Ganymede (moon) has an ionosphere associated with its atmosphere is unresolved. 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, after the mythological Ganymede, cupbearer of the Greek gods, kidnapped by Zeus for the purpose. Beginning with "Pioneer 10", several spacecraft have explored Ganymede. The Voyager probes, "Voyager 1" and "Voyager 2", refined measurements of its size, while "Galileo" discovered its underground ocean and magnetic field. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer
Ganymede (moon) Relatively young craters with rays of ejecta are also visible. Ganymedian craters are flatter than those on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust, which can (or could) flow and thereby soften the relief. Ancient craters whose relief has disappeared leave only a "ghost" of a crater known as a palimpsest. One significant feature on Ganymede is a dark plain named Galileo Regio, which contains a series of concentric grooves, or furrows, likely created during a period of geologic activity. Ganymede also has polar caps, likely composed of water frost.