№ 12 Moon2026.04.30
Ganymede (moon)
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Ganymede

Distance
orbits Jupiter in roughly seven days
Size
largest and most massive moon in the Solar System; larger than the planet Mercury
Discovered
known since antiquity (not specified in provided material)

Ganymede is the largest and most massive moon in the Solar System. Its internal ocean potentially contains more water than all of Earth's oceans combined.

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.

A moon larger than Mercury, with an iron-rich liquid core—and an ocean that potentially holds more water than all of Earth’s oceans combined.
Unseen
No observational guidance is provided in the supplied material, and no naked-eye or amateur viewing details can be stated from it.