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Answer: Ganymede, a moon of Jupiter, is the largest moon in the solar system, bigger than the planet Mercury and the dwarf planet Pluto. It is the only moon known to have its own magnetic field, and there is strong evidence it hides an underground saltwater ocean.

Key facts

• Magnetic field and auroras: a metallic iron core deep inside Ganymede generates the moon's magnetic field. The field causes auroras, bright ribbons of glowing gas that circle the moon's poles.
• Layers: a shell of rock surrounds the core, and another icy shell surrounds the rock.
• Ocean: the underground saltwater ocean may hold more water than all the water on Earth's surface. Ganymede might even have ice and oceans stacked up in several layers like a club sandwich.
• Atmosphere: there is evidence Ganymede has a tenuous oxygen atmosphere.

Exploration

Pioneer 10 had its closest encounter with Jupiter on Dec. 3, 1973, and was the first spacecraft to obtain close-up images of Jupiter and three of its largest moons, including Ganymede. Several NASA probes have explored Jupiter and its moons since. The Juno spacecraft was most recent, taking detailed photographs of Ganymede in June 2021 that show craters, distinct dark and bright terrain, and long structural features possibly linked to tectonic faults.

Place in the Jupiter system

Ganymede is one of the four Galilean satellites, first observed by Galileo Galilei in 1610.

Claims

• id: C1 claim: Ganymede is the largest moon in the solar system. source: S2 evidence: "Ganymede is Jupiter's largest moon, and the largest moon in our solar system."
• id: C2 claim: Ganymede is the only moon known to have its own magnetic field. source: S1 evidence: "Ganymede is the only moon known to have its own magnetic field"
• id: C3 claim: A metallic iron core generates the magnetic field. source: S1 evidence: "Deep inside Ganymede, there's a metallic iron core that generates the moon's magnetic field."
• id: C4 claim: There is evidence of a tenuous oxygen atmosphere on Ganymede. source: S1 evidence: "There is evidence Ganymede has a tenuous oxygen atmosphere."

Sources

• [S1] Ganymede. NASA. https://science.nasa.gov/jupiter/moons/ganymede/ . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Jupiter Moons. NASA. https://science.nasa.gov/jupiter/jupiter-moons/ . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_msb0swjr,
  author = {{iinventedlight}},
  title = {{Ganymede: the largest moon in the solar system}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/ganymede-moon/},
  note = {Permanent ID iil:note:msb0swjr. License: CC-BY-4.0.},
}

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