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
Titan (moon) in diameter, 1.06 times that of the planet Mercury, 1.48 that of the Moon, and 0.40 that of Earth. Before the arrival of "Voyager 1" in 1980, Titan was thought to be slightly larger than Ganymede (diameter ) and thus the largest moon in the Solar System; this was an overestimation caused by Titan's dense, opaque atmosphere, which extends many kilometres above its surface and increases its apparent diameter. Titan's diameter and mass (and thus its density) are similar to those of the Jovian moons Ganymede and Callisto. Based on its bulk density of 1.88 g/cm, Titan's composition is half