Which Galilean moon is the largest moon in the Solar System?

Ganymede is the largest moon in the Solar System.

Ganymede is a natural satellite of Jupiter and is larger in diameter than Mercury, although Mercury is much more massive. It is also the most massive moon known, making it the clear answer among the Galilean moons listed here. Jupiter’s other major Galilean satellites are Io, Europa, and Callisto.

Galileo Galilei observed Ganymede in January 1610, during his pioneering telescopic observations of Jupiter. Simon Marius independently observed the moon around the same time. Ganymede orbits Jupiter roughly once every seven days and participates in an orbital resonance with Io and Europa.

Its size is not its only distinction. Ganymede is the only moon known to generate its own intrinsic magnetic field, probably through motion in its liquid metallic core. Evidence also indicates a deep subsurface ocean. A common mix-up is Titan, Saturn’s largest moon: Titan has a thicker atmosphere, but Ganymede is larger.