Ganymede

Identity and discovery

Ganymede is Jupiter’s natural satellite and the largest moon in the Solar System, with a diameter exceeding Mercury’s. NASA dates Galileo Galilei’s discovery to January 7, 1610. It is third from Jupiter among the four Galilean moons. Its identifier is NAIF 503; 599 identifies Jupiter, while MOON-ATLAS is an internal catalog key, not an official designation.

Size and physical parameters

JPL lists a mean radius of 2631.20 ± 1.70 km, giving a calculated equivalent diameter of 5262.40 ± 3.40 km. Mean density is 1.9416 ± 0.0038 g/cm³ and gravitational parameter GM is 9887.83275 ± 0.00247 km³/s². GM is not mass in kilograms; the tabulated density is derived from GM and radius. These are whole-body properties, not the composition of individual layers.

Orbit and rotation

JPL’s JUP365 mean elements give a semimajor axis of 1070400 km and a period of 7.155588 days, in the Laplace plane at epoch 2000-01-01.5 TDB. The semimajor axis is measured from Jupiter’s center, not above its clouds. Mean elements are not ephemerides for precise position calculations. Ganymede rotates synchronously, keeping the same side toward Jupiter. Its resonance with Europa and Io links approximately one Ganymede orbit to two Europa orbits and four Io orbits.

Interior and ocean

NASA describes a metallic iron core, rocky mantle and icy shell. Galileo’s magnetic observations and the auroral motion observed by Hubble support a subsurface saltwater ocean. Estimates of an ocean about 100 km thick beneath roughly 150 km of ice are model interpretations, not direct soundings. Models with several ice and water layers are also studied. Possible contact between water and rock matters for astrobiology, but does not establish the presence of life.

Surface and environment

Dark, heavily cratered terrain contrasts with bright regions cut by grooves and ridges, interpreted as signs of tectonic deformation. Hubble provides evidence for a thin oxygen atmosphere, not a dense Earth-like atmosphere. In 1996 Galileo discovered Ganymede’s intrinsic magnetic field, embedded within Jupiter’s field. Its auroras respond to that changing environment and help probe the interior.

Exploration and open questions

Juno’s June 7, 2021 flyby collected images and spectral data. NASA reports salts and organic compounds observed by JIRAM in results published in 2023. These are surface observations, not an ocean sample or a detection of organisms. Formation from leftover material around young Jupiter is the scenario NASA describes. Reconstructing the distribution of internal water and the history of the terrains requires combining models and observations; these profile values are not new discoveries from October 2026.