Jupiter has four main moons, which were discovered by Galileo, as well as many smaller ones (47 Jovian moons are currently known). Ganymede is Jupiter's largest moon and the largest known planetary satellite in the solar system. It is larger than the planets Mercury and Pluto, and is an icy world with some regions scarred by impact craters and others with parallel ridges and grooves formed by unknown tectonic proceses. This photograph, taken by Voyager 1, shows the hemisphere of Ganymede which points permanently away from Jupiter. Ganymede has its own magnetic field, and is believed to have a molten iron core. NASA's two Voyager spacecraft were launched in 1977 to explore the planets in the outer solar system.
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