Short answers, pulled from the story.
Ganymede is a natural satellite of Jupiter and the largest and most massive moon in the Solar System, with a diameter of about 5,270 km, making it larger than the planet Mercury. It is the only moon known to possess an internally generated magnetic field, and its subsurface ocean may contain more water than all of Earth's oceans combined.
Galileo Galilei first observed Ganymede with a telescope on the 7th of January 1610, and by the 15th of January he concluded it was orbiting Jupiter. Simon Marius also claimed discovery of the Galilean satellites independently. Galilean credit is shared between both astronomers.
Ganymede was named by Simon Marius, following a naming proposal by Johannes Kepler, after Ganymede, a Trojan prince in Greek mythology whom Zeus, taking the form of an eagle, carried off to serve as cupbearer to the gods. The name was not in common astronomical use until the mid-20th century; earlier literature referred to the moon as Jupiter III.
Yes. In March 2015, measurements from the Hubble Space Telescope confirmed a subsurface saltwater ocean by tracking how the ocean's influence on Ganymede's magnetic field shifted its polar auroras. A 2014 analysis suggested the ocean may consist of multiple layers separated by different phases of ice, with the deepest liquid layer resting against the rocky mantle at a depth of roughly 800 km.
Ganymede's magnetic field is generated by convection in its liquid iron-nickel-rich core, the same basic dynamo process that produces Earth's magnetic field. Its equatorial field strength is 719 plus or minus 2 nanoteslas, about three times greater than Mercury's magnetic moment. Why the core remains fluid enough to sustain a dynamo is still debated; tidal heating from past orbital resonances may have kept the core warm.
Pioneer 10 flew by in 1973 and Pioneer 11 in 1974, followed by Voyager 1 and Voyager 2 in 1979. The Galileo spacecraft made six close flybys between 1996 and 2000, with its G2 pass coming within 264 km of the surface. Juno conducted flybys in 2019 and in June 2021, the latter at a distance of 1,038 km. ESA's JUICE, launched on the 14th of April 2023, is planned to become the first spacecraft to orbit Ganymede, arriving in 2032.