This color reconstruction of part of Ganymede’s northern hemisphere, taken by Voyager 2, encompasses an area about 1300 kilometers (800 miles) across. It shows part of a dark, densely cratered region that contains numerous craters, many with central peaks. The large bright circular features have little relief and are probably the remnants of old, large craters that have been annealed by the flow of icy material near the surface. The gradually curving lines that press through the dark region suggest the presence of a large impact basin to the southwest, which has been obliterated by the subsequent formation of younger grooved terrain.
3/5/79 165,000 km (103,000 mi)
A broad, north-south strip of grooved terrain on Ganymede, offset by a traversing fault in the upper part of the picture, is shown in this Voyager 1 photograph. There are several other perpendicular fault lines farther down on the fault. Within the major light stripes, the more closely spaced, shallow grooves run parallel to the boundaries of the stripes. The larger striped features divide the cratered terrain into isolated polygons several hundred to about 1000 kilometers (600 miles) across.
3/5/79 145,000 km (90,000 mi)
The grooved terrain at higher resolution emphasizes numerous interwoven linear features in this Voyager 1 picture, near the terminator on Ganymede. This suggests an early period in Ganymede’s history when the crust was active and mobile, resembling Earth’s plate tectonics in some ways. The causes of the extreme differences in crustal evolution between Callisto and Ganymede are under investigation. Combinations of radioactive heating and a greater degree of tidal heating for Ganymede are possibilities.
7/9/79 100,000 km (62,000 ml)
This mosaic of Ganymede, composed of photographs taken by Voyager 2, shows numerous impact craters, many with bright ray systems. The rough terrain at the lower right is the outer portion of a large, fresh impact basin that postdates most of the other terrain. The dark patches of heavily cratered terrain (right center) are probably ancient mixtures of ice and rock formed prior to the grooved terrain. The large rayed crater at the upper center is about 150 kilometers (95 miles) in diameter.