A salty ocean is lurking beneath the surface of Jupiter’s largest moon, Ganymede, scientists using the Hubble Space Telescope have found. The ocean on Ganymede — which is buried under a thick crust of ice — could actually harbour more water than all of Earth’s surface water combined, according to NASA officials. Scientists think the ocean is about 60 miles (100 kilometers) thick, 10 times the depth of Earth’s oceans. Scientists are particularly interested in learning more about watery worlds because life as we know it depends on water to thrive. Ganymede’s surface shows signs of flooding. Young parts of the moon may have been formed by water bubbling up from the interior of the moon through faults or cryo-volcanos at some point in the moon’s history.
Scientists have long suspected that there was an ocean of liquid water on Ganymede — the largest moon in the solar system, at about 3,273 miles (5,268 kilometers) across. The Galileo probe measured Ganymede’s magnetic field in 2002, providing some data supporting the theory that the moon has an ocean. Scientists used Hubble to monitor Ganymede’s auroras, ribbons of light at the poles created by the moon’s magnetic field. The moon’s auroras are also affected by Jupiter’s magnetic field because of the moon’s proximity to the huge planet. When Jupiter’s magnetic field changes, so does Ganymede’s. Researchers were able to watch the two auroras “rock” back and forth with Hubble. Ganymede’s aurora didn’t rock as much as expected, so by monitoring that motion, they concluded that a subsurface ocean was likely responsible for dampening the change in Ganymede’s aurora created by Jupiter. Astronomers might be able to detect oceans on planets near magnetically active stars using similar methods.
Jupiter’s moons are popular targets for future space missions. The European Space Agency is planning to send a probe called JUICE — short for JUpiter ICy moons Explorer — to Jupiter and its moons in 2022. JUICE is expected to check out Europa, Callisto and Ganymede during its mission. NASA also has its eye on the Jupiter system. Officials are hoping to send a probe to Europa by the mid-2020s.