'Jupiter's moon Europa may be a habitable world. Evidence points strongly to a global subsurface ocean beneath an ice shell. The paucity of large craters argues for a surface age of only 60 million years, implying that Europa is still geologically active. Tidal flexing and nonsynchronous rotation of the floating ice shell generate stresses that can fracture and deform the surface to create Europa's troughs, ridges, and bands. Europa's astonishing geology and astrobiological potential make it a top priority for spacecraft exploration.'