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The largest satellite of Jupiter, discovered by Galileo Galilei on January 7, 1610. It has a diameter of 5262 km and a mass of 1023 kg ∿1,5 ∙. The semi-major axis measuring 1.07 million km, the orbital period of 7.16 days. Its density suggests that it is composed for about half ice and for the other half of rocky material. The reflectivity of the surface of Ganymede is, on average, about 40%. It is however not uniform: stand on the surface of the satellite, the darkest and lightest areas. These latter are crossed by long chains hilly high few hundred meters. The dark regions are different from light areas because they are rich in craters in them, even the density of impact craters reaches, as in the lunar highlands, the ‘saturation’ (ie, the number of craters can not rise further because the new craters erase some of the previous ones). The morphological characteristics of the surface of Ganymede imply that the satellite has had an intense geological activity, at least during the first billion years of his life. The color difference between the older and younger regions depend regions by the fact that the latter are covered by more ‘clean ice’ because more recently erupted and, therefore, less contaminated by the debris produced by the impacts of meteorites. Ganymede is the largest natural satellite of the planet Jupiter and the largest of the solar system; It exceeds in size the same Mercury. Ganymede completes an orbit around Jupiter in just over seven days and is in orbital resonance 1: 2: 4 with Europa and Io, respectively. Composed mainly of silicates and ice water, it is totally different with a molten iron core. It is believed that an ocean of salt water exists at about 200 km depth from the surface, between two ice layers.