Ganymede is a moon of Jupiter and the largest and most massive satellite in the Solar System. It is the largest Solar System object without a substantial atmosphere and also the only moon in the Solar System with a substantial magnetic field. Like Titan, Saturn's largest moon, it is larger than the planet Mercury but, due to its lower density, has somewhat less surface gravity than Mercury, Io, or the Moon. Ganymede is composed of silicate rock and water in approximately equal proportions. It is a fully differentiated body, with an iron-rich liquid core and an internal ocean. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with Europa and Io. This image, a composite of three photographs taken by the NASA space probe Juno during a flyby in 2021, depicts the northern hemisphere of Ganymede roughly centered around the prime meridian.Photograph credit: NASA / JPL-Caltech / SwRI / MSSS / Kevin M. Gill