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.] [Illustration: 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.] [Illustration: 7/8/79 85,000 km (53,000 mi) Curved troughs and ridges in this high-resolution Voyager 2 photograph of Ganymede are the distinctive characteristics of an enormous, ancient impact basin. The basin itself has been eroded by later geologic processes; only the shock ring features are preserved on the ancient surface. Near the bottom of the picture these curved markings are perforated with the younger, grooved terrain.] [Illustration: 7/7/79 2.3 million km (1.4 million mi) Callisto, only slightly smaller than Ganymede, has the lowest density of all the Galilean satellites, implying that it has large amounts of water in its bulk composition. Its surface is darker than the other Galilean satellites, although it is still twice as bright as our Moon. This Voyager 2 image shows Callisto to have the most heavily cratered and, therefore, the oldest surface of the Galilean satellites, probably dating back to the period of heavy meteoritic bombardment ending about four billion years ago.] [Illustration: 3/6/79 350,000 km (217,000 mi) The prominent concentric ring structure shown in this Voyager 1 four-picture mosaic of Callisto is believed to be a large impact basin, similar to Mare Orientale on the Moon and Caloris Basin on Mercury. The bright circular spot is about 600 kilometers (360 miles) across, and the outer ring is about 2600 kilometers (1560 miles) across. This is the first recognized basin in the Jovian system and supports the assumption that Callisto’s surface is old. The lack of high ridges, ring mountains, or a large central depression suggests that the impacting body caused melting, some flow, and shock waves, and that the refreezing occurred in time to preserve the concentric shock rings.]