The solar system’s largest moon, Ganymede, in orbit around Jupiter, harbors an underground ocean containing more water than all the oceans on Earth.
Scientists were already fairly confident in the ocean’s existence, based on the moon’s smooth icy surface—evidence of past resurfacing by the ocean—and other observations by the Galileo spacecraft, which made a handful of flybys in the 1990s. But new observations by the Hubble Space Telescope remove any remaining doubt. Ganymede now joins Jupiter’s Europa and two moons of Saturn, Titan and Enceladus, as moons with subsurface oceans—and good places to look for life. Learn more here or here.