Jupiter’s three largest moons are thought to harbour oceans of liquid water beneath their icy crusts. Juice will investigate whether life could ever have emerged in these oceans. The mission’s primary scientific target will be Ganymede, the largest moon in the Solar System and the only one to generate its own
magnetic field. Scratched and scored with reddish-brown scars, Europa may vent water vapour to space via ‘plumes’ and geysers. Heavily cratered Callisto, a remnant of the early Jovian system, may contain a salty subsurface ocean.