9. Gilgamesh Basin, Southern Hemisphere, Ganymede
The crust of Jupiter's moon Ganymede (the largest moon in the solar system) is composed mainly of water ice. Situated on the plains of its southern hemisphere is the remarkable Gilgamesh basin, a multiring impact basin nearly 600 kilometers in diameter. This image, taken by the Galileo spacecraft, is illuminated from the left, and shows only the western two-thirds of the structure. The 144-kilometer-wide circular central depression is flanked by a broad rugged annulus of comparable width. This annulus exhibits arcuate structural features suggestive of the terraces observed in lunar impact basins. Radial features emanating from the rim of the broad ring are probably surface scour marks associated with the emplacement of debris ejected from the impact site during crater formation. Numerous 10–30-kilometer impact craters of various sizes and degradation states are seen on the surface of Gilgamesh, indicating that the large basin is a relatively ancient feature.
Mosaic of Voyager 2 images. Image processing by Paul Schenk, Lunar and Planetary Institute.