher-albedo
zone surrounding Doh probably represents ejecta depos-
its, including impact melt.
As crater diameters on Callisto increase above 100 km,
the relative depth decreases, which may be due to viscous
relaxation of the ice. Relatively young large craters are
identi
ed by concentric faults or grabens, or remnants of
their rims. Two such craters, Lofn and Bran, show no
evidence of a central pit or dome but have very
at
oors.
This morphology could have resulted from a cluster of
impacts, much like a shotgun blast, or they could be a
form that is transitional to multi-ring structures.
Catenae, or crater chains, are seen in many areas of
Callisto
(Fig. 8.47)
. The longest found thus far is Gipul
Catena, which stretches some 620 km across the surface
and includes craters as large as 40 km. Two models of
formation have been suggested for catenae. Initially,
Callisto
'
s catenae were thought to be secondary craters
ejected from primary craters. However, except for two
small chains radial to Valhalla and a few other cases,
-
Figure 8.45. A mosaic of Galileo images (outlined) taken at 410m
per pixel, set in a background of Voyager coverage of part of the
Valhalla multi-ringed structure, showing the central bright zone (left
side), the inner ring zone, and the outer fracture zone (NASA ASU IPF
1148).
since its initial formation, partly owing to its lack of tidal
interactions
(Fig. 8.3)
.
The most prominent feature on Callisto is a multi-ring
impact structure named Valhalla
(Fig. 8.45)
. Valhalla con-
sists of a bright central zone (palimpsest) about 600 km
wide, surrounded by concentric rings extending outward
for nearly 2,000 km from the center of the structure. The
bright central zone probably marks the initial crater imme-
diately following impact. At least seven other multi-ring
features have been recognized on Callisto, including
Asgard with its central bright zone 230 km wide.
Tectonic and cryovolcanic features other than those asso-
ciated with impacts have not been detected. Thus,
Callisto
'
s surface consists of ancient heavily cratered ter-
rain composed of ice and rocky material with large, super-
posed multi-ring impact structures.
8.7.2 Impact features
Callisto
'
s impact craters
(Fig. 8.46)
have morphologies
similar to those on Ganymede. The smallest craters are
simple bowl-shaped to
at-
oored depressions that range
