Ganymede is the biggest moon in the Solar System with a diameter of 5,268km. It is the 7th planet from Jupiter and completes an orbit in around 7 days.
Ganymede has three main layers that make it up, a metallic liquid iron core, a shell of rock that surrounds the core and a shell of ice that surrounds the rock.
The craters on Ganymede are quite flat. This is probably due to the gradual adjustment of the soft icy surface.
Ganymede also has a thin atmosphere of Oxygen, but this is far too thin to support life.
Ganymede is the only moon to have its own magnetic field. This is believed to have been generated by convection currents in the iron core.