Later, when measuring Europa’s magnetic field, the spacecraft found global signs of an electrically conductive layer beneath the surface, precisely where a tidally heated reservoir of salty, highly conductive seawater would be.

If Europa is alive, if some biology dwells within those dark waters, the implications would be even more staggering than finding life on Mars. Our gaze would turn to Jupiter’s Ganymede next, and to Callisto, along with Saturn’s Titan and Enceladus, and perhaps even the dwarf planets such as Ceres and Pluto, all of which also likely harbour substantial subsurface reservoirs, heated through some combination of tides and radioactive decay.