Ever since 1979, when pictures sent back from the two Voyager probes revealed that one of Jupiter's moons, Europa, was swathed in cracked ice, scientists have suspected the existence of an ocean of water beneath its surface. But the evidence was only circumstantial. Europa's icy surface had fewer craters than Jupiter's other moons, which suggested that it was being constantly renewed and remoulded. Also, the scars and fissures on Europa's surface looked as though they had been filled in by upwellings from beneath.
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