Ganymede is the largest moon of Jupiter's 63 known moons and is the also the largest moon in our solar system
Astronomer first Galileo Galilee observed Ganymede in 1610.
Ganymede was named, shortly thereafter by Simon Marius, after the favored cup bearer of the Greek gods who was favored by Zeus, and is the only one of Jupiter's Galilean moons named after a male figure
Scientists speculate that most likely Ganymede is mainly composed of a rock core with a water/ice mantle and a crust thick layer of rock and ice.
While it was thought that Ganymede had no known atmosphere, the Hubble Space Telescope recently detected an ozone layer on its surface. This ozone is produced when trapped charged particles, rain down from Jupiter's magnetic field on to the surface of the moon. When these charged particles pierce the icy surface, the particles of water are then disrupted leading to the production of the ozone. This chemical progression hints that Ganymede probably has a thin fragile oxygen atmosphere.
Ganymede has craters mountains valleys, and lava flows, and is dappled by both light and dark regions, which are heavily cratered. The bright regions is grooved with troughs and ridges, these features form intricate patterns and are a few hundred meters high, and are thousands of miles in length. This is speculated that by tension from moons geographic movement. In reality the real reason is still unknown; however the local crust spreading does appear to have taken place causing the crust to shear and separate.
The most prevalent feature on Ganymede is Galileo Regio, a dark plain region that is a remnant of an early impact but has been long since masked by geographical movement.
The diameter of Ganymede is 3,280 miles, and the low density, suggests that the core takes up about 50% of the moons diameter.
It is said that if Ganymede orbited the Sun instead of Jupiter it could be technically classified as a planet.