The largest satellite of any planet in the solar system is Ganymede, one of Jupiter’s moons. Measuring 3273 miles (5268 km) across, it is by volume a bit larger than Mercury, about half the size of Mars, and about three and a half times the size of our own moon.
Ganymede has a metallic core that supports a magnetic field, an ice and silicate mantle, and a thick crust that is mostly ice. Its very thin oxygen atmosphere is produced by high-energy particles that break up surface water molecules.
Source: The Learning Kingdom’s Cool Fact of the Day for August 14, 1998