Moon of Jupiter
Diameter: 5268 km
Distance: 1070400 km (mean)
Period: 7.1546 days
Eccentricity: 0.001
Inclination: 0.17°
Magnitude: 4.6

Discovered in 1610 by G. Galileo, Ganymede is the third and largest of Jupiter's 'Galilean Satellites', and the largest known moon in the solar system with 0.413 Earth radii. Ganymede is composed of equal amounts of silicate rock and water ice under which a saltwater ocean is believed to exist about 200 km below. Its surface is composed of old impact craters and regions of grooves and ridges with tidally forced tectonic activity. Ganymede is the only moon known to have a magnetossphere. March 2015: NASA's Hubble Space Telescope has the best evidence yet for an underground saltwater ocean on Ganymede, Jupiter's largest moon. The subterranean ocean is thought to have more water than all the water on Earth's surface. Hubble was used to watch aurorae glowing above the moon's icy surface. The aurorae are tied to the moon's magnetic field, which descends right down to the core of Ganymede. A saline ocean would influence the dynamics of the magnetic field as it interacts with Jupiter's own immense magnetic field, which engulfs Ganymede.