Ganymede

It is jupiter's largest moon and also the solar system's largest moon at 5,268.2 km in diameter, which is even larger than Mercury. Despite its diameter being almost 400 km more than Mercury, it is only 1/3 the mass of mercury due to its low density.

History
Ganymede formed from a disk of gas gathered by Jupiter's gravity. It formed along with the other Galilean Moons around 4.503 billion years ago. Ganymede was probably the first to form, as it is very massive, and the largest is often the oldest.

Surface
Ganymede has an rocky-icy surface with 2 main sections. The light sections and the dark sections. The light sections are bright, icy, smooth, and less cratered. The dark sections are dark, rocky, rough, and very createred.

Ocean
Below the surface is a possibly salty ocean. The ice there didn't freeze because of tidal heating and heat from ganymede's core.

Magnetic field
Ganymede is the only moon in the solar system to have a magnetic field. While ganymede doesn't have a liquid outer core (which is normally the thing that creates magnetic fields), the best estimates say the magnetic field is caused by extreme tidal heat from Jupiter which melts parts of the core and causes a magnetic field to form.

Life?
Ganymede is a good candidate for alien life in the solar system, but not one of the best, as Europa and Enceladus is a better candidate for life.