Jupiter’s moon Ganymede (“GAN uh meed”), and it is the largest moon in our solar system. Ganymede was discovered by Galileo Galilei on January 7, 1610.
For an Earth-based observer, it always appears in the same spot during each Jovian rotation; at the same time every other satellite would appear to move around their orbit. This suggested that it might be a spherical object like Earth’s moon, and not a more complex shape like a ring or doughnut due to its higher-quality image compared to those observed for other planets.
Ganymede is almost twice the size of Earth’s moon. It’s bigger than Mercury and Pluto.
Ganymede has a rocky core, but most of it is an icy shell with very deep craters which means something like 4% of its volume (mass) consists of water ice. For comparison, Earth’s moon is about 3x smaller in diameter than Ganymede–but the latter only reflects much less light back to space because it has nothing like Earth’s atmosphere to trap much heat-causing infrared radiation.