A fresh, icy crust hides a deep, enigmatic ocean. Plumes of water burst through cracks in the ice, shooting into space. An intrepid lander collects samples and analyses them for hints of life.

ESA has started to turn this scene into a reality, devising a mission to investigate an ocean world around either Jupiter or Saturn. But which moon should we choose? What should the mission do exactly? A team of expert scientists has delivered their findings.