Jupiter’s icy moon Ganymede is the largest moon in our solar system, even bigger than the planet Mercury, and the dwarf planet Pluto. There’s strong evidence that Ganymede has an underground saltwater ocean that may hold more water than all the water on Earth's surface.
Ganymede is the only moon known to have its own magnetic field – something typically found on planets like Earth. The magnetic field causes auroras, or bright ribbons of glowing gas, that circle the moon’s poles.
Images taken by NASA’s Juno spacecraft on June 7, 2021, provided a fresh look at features on Ganymede’s surface including craters, clearly distinct dark, and bright terrain, and long structural features possibly linked to tectonic faults. Deep inside Ganymede, there’s a metallic iron core that generates the moon's magnetic field. A shell of rock surrounds the core, and another icy shell surrounds the rock.
Ganymede was discovered by Galileo Galilei on Jan. 7, 1610. The discovery, along with three other Jovian moons, was the first time a moon was discovered orbiting a planet other than Earth. Galileo's discovery eventually led to the understanding that planets orbit the sun, instead of our solar system revolving around Earth.
Galileo called this moon Jupiter III. When the numerical naming system was abandoned in the mid-1800s, the moon was named after Ganymede, a Trojan prince in Greek mythology. Zeus, a counterpart of Jupiter in Roman mythology, carried Ganymede, who had taken the form of an eagle, to Olympus, where he became a cupbearer to the Olympian gods and one of Zeus' lovers.