Ganymede is the largest natural satellite of Jupiter and in the Solar System. It is also the only known moon having a magnetosphere. Actually, this natural satellite is 8% larger than Mercury, but it has only 45% of its mass. Ganymede's diameter is 2% larger than that of Titan, the largest moon of Saturn and the second-largest natural satellite of the whole Solar System. By the way, Titan is a moon which has a dense atmosphere, and that is also unique for our planetary system.


Ganymede

More Info: https://en.wikipedia.org/wiki/Ganymede_(moon)