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A new international study co-led by Aix-Marseille University and Southwest Research Institute (SwRI) reveals that the striking contrast in the water contents of Jupiter’s Galilean moons was established at birth, as they formed around the gas giant. Within Jupiter’s circumplanetary disk, hydrated materials forming Europa remained water-rich, while the same materials dried up when crossing the dehydration line before reaching Io, producing an intrinsically arid moon.