Short answers, pulled from the story.
Galileo Galilei first observed Ganymede on the 7th of January 1610. He initially saw three faint points of light near Jupiter before realizing they were moons moving relative to the planet over subsequent nights.
Ganymede has an iron-rich liquid core surrounded by a silicate mantle and outer layers of water ice. Scientists suspect a subsurface ocean exists beneath the icy crust that potentially contains more water than all of Earth's oceans combined.
The Galileo spacecraft discovered that Ganymede possesses a permanent magnetic field independent of Jupiter's influence. This intrinsic magnetosphere is three times stronger than Mercury's magnetic moment despite Ganymede being much smaller.
Dark cratered regions cover about one-third of the moon and date back four billion years. Lighter grooved terrain spans roughly two-thirds of the surface and formed slightly less than 4 billion years ago through tectonic activity.
The JUICE mission launched on the 14th of April 2023 and plans to enter orbit around Ganymede by 2032. The spacecraft is scheduled to impact the moon in February 2034 after completing its orbital operations.