Ganymede [gan-uh-meed] is a moon of Jupiter and the largest satellite in the Solar System. It has a diameter of 5,268 km (8% larger than Mercury), orbits Jupiter in roughly seven days, and is composed of approximately equal amounts of silicate rock and water ice. A saltwater ocean is believed to exist nearly 200 km below Ganymede’s surface, sandwiched between layers of ice. Ganymede is the only satellite in the Solar System known to possess a magnetosphere, likely created through convection within its liquid iron core. The meager magnetosphere is buried within Jupiter’s much larger magnetic field and connected to it through open field lines. The satellite has a thin oxygen atmosphere that includes O, O2, and possibly O3 (ozone).

Ganymede’s discovery is credited to Galileo Galilei, who was the first to observe it on January 7, 1610. The satellite’s name was soon suggested by astronomer Simon Marius, for the mythological Ganymede, cupbearer of the Greek gods and Zeus’s beloved. A joint NASA/ ESA mission to Jupiter’s icy moons (including Ganymede), the Europa Jupiter System Mission (EJSM), is proposed for a launch in 2020.

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