Gilgamesh is a large multi-ring impact basin on Jupiter's largest moon Ganymede. Named after Gilgamesh, a hero in ancient Mesopotamian mythology and protagonist of the Epic of Gilgamesh, the impact structure was observed in detail by the Voyager 2 and Galileo missions, enabling its characteristics to be studied in detail. With an estimated diameter of around , it is one of the largest known impact basins on Ganymede.
On 9 July 1979, the Voyager 2 spacecraft encountered Jupiter and its system of moons. During its flyby of Ganymede, it imaged two-thirds of Gilgamesh and the surrounding terrain to the east at a resolution of per pixel. Later, the Galileo orbiter acquired low-resolution imagery of Gilgamesh's eastern ejecta blanket and low phase angle images of its central basin. The crater was named after the Ancient Mesopotamian character and hero Gilgamesh, protagonist of the Epic of Gilgamesh. The name was adopted by the International Astronomical Union (IAU) in 1979.
Gilgamesh is approximately in diameter, making it the largest fully preserved impact structure on Ganymede. It is a multi-ring impact basin, with at least 4 concentric rings surrounding a smooth central plain across. The central plain domes upward by up to , and several small hills are arranged in a diameter arc surrounding the center. Surrounding the central plain is a discontinuous inward-facing scarp averaging high, marking the inner boundary of a annular region of hummocky terrain hosting discontinuous concentric ridges and angular mountains. Another prominent inward-facing scarp marks the outer boundary, averaging 1 km in height and about in diameter. Some segments of this scarp are quasi-linear, suggesting that pre-existing terrain may have influenced its shape. The location and prominence of the outer scarp suggests it is likely Gilgamesh's rim. The angular mountains are long, and their long axes point radially toward the basin center. Structurally, Gilgamesh resembles Argyre Planitia on Mars, which also exhibits numerous discontinuous concentric ridges.
Gilgamesh is surrounded by a ring of mottled terrain where the pre-existing grooved terrain has been covered or obliterated by its ejecta blanket. A vast field of secondary craters and crater chains extends up to from the basin. Most of the secondary craters and crater chains are concentrated to the north and south of the basin; no secondary craters are identifiable to Gilgamesh's west, suggesting that it was formed by an oblique impact. Many of the secondary craters are larger than a few kilometers in size and are thus complex craters A 2018 study led by planetary scientist Kelsi N. Singer calculated the theoretical size of fragments blasted out by Gilgamesh's formative impact event, yielding a maximum fragment size of . This maximum fragment size implies that secondary craters formed by Gilgamesh's impact event are no larger than in diameter.
Gilgamesh, by the law of superposition, is younger than the surrounding grooved terrain since no grooves cut into its structure. In combination with the crater counting method on younger craters superposed on the basin rim, Gilgamesh's constrained age likely lies between 3.5 and 3.2 billion years (Gyr). The projectile that formed Gilgamesh is estimated to have been around in size. Gilgamesh is structurally distinct from large impact basins on Callisto such as Valhalla, which have organized incomplete concentric rings and moderate topographical relief. Gilgamesh's closer resemblance to large impact basins on Earth's Moon indicates that at the time of its formation, Ganymede's crust was thicker and more rigid than Callisto's at the time of Valhalla's formation. The preservation of Gilgamesh's structure post-formation may be partially due to its location in Ganymede's polar regions where the crust cooled more rapidly, preventing viscous relaxation.