Larger than the planet Mercury, Ganymede is one of four moons discovered by Galileo in 1610, easily visible in small telescope and large binoculars. The subsurface ocean confirmed by Hubble is believed to be at least 60 miles thick, containing more water than all of Earth's ocean's combined. As such, Ganymede joins a growing list of planets and moons in Earth's solar system, including Jupiter's moon Europa and Saturn's moon Enceladus, that are known to harbor vast reservoirs of liquid water. The latest findings using the Hubble Space Telescope build on earlier observations by NASA's Galileo spacecraft that showed Ganymede...