The first detailed imagery of the Galilean satellites of Jupiter was by the Voyager I mission in 1979. Outermost Callisto turned out to be what everyone expected, a dark, heavily cratered icy ball.
And that was about the last thing anyone saw in the outer Solar System that looked like we expected.
Ganymede, which nudged out Saturn's satellite Titan for the title of largest satellite in the Solar System by just a few kilometers, turned out to be the only body apart from Earth to show evidence of large scale horizontal crustal movements. Europa turned out to be criss-crossed by cracks and have almost no craters, but Io was the most surprising of all. It had no craters at all, a bizarre colored surface that many likened to a pizza, and the only active volcanism seen anywhere but Earth.
By custom, all objects discovered in historic time have a feature on them named for its discoverer. On Ganymede, the large dark areas are given the designation Regio (Latin: region) and the largest is named Galileo Regio. Others (Barnard, Perrine, Nicholson, Melotte) are named after discoverers of Jupiter's smaller moons. Marius Regio is named for Simon Marius, the astronomer who suggeted the mythological names used for the Galilean satellites. Galileo, no dummy, wanted to call them the "Medicean stars," hoping to gain favor and patronage from the powerful Medici family, but nobody outside of Florence agreed.
The Galilean satellites were the first new objects in the Solar System ever discovered. As satellites, they should be named after figures associated with Jupiter but subordinate to him. If you know your mythology, right away you know that rules out his wife Juno (she did get an asteroid in her honor). Jupiter's favorite hobby, after ruling Olympus and hurling thunderbolts, was having love affairs, and Juno's favorite hobby was breaking them up and generally inflicting awful vengeance on the lovers. The four Galilean satellites were named for lovers of Jupiter. Ganymede was a boy. The Greeks and Romans were okay with that. The number of small satellites eventually became too great for even Jupiter's prodigious libido, so female mythological names in general are now used.
Features other than large regions on Ganymede are named for figures from Egyptian and Middle Eastern myths.
Ganymede's history consists of three episodes: formation of crust, widespread tectonic activity, and later cratering. Its history is divided into three epochs:
Dark Terrain: The oldest crust on Ganymede. Fractured into large rafts, many of which can be fitted back together. Despite the tantalizing similarity to plate tectonics, there's no evidence of subduction or axial spreading centers. Subdivided into cratered, lineated, and undifferentiated.
Bright Terrain: Belts of lineated terrain separating blocks of dark crust. There are numerous belts cutting and intersecting each other. Subdivided into grooved, subdued and irregular. Grooved terrain has prominent relief, subdued terrain has faint relief, and irregular terrain is a mixture of the two on scales too small to resolve.
Reticulate Terrain: Terrain with criss-crossing grooves.
Palimpsests: Ancient impact craters mostly flattened by crustal relaxation. They typically consist of a bright circular region, often with concentric fractures or lineaments, and sometimes a smooth inner core.
Gilgamesh Basin: Largest impact crater on Ganymede. Consists of a smooth outer annulus, a wide ring of blocky topography, and a smooth inner plain.
Collins, G.C., Patterson, G.W., Head, J.W., Pappalardo, R.T., Prockter, L.M., Lucchitta, B.K., and Kay, J.P., 2013, Global geologic map of Ganymede: U.S. Geological Survey Scientific Investigations Map 3237, pamphlet 4 p., 1 sheet, scale 1:15,000,000, http://dx.doi.org/10.3133/sim3237. These maps were created from the accompanying on-line geospatial database using Global Mapper v. 14.
Created 11 April 2014, Last Update
25 February 2015
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