It now appears that, of the many moons of Jupiter and Saturn, two of them may have oceans beneath their icy exteriors. Scientists studying Jupiter's moon Ganymede - the largest moon in the solar system and the only one with its own magnetic field, which frequently sparks aurorae - used the Hubble space telescope to detect ultraviolet light emitted by the aurorae, which were less active than expected, given the moon's magnetic field. Researchers propose the field is being counteracted by an electrically conductive saltwater ocean beneath the crust.
