Short answers, pulled from the story.
Ganymede was discovered on the 7th of January 1610 by Galileo Galilei and Simon Marius. Galileo Galilei observed the moon while pointing his telescope toward Jupiter, and Simon Marius claimed to have observed the moons independently.
Ganymede is the only natural satellite in the Solar System to possess an internally generated magnetic field. This magnetic field creates a tiny magnetosphere embedded within Jupiter's massive one and is generated by an iron-rich core.
Ganymede's subsurface ocean may contain more water than all of Earth's oceans combined. This ocean is sandwiched between layers of ice and is likely several kilometers deep.
Ganymede participates in a complex orbital resonance known as the Laplace resonance with Europa and Io. For every orbit of Ganymede, Europa orbits twice and Io orbits four times.
The European Space Agency's JUICE mission is set to enter orbit around Ganymede in 2032. The mission was launched on the 14th of April 2023 and is intended to perform its first flyby of Ganymede in 2031.