Geology Reference
In-Depth Information
Figure 8.48. A Galileo mosaic showing the Asgard multi-ringed
structure. Utgard is a degraded multi-ring structure partly obscured
by the fresh, bright-rayed crater, Burr. Tornarsuk crater is east of the
central zone of Asgard (NASA ASU IPF 1041).
Figure 8.49. A moderate-resolution (410m per pixel) image of the
Valhalla outer ring zone centered about 27° N, 27.3° W, showing
sinuous troughs, or grabens (NASA ASU IPF 1148).
impact craters, which might have weakened the ice locally
and in
uenced the development, location, and orientation
of the troughs during and following the Valhalla impact.
The inner ridge and trough zone of Valhalla extends
~950 km from the center of the structure and is character-
ized by asymmetric ridges with the steep
flanks facing
outward. Many of the ridges consist of a series of bright
knobs. The troughs are as long as several hundred kilo-
meters and as wide as 20 km. They are slightly sinuous
and appear to be grabens, similar to those of the outer
zone. Valhalla
'
s 360 km central zone has a mottled appear-
ance and is moderately cratered
(Fig. 8.50)
. Numerous
dark-halo craters suggest penetration into a darker sub-
strate at depths of 1 km or less. The central zone shows
remnants of degraded crater rims and small knobs of
unknown origin, all set in a background of smooth, low-
albedo plains. Although it has been suggested that some of
the plains associated with Valhalla are cryovolcanic, there
are no indications of
flows or similar features. Impact may
have occurred in thin, rigid ice overlying liquid water that
rapidly
filled the crater to form the plains.
The Asgard multi-ringed structure
(Fig. 8.48)
includes
central plains surrounded by discontinuous, concentric
scarps and an outer zone characterized by sinuous troughs.
The central plains are about 200 km across and are bright,
partly due to ejecta from a more recent impact crater, Doh,
which apparently excavated ice-rich (brighter) subsurface
materials.
The Adlinda multi-ringed structure is in the southern
hemisphere of Callisto, where it is de
ned by sets of
concentric lineaments and troughs, the most prominent
of which is a nearly continuous trough ~520 km long on
the southwest part of the structure. The Adlinda structure
has a lower frequency of superposed impact craters than
the surrounding terrain, suggesting its relative youth.
8.7.3 Gradation features
One of the surprises revealed by Galileo images of
Callisto was the paucity of small impact craters in some
areas.
Figure 8.50
compares terrains within Valhalla and
an area at about the same latitude west of Valhalla at the
same scale and resolution. Although the viewing geome-
try and illumination are not quite the same, it is clear that
small craters are present in some areas but absent in
