Ganymede

Ganymede is a natural satellite of Jupiter. It is larger than the planet Mercury, yet has somewhat less surface gravity than Mercury, Io, or Earth's Moon because its density is lower than those three; it orbits Jupiter in roughly seven days and participates in a 1:2:4 orbital resonance with Europa and Io, respectively.

The provided source material describes Ganymede’s physical and orbital properties, its composition, and the presence of an internally generated magnetic field, but it does not name the instruments, spacecraft, or observational techniques by which these facts were established. It does state that Ganymede’s magnetic field is probably created by convection within an iron-rich, liquid metallic core and influenced by tidal forces from Jupiter’s far greater magnetic field, and that whether Ganymede has an ionosphere associated with its thin oxygen atmosphere is unresolved.