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, published online today in the Journal of Geophysical Research: Space Physics, 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. Ceres, the largest object in the asteroid belt, may also have a subsurface ocean. The new results come from Hubble’s observations of Ganymede’s magnetic field, which produces two auroral belts (pictured) that can be detected in the ultraviolet. Because of interactions with Jupiter’s own magnetic field, these belts rock back and forth. However, there is a third magnetic field in the mix—one emanating from the electrically conductive, saltwater ocean and induced by Jupiter’s field—that counterbalances Jupiter’s field and reduces the rocking of the auroral belts. The Hubble study suggests that the ocean can be no deeper than 330 kilometers below the surface.