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termed palimpsests (Fig. 8.35) . These are circular patches
of bright terrain as wide as several hundred kilometers,
found principally in dark terrain. The term was borrowed
from historic literature in which palimpsest refers to traces
of writing preserved on materials such as parchment. In a
form of recycling in the Middle Ages, such material was
reused by scraping the original
seldom complete, and some of the writing was still visible.
Similarly, palimpsests on Ganymede preserve some of the
ancient landforms that have been incompletely overwrit-
ten by more recent processes.
Palimpsests range in size from 50 to 400 km in diameter
and occur as circular-to-oval patches with little topo-
graphic expression. Most have a central bright zone com-
posed of relatively clean ice excavated from the
subsurface. From impact crater mechanics, this suggests
that the icy crust was about 10 km thick at the time of
impact and was underlain by water or slush.
text, but erasure was
craters (Fig. 8.36) , which have pronounced dome-shaped
floors, are probably related to palimpsests.
Impact basins are also present on Ganymede. For exam-
ple, Gilgamesh consists of a central depression 150 km
wide, filled with smooth plains and surrounded by rugged,
mountainous ejecta deposits. A distinct ring fracture zone
found at 275 km radius probably represents the boundary
for the excavation of the basin. Chains of secondary
craters and individual secondaries can be traced to dis-
tances of 1,000 km.
Dome
8.6.3 Tectonic features
Figure 8.29. The boundary between the furrowed dark terrain of
Galileo Regio (upper right) and the younger bright terrain of Uruk
Sulcus (lower left): furrows of the Lakhmu Fossae system trend
subparallel to the northern boundary of Uruk Sulcus, while the Zu
Fossae cut them obliquely; crater forms include bright rays, central
pits, and dark floors. The inset shows high-resolution Galileo images
and details of the bright terrain (NASA PIA00705).
The oldest surface features on Ganymede are sets of
semi-concentric furrows (Fig. 8.37) , which are consid-
ered to represent structural adjustments of the icy crust to
impacts. However, because much of the dark terrain is
disrupted by bright terrain, the furrows are incomplete
Figure 8.30. Impact crater depth
diameter data
for Galilean satellites compared with the Moon.
T1, T2, and T3 identify changes in morphology and
shape. T1 shows simple
-
complex transition. T2
shows change from complex to central pit and
dome morphology. T3 shows transition to large
basins or multi-ring morphologies (from Schenk
et al., 2004 ).
-
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