Ganymede and Callisto 463
FIGURE 10 This irregular pit is one of several found along the edges of smooth terrain in Sippar
Sulcus on Ganymede. Faint curved ridges within the pit suggest flow folding in the surface of a
cryovolcanic flow emanating from the closed end of the pit and flowing out the open end, into the
surrounding smooth terrain.appears to be more moderate. Circumstantial evidence ex-
ists for contractional deformation in a few areas, but it is
not widespread.
These observations force us to ask how the crust of
Ganymede could have undergone a large amount of ex-
tension with very little evidence for contraction to balance
it out. There are a few possible solutions to this conun-
drum. One solution is that Ganymede actually expanded
during the formation of grooved terrain. Differentiation of
Ganymede’s interior or melting of high pressure ices can
serve to increase the volume of the satellite, leading to an
increase in surface area and thus stretching of the crust.FIGURE 11 Views of bright grooved terrain on Ganymede. (a)Voyager 2imaged this region of
grooved terrain in Uruk Sulcus; (b)Galileoimaged these grooves in the central part of (a), shown at
ten times the scale.