Gilgamesh (crater)

Gilgamesh

A mosaic of Gilgamesh constructed from Voyager 2 imagery
Feature type	Multi-ring impact basin
Location	Ganymede
Coordinates	62°50′S124°50′W / 62.84°S 124.83°W / -62.84; -124.83 [1]
Diameter	~590 km (370 mi) [2] : 433
Depth	<2 km (1.2 mi) [2] : 435
Eponym	Gilgamesh, Mesopotamian hero

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 590 kilometres (370 mi), it is one of the largest known impact basins on Ganymede.

Observation and naming

On 9 July 1979, the Voyager 2 spacecraft encountered Jupiter and its system of moons. ^[3] During its flyby of Ganymede, it imaged two-thirds of Gilgamesh and the surrounding terrain to the east at a resolution of 550 metres (1,800 ft) 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. ^{[2] : 434  [4] : 867} 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. ^[1]

Characteristics

Gilgamesh is approximately 590 kilometres (370 mi) 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 150 kilometres (93 mi) across. The central plain domes upward by up to 500 metres (1,600 ft), ^{[5] : 42} and several small hills are arranged in a 50 kilometres (31 mi) diameter arc surrounding the center. ^{[6] : 391} Surrounding the central plain is a discontinuous inward-facing scarp averaging 1 kilometre (0.62 mi) high, marking the inner boundary of a 225-kilometre-wide (140 mi) annular region of hummocky terrain hosting discontinuous concentric ridges and angular mountains. Another prominent inward-facing scarp marks the outer boundary of this zone, averaging 1 km in height and about 585 kilometres (364 mi) 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. ^{[2] : 433–434} Structurally, Gilgamesh resembles Argyre Planitia, which also exhibits numerous discontinuous concentric ridges. ^{[5] : 42}

Surrounding Gilgamesh is a 200-kilometre-wide (120 mi) ring of mottled terrain where the pre-existing grooved terrain has been covered or obliterated by Gilgamesh's ejecta deposit. ^{[2] : 434} A vast field of secondary craters and crater chains extends up to 400–1,000 kilometres (250–620 mi) from the basin. ^{[6] : 391} Most of the secondary craters and crater chains are concentrated to the north and south of the basin; ^{[4] : 867} no secondary craters are identifiable to Gilgamesh's west, suggesting that it was formed by an oblique impact. ^{[2] : 434} Many of the secondary craters are larger than a few kilometers in size and are thus complex craters ^{[4] : 870} 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 360 metres (1,180 ft). This maximum fragment size implies that secondary craters formed by Gilgamesh's impact event are no larger than 2–3 kilometres (1.2–1.9 mi) in diameter. ^{[4] : 877–879}

Gilgamesh, by the law of superposition, is younger than the surrounding grooved terrain, as no grooves cut into its structure. ^{[6] : 484} 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). ^{[6] : 517} The projectile that formed Gilgamesh is estimated to have been around 60 kilometres (37 mi) in size. ^{[2] : 451} 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. ^{[6] : 484} 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. ^{[6] : 505}

See also

• Mare Orientale – a well-preserved multi-ring impact basin on the Moon
• Burney – a multi-ring impact basin on Pluto

References

1. "Gilgamesh". Gazetteer of Planetary Nomenclature. USGS Astrogeology Research Program. (Center Latitude: –62.84°, Center Longitude: 124.83°; Planetographic, +West)
2. Bagenal, Fran; Dowling, Timothy E.; McKinnon, William B., eds. (2004). Jupiter: The Planet, Satellites and Magnetosphere. Cambridge University Press. Bibcode:2004jpsm.book.....B. ISBN   978-0521035453.
3. "Voyager 2". NASA's Solar System Exploration website. Archived from the original on 20 April 2017. Retrieved 4 December 2022.
4. Singer, Kelsi N.; McKinnon, William B.; Nowicki, L. T. (September 2013). "Secondary craters from large impacts on Europa and Ganymede: Ejecta size–velocity distributions on icy worlds, and the scaling of ejected blocks". Icarus. 226 (1): 865–884. Bibcode:2013Icar..226..865S. doi:10.1016/j.icarus.2013.06.034.
5. Thomas, P. G.; Forni, O. P.; Masson, P. L. (January 1986). "Geology of Large Impact Craters on Ganymede: Implications on Thermal and Tectonic Histories". Earth, Moon, and Planets. 34 (1): 35–53. Bibcode:1986EM&P...34...35T.
6. Morrison, David, ed. (1982). Satellites of Jupiter (PDF). University of Arizona Press. ISBN   978-0-8165-5451-5. LCCN   81013050.