Ganymede is a satellite of Jupiter and the largest moon in the Solar System with a diameter of 5,265 km. At 3,280 miles wide,it is 8% larger than that of the planet Mercury and and almost as large as Mars. Its complex icy surface is criss crossed by dark regions covered with craters and lighter areas lined with grooves and ridges. The second biggest moon of the Solar System is Titan. Ganymede's diameter is 2% larger than that of Titan.

Galileo Galilei discovered it on 7 January 1610. The discovery, along with three other Jovian moons, was the first time a moon was discovered orbiting a planet other than Earth. The discovery of the four Galilean satellites eventually led to the understanding that planets in our solar system orbit the sun, instead of our solar system revolving around Earth. The satellite's name was soon suggested by astronomer Simon Marius, for the mythological Ganymede, cupbearer of the Greek gods and Zeus's lover.

Ganymede is composed of approximately equal amounts of silicate rock and water ice. It is a fully differentiated body with an iron-rich, liquid core. It is the only satellite in the Solar System known to possess a magnetosphere, likely created through convection within the liquid iron core.

The satellite has a thin oxygen atmosphere that includes O, O2, and possibly O3 (ozone). Atomic hydrogen is a minor atmospheric constituent. Whether the satellite has an ionosphere associated with its atmosphere is unresolved. Ganymede orbits Jupiter at a distance of 1,070,400 km, third among the Galilean satellites and completes a revolution every seven days and three hours. Ganymede has been visited by several spacecraft as they orbited or flew by Jupiter. Pioneer 10 and 11 approached the moon in 1973 and 1974. They returned the first images of the satellite, but they lacked detail to say the least. Voyager 1 and 2 provided better measurements of the moon’s size, elevating it to the largest in the Solar System. Galileo discovered the magnetic field and the subsurface ocean.

New Horizons made a fly by in 2007 returning topography and composition maps of the moon. Ganymede even has a very thin oxygen atmosphere as well as its very own magnetosphere, generated by a molten core made up of heavy conductive metals. It’s been said that if Ganymede were to be orbiting the sun instead of Jupiter, it would easily be classified as a planet in its own right.The average density of Ganymede, 1.936 g/cm3. Ganymede is most likely composed of a rocky core with a water/ice mantle and a crust of rock and ice. Its slow density of 1.94 gm/cm3, indicates that the core takes up about 50% of the satellite's diameter. Ganymede's mantle is most likely composed of ice and silicates, and its crust is probably a thick layer of water ice.